Simple DIY Induction Heater - RMCybernetics

September 11, 2017 | Author: Indra Levyprawira | Category: Inductor, Capacitor, Field Effect Transistor, Power Supply, Transformer
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A DIY Induction Heater This great little project demonstrates the principles of high frequency magnetic induction. The circuit is very simple to build and only uses a few common components. With the induction coil shown here the circuit draws about 5A from a 15V supply when a screwdriver tip is heated. It takes approximately 30 second for the tip of the screwdriver to become red hot! The control circuit uses a method known as ZVS (zero voltage switching) to activate the transistors which allows for an efficient transfer of power. In the circuit you see here, the transistors barely get warm due to the ZVS method. Another great thing about this device is that it is a self resonant system and will automatically run at the resonant frequency of the attached coil and capacitor.

How Does Induction Heating Work? When a magnetic field changes near a metal or other conductive object, a flow of current (known as an eddy current) will be induced in the material and will generate heat. The heat generated is proportional to the current squared multiplied by the resistance of the material. The effects of induction are used in transformers for converting voltages in all sorts of appliances. Most transformers have a metallic core and will therefore have eddy currents induced into them when in use. Transformer designers use different techniques to prevent this as the heating is just wasted energy. In this project we will directly make use of this heating effect and try to maximise the heating effect produced by the eddy currents.

If we apply a continuously changing current to a coil of wire, we will have a continuously changing magnetic field within it. At higher frequencies the induction effect is quite strong and will tend to concentrate on the surface of the material being heated due to the skin effect. Typical induction heaters use frequencies from 10kHz to 1MHz.

DANGER: Very high temperatures can be generated with this device!

The Circuit The circuit used is a type of collector resonance Royer oscillator which has the advantages of simplicity and self resonant operation. A very similar circuit is used in common inverter circuits used for powering fluorescent lighting such as LCD backlights. They drive a center tapped transformer which steps up the voltage to around 800V for powering the lights. In this DIY induction heater circuit the transformer consists of the work coil and the object to be heated. The main disadvantage of this circuit is that a center tapped coil is needed which can be a little more tricky to wind than a common solenoid. The center tapped coil is needed so that we can create an AC field from a single DC supply and just two N-type transistors. The center of the coil is connected to the positive supply and then each end of the coil is alternately connected to ground by the transistors so that the current will flow back and forth in both directions. The amount of current drawn from the supply will vary with the temperature and size of the object being heated.

From this schematic of the induction heater you can see how simple it really is. Just a few basic components are all that is needed for creating a working induction heater device. R1 and R2 are standard 240 ohm, 0.6W resistors. The value of these resistors will determine how quickly the MOSFETs can turn on, and should be a reasonably low value. They should not be too small though, as the resistor will be pulled to ground via the diode when the opposite transistor switches on. The diodes D1 and D2 are used to discharge the MOSFET gates. They should be diodes with a low forward voltage drop so that the gate will be well discharged and the MOSFET fully off when the other is on. Schottky diodes such as the 1N5819 are recommended as they have low voltage drop and high speed. The voltage rating of the diodes must be sufficient to withstand the the voltage rise in the resonant circuit. In this project the voltage rose to as much as 70V. The transistors T1 and T2 are 100V 35A MOSFETs (STP30NF10). They were mounted on heatsinks for this project, but they barely got warm when running at the power levels shown here. These MOSFETs were chosen due to having a low drain-sorce resistance and fast response times.

The inductor L2 is used as a choke for keeping the high frequency oscillations out of the power supply, and to limit current to acceptable levels. The value of inductance should be quite large (ours was about 2mH), but also must be made with thick enough wire for carrying all the supply current. If there is no choke used, or it has too little inductance, the circuit

might fail to oscillate. The exact inductance value needed will vary with the PSU used and your coil setup. You may need to experiment before you get a good result. The one shown here was made by winding about 8 turns of 2mm thick magnet wire on a toroidal ferrite core. As an alternative you can simply wind wire onto a large bolt but you will need many more turns of wire to get the same inductance as from a toroidal ferrite core. You can see an example of this in the photo on the left. In the bottom left corner you can see a bolt wrapped with many turns of equipment wire. This setup on the breadboard was used at low power for testing. For more power it was necessary to use thicker wiring and to solder everything together. As there were so few components involved, we soldered all the connections directly and did not use a PCB. This was also useful for making the connections for the high current parts as thick wire could be directly soldered to the transistor terminals. In hindsight it might have been better to connect the induction coil by screwing it directly to the heatsinks on the MOSFETs. This is because the metal body of the transistors is also the collector terminal, and the heatsinks could help keep the coil cooler. The capacitor C1 and inductor L1 form the resonant tank circuit of the induction heater. These must be able to withstand large currents and temperatures. We used some 330nF polypropylene capacitors. More detail on these components is shown below.

The Induction Coil and Capacitor The coil must be made of thick wire or pipe as there will be large currents flowing in it. Copper pipe works well as the high frequency currents will mostly flow on the outer parts anyway. You can also pump cold water through the pipe to keep it cool. A capacitor must be connected parallel to the work coil to create a resonant tank circuit. The combination of inductance and capacitance will have a specific resonant frequency at which the control circuit will automatically operate. The coil-capacitor combination used here resonated at around 200kHz. It is important to use good quality capacitors that can withstand large currents and the heat dissipated within them otherwise they would soon fail and destroy your drive circuit. They must also be placed reasonably close to the work coil and using thick wire or pipe. Most of the current will be flowing between the coil and capacitor so this wire must be thickest. The wires linking to the circuit and power supply can be slightly thinner if desired. This coil here was made from 2mm diameter brass pipe. It was simple to wind and easy to solder to, but it would soon start to deform due to excess heating. The turns would then touch, shorting out and making it less effective. Since the control circuit stayed relatively cool during use, it seemed that this

could be made to work at higher power levels but it would be necessary to use thicker pipe or to water cool it. Next the setup was improved to tolerate a higher power level...

AVAILABLE PARTS: Resistors, Diodes, Capacitors, MOSFETs, Heatsinks, Ceramic support, 4mm Brass Pipe, 4mm Copper Pipe, Clear PVC Tubing, Large Bolt, 30A Cable, 12V Water Pump, 12V Regulator, Current Meter, Volt Meter, Power Supply

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Pushing it Further The main limitation of the setup above was that the work coil would get very hot after a short time due to the large currents. In order to have larger currents for a longer time, we made another coil using thicker brass tubing so that water could be pumped through when it was running. The thicker pipe was harder to bend, especially at the center tapping point. It was necessary to fill the pipe with fine sand before bending it as this prevents it from pinching at the sharp bends. It was then cleared out using compressed air.

The induction coil was made in two halves as shown here. They were then soldered together and a small piece of pvc pipe was used to connect the central pipes so that water could flow through the whole coil.

Less turns were used in this coil so that it would have a lower impedance and therefore sustain higher currents. The capacitance was also increased so that the resonant frequency would be lower. A total of six 330nF capacitors were used to give a total capacitance of 1.98uF.

The cables connecting to the coil were just soldered onto the pipe near the ends, just leaving room for fitting some PVC pipe. It is possible to cool this coil simply by feeding water through directly from the tap but it is better to use a pump and radiator to remove the heat. For this, an old fish tank pump was placed in a box of water and a pipe fitted the outlet nozzle. This pipe fed to a modified computer CPU cooler which used three heat-pipes to move the heat. The cooler was converted into a radiator by cutting the ends off the heat pipes and then linking them with PCV pipes to the the water would flow through all 3 heatpipes before exiting and going back to the pump. If you do cut some heatpipes yourself, make sure to do it in a well ventilated area, and not indoors as they contain volatile solvents that can be toxic to breathe. You should also wear protective gloves to prevent skin contact.

This modified CPU cooler was very effective as a radiator and allowed the water to remain quite cool. Other modifications needed were to replace the the diodes D1 and D2 with ones rated for higher voltages. We used the common 1N4007 diodes. This was because with the increased current there was a larger voltage rise in the resonant circuit. You can see in the image here that the peak voltage was 90V (yellow scope trace) which is also very close to the 100V rating of the transistors. The PSU used was set to 30V so it was also neccesary to feed the voltage to the transistor gates via a 12V voltage regulator. When no metal was inside the work coil, it would draw about 7A from the supply. When the bolt in the photo was added, this went up to 10A and then gradually dropped again as it heated up beyond curie temperature. It would certainly go over 10A with larger objects, but the PSU used has a 10A limit. You can find a suitable a 24V, 15A PSU in our online shop. The bolt you can see glowing red hot in the photo took about 30 seconds to reach maximum temperature. The screwdriver in the first image could now be heated red hot in about 5 seconds. In order to go to higher power than this, it would be necessary to use different capacitors or a larger array of them so that the current was more distributed between them. This is because the large currents flowing and high frequencies used would heat the capacitors significantly. After about 5 minutes of use at this power level the DIY induction heater needed to be switched off so that they could cool down. It would also be necessary to use a different pair of transistors so that they could withstand the larger voltage rises. In all this project was quite satisfying as it produced a good result from just a simple and inexpensive circuit. As it is, it could be useful for hardening steel, or for soldering small parts. If you decide to make your own induction heater project, please post your photos below. Please read through the other comments before making your own as it could save you time later on. If you wish to simulate this project for testing different inductance values or transistor choices, please download LTSpice and run this DIY Induction Heater Simulation (Right click, Save as)

Troubleshooting If you have trouble getting this working, here are a few tips to help troubleshoot your home made induction heater project.... PSU (Power Supply) If your PSU is unable to deliver a large surge of current when the induction heater is powered on, then it will fail to oscillate. The voltage from the supply will drop during that moment (although the PSU may not display this) and this will prevent the transistors from switching correctly. To help with this problem, you can place several large electrolytic capacitors in parallel with the supply. When charged they will be able to deliver a large surge current to your circuit. A good powerful supply would be our 24V 15A DC PSU. Choke (inductor L2) This limits the power to your induction heater. If yours is not oscillating, then you may need more inductance to prevent voltage drop in your PSU. You will need to experiment with how much inductance you need. Better to have too much, than too little as this will only limit the power of the heater. Too little may mean it wont work at all. Wiring Keep the connecting wires short to reduce stray inductance and interference. Long wires add unwanted resistance and inductance to the circuit and can result in unwanted oscillations or poor performance. Our 30A power cable is well suited to this. Components The transistors chosen must have a low voltage drop / on-state resistance otherwise they will overheat, or even prevent the system from oscillating. IGBTs will proabbaly not work, but most MOSFETs with similar ratings should be OK. The capacitors must have a low ESR (resistance) and ESL (inductance) so they can tolerate the high current and temperatures. The diodes should also have a low forward voltage drop so that the transistors switch off correctly. They should also be fast enough to work at the resonant frequency of your induction heater. Powering it up When switching it on, do not have metal within the heating coil. This can lead to larger current surges which could prevent the oscillation from starting as mentioned above. Also do not try to heat large amounts of metal. This project is only suitable for small induction heaters. If you want to control or gradually turn up the power, you can use one of our power pulse modulator circuits. See post 5108 below for details. Brain You will need a brain that functions reasonably well to make this project safely. If you are one of those people who is known by your friends as "a bit thick", then congratulations on being able to use a computer and navigate to this page, but unfortunately this project is not for you. It can be very dangerous to build an induction heater, so if you are new to electronics, you should get someone to help you make it. Approach things logically; If it is not working, check the components used are not faulty, check connections are correct, read this whole article and all the comments, search Google if you do not understand any of the terms, or read through our Learn Electronics section. Remember: Hot things will burn you and can set things on fire; Electricity can electrocute you and also cause fire. Put safety first. Rusdi - Sunday, 30th October 2011 1:01am - #4646

I intend to buy 3sets of all the parts from u guys to build this, but I am a newbie in electronics, is there any special considerations to build this other than then current and voltage hazard? And whats with the osciloscope? anything special with the osciloscope, like it has to be grounded in some particular way or anything else? Or any other particular considerations? Surastyo - Sunday, 30th October 2011 3:46am - #4648

I build this today and ended with burnt FET. I use FQP60N06. I use 6 turn and 4 x 470nF capacitors. How can it happen? Should I use 7812 to feed Gate? Can you show the drawing? Or wrong pin connection of FET? is it G, D, S if read from front side? Thanks RMCybernetics - Sunday, 30th October 2011 11:01am - #4649

Rusdi, There is also the temperature hazard, and then potentially chemical hazards if components get burned. The scope is not needed, but was useful for seing the waveforms and frequency. The scope ground should be the same ground as your PSU. you can then measure at the MOSFET gate, or collector terminals. If you measure at the collector, remember that the voltage could be much higher than the supply voltage so set the scope accordingly. Surastyo. I can't seem to find the datasheet for that part number. What are its ratings? They are typically set as GDS, but you need to check the manufacturers datasheet. I also do not see any large inductor (L2) in your picture. I've attached a diagram showing the 7812 voltage regulator being used. Garudadidada - Sunday, 30th October 2011 8:51pm - #4650

DELETED: No sending of secret messages in Indonesian or any other language. Surastyo - Monday, 31st October 2011 3:20am - #4652

FQP60N06: I think its 60V, 18A.its chinnese and hardly to find the data sheet. The large inductor (L2)is behind the CAPs. see my new pic. Thanks for 7812 diagram. I'll try another other MOSFET I have, while waiting the the order from RMCybernetics arrived. BTW what is modification needed to make it a "Induction Furnace" to melt aluminum (700 deg. C)? Thanks RMCybernetics - Monday, 31st October 2011 1:06pm - #4654

I think 60V is much too low of a voltage rating for the MOSFETs. THey are probably being destroyed by over voltage. To get hotter temperatures, you need to use more powerful transistors and capacitors, and then use a higher supply voltage. Rusdi - Monday, 31st October 2011 5:10pm - #4655

This is something else to use this as an induction furnace, but could you recommend stronger caps and transistors and how about the higher voltage power supply? I'd like to buy them from you, rmc Rusdi - Wednesday, 2nd November 2011 8:26am - #4656

Rmcybernetics, could u recomend some "higher" transistors and caps? I intend to buy some sets from u but I think ill go for the "higher" components if they exist in ur site and with ur recomendations. RMCybernetics - Wednesday, 2nd November 2011 9:43am - #4657

We don't have anything else available at the moment. You would just have to look for components with larger voltage and current ratings. Martin - Tuesday, 8th November 2011 3:42pm - #4659

I've tried building an induction heater based on your schematic and it works quite well. However, my heater won't heat up to 'glowing point' (maybe due to my power supply, it's only 30V, 5A) and, a bigger problem, my FETs tend to become pretty hot. When i look at my coil voltage with a scope it's way more noisy and irregular than what you are having. Do you have any clue about what might be going on here? joco - Tuesday, 8th November 2011 4:33pm - #4660

FET's going warm....Amps...up like hell...what is going on? I used irfpb4615, 4 pcs of 330n capacitor...the rest the same as yours...oh...with a 25V supply. RMCybernetics - Tuesday, 8th November 2011 6:24pm - #4661

Martin, If you are not getting nice sinusoidal oscillations, this could be due to having too little, or even too much inductance in your choke (L2). You may need to experiment with different choke designs to find something that works well with your other parts. Your work coil should ideally be only a little larger than the object to be heated as this will maximise the concentration of magnetic flux. If that does not help, add a photo of your scope showing the gate voltage, and drain voltage waveforms. Joco, I can't find that part number. Did you make a typo?

Tohooloo - Tuesday, 8th November 2011 9:06pm - #4662

What is the maximum capacitance for C1.

joco - Wednesday, 9th November 2011 6:45am - #4663

sorry for part number. it's irfb4615

RMCybernetics - Wednesday, 9th November 2011 8:38am - #4664

Tohooloo, C1 can be as large as you like. The combination of capacitance of C1 and the inductance of L1 form a resonant circuit. A larger capacitance or inductance will result in a lower frequency. Joco, That MOSFET seems ok. Try using a larger choke (L2). deyan - Friday, 11th November 2011 11:35pm - #4666

To get faster heating do i need higher or lower frequency RMCybernetics - Saturday, 12th November 2011 9:55am - #4667

Neither. You need higher power. deyan - Sunday, 13th November 2011 5:59pm - #4668

i am usinga 12 volt 10 amp power suply my trnsistor are p50n06 R1 R2 the dides and L2 are the same as yours. Is my suply weak because i cant get i to gloing point and a nother thing my car are burning in about a minute or so they are reatetd for 250-275 volts what shuld i change to get better reasults? Thank you. deyan - Monday, 14th November 2011 5:14pm - #4669

can i use a IGBT such as BUP203 as the transistors they are reated for 1000volts and if yes are there any difernces in the schematic? Stephen - Thursday, 8th December 2011 8:35pm - #4680

What voltage rating are your capacitors?

RMCybernetics - Sunday, 11th December 2011 4:32pm - #4684

Deyan, It sounds like your capacitors have too much internal resistance (ESR). You need something that is better for high power, they are typically much larger in size for the same capacitance value. There are many types of transistors that may work, but it is up to you to test if the ones you have are ok. I suspect that a high voltage IGBT will have too large of a voltage drop between collector and emmiter which would mean that the opposite transistor would not switch off properly. I used these 100V, 35A MOSFETs which worked well. Stephen, I used these 1000V, 330nF capacitors. Rob - Monday, 12th December 2011 8:01am - #4686

I sent an email, but I think I should just ask here. Would these parts work for this project? : 240 OHM 5W 5% METAL OXIDE 1N4007 FDA69N25 N-CH MOSFET 250V 69A TO-3P FILM Capacitor 2UF 700VDC B32794D3205K Also, will this circuit be able to handle 55V at 10A (with upgrades)? -Cheers RMCybernetics - Wednesday, 14th December 2011 11:59am - #4687

I can only suggest you follow the instructions given. If you use other components, they may or may not work. That is up to you to work out. Surastyo - Friday, 16th December 2011 11:10am - #4688

Hi again I try new coil, new caps, and new Mosfet. I burtn 5 or 6 of mosfets, it's happen because I didn't realize that one diode is sorted so HV directly goes to G pin. Now I use 30A, 30V power supply, IRFP260, 6A diode, 7812, 7T copper pipe coil,etc.I am quiet happy with the result, but it seems take to long to heat a thing. I plant to melt aluminum with it. my question are: 1. is coil turn, coil diameter, capacitor size affect heating power, or it just shaping the wave? 2. my wave is slithly different from yours, why? 3. is it ok to increase the V+ to 50V or 90V to increase power (yes I'll count the mosfet and diode power rate) ? 4. how to trim this to be most efficient? is wave form can determine that? let say clear simetrical wave is most efficient? 5. can we create sinusoidal wave? 6. is sinusoidal wave the most efficient wave for induction heating? Ohh... I hope it's not to much.. Thanks Surastyo - Friday, 16th December 2011 11:12am - #4689


it makes things hot 100 or 200 deg C maybe.. here the other

RMCybernetics - Friday, 16th December 2011 11:39am - #4690

1. All those factors will vary the performance. The number of turns will determine the impedance of the coil and therefore how much current will flow for a given frequency. Less turns will allow for higher peak current, but too few and the transistors will blow. The coil diameter should ideally be just slightly larger than the workpiece that is to be heated. This maximises the field coupling and is more efficient. The capacitor size will also determine peak current. larger is better, but not too large or the resonant frequency will become too low. Physically larger capacitors will have low internal resistance too so they will perform much better. The bank of caps in your picture look way to small for the amount of metal you are trying to heat. 2. You should get a nice half sine waveform. You need to experiment witht the inductor (L2). Try different numbers of turns until you see some improvement. You could also add 15V Zener diodes between the gate and source pins of each transistor to help protect them from voltage spikes. 3. V+ can be as high as you like, just make sure your components are sufficioent to handle the voltage rise in the coil. You will also need to supply the gates from a stable 12V source.

4,5,6. See other answers.

Nicollas - Tuesday, 20th December 2011 12:10am - #4691

Hello I wonder if VDC power supplies used to feed the coil L2 and Gates/MOSFETS are two power suplies separates. RMCybernetics - Tuesday, 20th December 2011 11:08am - #4692

You can use a single power supply, or seperate ones. If using a single supply it should be at most 15V, or 30V if you use the regulator as shown in another post. Nicollas - Wednesday, 21st December 2011 8:21pm - #4696

HELLO, MY HILL ​​LOOP, BUT WHEN PUT IN POWER SUPPLY CIRCUIT, He behaves like a short circuit and the voltage drops too, IF I DISCONNECT THE CENTER OF TENSION COIL Normalize, BUT NOT THE NOTHING HAPPENS RMCybernetics - Friday, 23rd December 2011 10:50am - #4697

Your power supply is not able to deliver enough current. You need a bigger supply, or many more turns on the work coil or choke. Rob - Sunday, 25th December 2011 8:40am - #4698

Will this circuit work properly with a power supply of 12v at 30A? RMCybernetics - Wednesday, 28th December 2011 2:25pm - #4699

Yes Nicollas - Thursday, 29th December 2011 8:55pm - #4701

I would like to know a way to reduce current consumption, because my source is 10 amps. RMCybernetics - Wednesday, 4th January 2012 11:15am - #4705

The current will depend on the voltage you apply, and the load impedance. More turns on your work coil, smaller capacitors, and larger choke inductance will reduce current. Rob - Thursday, 12th January 2012 5:01am - #4707

Just posting to share that I've successfully made my own heater using the info here. Some details of what I used: - ~.3uH work coil -12v 30A power supply -IRFB59N10DPBF mosfets -4.2uf capacitor bank (100A pulse) -2mH choke I posted a Video Response to the original video: -Thanks for sharing this information. RMCybernetics - Thursday, 12th January 2012 2:33pm - #4708

Nice job. Thanks for sharing Rob. james - Friday, 13th January 2012 11:00pm - #4709

Hello, Would it be possible for you to show a bit clearer picture of the physical connections of the transistors themselves and to also indicate which pins are what for the electronically challengened among us ? Chanil - Saturday, 14th January 2012 12:56am - #4710

Hi, is it possible to run this on 15V at 1A? For my application I only need it to achive 200 degrees celcius. But it has to run for days or even weeks at a time(it's for worlds largest 3d ABS printer). I'm currently using every component that is recommended, but all of them are getting hot... I have already roasted a capacitor(used a wrong one, would it explain the rise in temperture of the mosfets?)In my tests I've powered them with a 19V 5A psu(Im using a 12v regulator for the gates). I'm planning to use multiple heatsinks with a small fan to provide cooling. So to conclude will it reach the required temperture if only using 15V at 1A? Will the components stay cool if it would work? And if not any suggestions? RMCybernetics - Sunday, 15th January 2012 12:08pm - #4713

James, If you Google the part numbers you will find the manufacturer pdf datasheets. These will tell you which pins provide a particular function. I think it is best you try working this out yourself as you will gain a better knowledge and understanding. You can also check out the section on the site for helping to learn electronics as it will explain a lot about the components and the physics of the electronics involved. Chanil, Yes, if you want to limit the current to 1A, then you must increase the impedance of the circuit. Using many more turns in your work coil and the choke will limit the current. Chanil - Friday, 27th January 2012 7:21pm - #4719

Check out my induction heater ^^. I had a board made for it. This is one of the tests for it. Note: the cam can't read higher temps than 270 degrees celcius. And for my application only 205-210 is required. It reaches about 400-500 degrees celcius in the video. RMCybernetics - Sunday, 29th January 2012 9:38pm - #4722

Chanil's video... akram - Tuesday, 31st January 2012 9:26am - #4729

I used power supply 110 volt 60 A i want to design a circuit i want to know the kind of mosfet transistor and diode and resistors and capacitors so we want a quotation for the material justin - Sunday, 5th February 2012 11:16pm - #4731

hi guys i built this thing and had a little truble it turned out to be the diodes i was useing (4007) and changed them to some smaller glass ones (don't know what they are) and with 1.8uf worth of caps it works great!! it was a lot of fun and i built two more to give away all the parts i had got from a broken flat screen tv. they run around 125k and i used IGBT. thanks for the project and a good rundown on it!!!!!! ad - Monday, 6th February 2012 4:27am - #4732

RMCybernetics, I really like your coil design! I'm working on a project where I need to have a tank that resonates at about 1 MHz. Do you have suggestions for modifying C1, L1, and L2 for that purpose? As long as the FETS can switch fast enough the rest of the circuit can likely remain the same; but I'm not sure whether I'm better of decreasing C or L to get the right resonance. if you had any references for choosing those values that would also be great! thanks, a.d. Eric - Friday, 10th February 2012 10:22pm - #4737

I've noticed some interesting behavior with this circuit and I am curious what you think. I used all the same components (The coil is similar and the caps are similar, same mosfet, diodes and resistors (I did use a 124 ohm resistors as well, with similar behavior)). Using a 30V 30A supply the circuit pulls a lot of Amps (15 to 30A) at a very low voltage and the supply simply can't increase the voltage beyond a few volts. Using a 25V .5A supply the circuit draws power at a low voltage, but I can increase the voltage. At about 5V (according to the analog meter on the supply, could be different given the load) the resonant circuit kicks in and we get a beautiful sine wave on the resonant circuit (but minimal heating). Using this I went back to the large supply. I connected a switch between the supply and the circuit and set the supply to 12V. Flipping the switch (providing a near instantaneous 12V) drove the sine wave just as before (now pulling a bit more current as the supply could handle it). So there

seems to be some issue at low voltages. It may explain why @joco was getting 'amps like hell' at low voltages. RMCybernetics - Monday, 13th February 2012 4:57pm - #4740

ad, You can reduce C, but you will need t make sure that the remaining capacitors you have are able to take all the current. The advantage of using lots of capacitors is that the current is shared between them. Eric, What you describes just sounds like it is due to the limitations of your PSU. Remember that voltage and current are directly related and will be affected by the impedance of your load. If your PSU shows low volts, and high amps, this indicates that the impedance of your circuit is low and your PSU is dropping the volts to keep the current within its ratings. Eric - Thursday, 16th February 2012 1:24am - #4744

RMC, What is odd is that the circuit has very low impedance at low voltage (1-2 volts) and normal impedance at 12 volts. This is using the same circuit and supply in both cases. The only difference is that in the 12 volt case I immediately apply the full 12 volts via a switch instead of hooking up the supply and turning up the voltage. Using my low amperage supply I was able to see that the harmonic circuit wasn't kicking in until about 5V. My guess is that at low voltages the harmonic L-C system isn't being driven and the DC load is passing through a single mosfet through the coil. In this case the impedance would be very low. I have observed just a single hot mosfet in this odd low voltage scenario. RMCybernetics - Saturday, 18th February 2012 3:52pm - #4748

Yes, it would seem that you just have DC flowing at low voltages javier - Wednesday, 14th March 2012 4:01am - #4759


Your MOSFET needs to be able to withtsand the peak current. Measure the DC current your coil draws from your supply whehn you apply 120V. I suspect you need more turns on your work coil and choke. david - Saturday, 17th March 2012 4:10pm - #4769

can you use 12 volts for this project?

Hampus - Thursday, 29th March 2012 12:42pm - #4771

Which one of the diods are you actualy using? The 40V or the 100V? RMCybernetics - Friday, 6th April 2012 9:04am - #4777

David, yes. Hampus, Both. The first version uses the 40V ones, later when more power is used, the 1000V ones are used. Benjamin - Monday, 9th April 2012 12:59am - #4780

RMC, I have managed to reproduce your induction heater using the same components and circuit, but mine isn't working. I powered it with a 19V 4,5Amps laptop power supply. Actually, all the voltage is shared between the two resistors and it's like no current flows in the control circuit. Do you have any idea about where the problem could come from ? Chris - Monday, 16th April 2012 7:54am - #4782

Hi there. I built a smaller induction heater following as closely as I could to yours, and after a few unfortunate accidents, i got it working and loved it. I am now upsizing to a 110 volt version from wall

power rectified through a bridge and a transformer with rectifier for 12 volts to the gates. My question is can I solely use the choke to limit current so that I could have a one turn work coil if I wanted, and if so, do I add that impedance to half or all of the work coil as it is center tapped. Thank you for your great work RMCybernetics - Monday, 16th April 2012 9:47am - #4784

Benjamin, Maybe it is the 19V supply. Make sure you use a regulator for the gates as shown in a previous post. Chris, Yes the choke will limit the current, but only as long as it is oscillating. You should work based on the impedance of half the work coil. MrLeeh - Wednesday, 18th April 2012 12:13pm - #4787

Dear RMCybernetics, dear users thanks for this smart and very simple way of inductional heating. I tried it and eventually it worked very well. We can heat things up to 350°C and higher. At first we had problems with the MOSFETS getting hot and also switching through at a certain power voltage level (around 8V). So we couldn' t get any more then 40W of heating power. Then we discoverred that the wires we used to connect the SOURCE to the ground were simply to long and at the high frequency they produced lot' s of noise. So we decided to use only one thick wire as connection to the ground of the source and part it short-wired to the two MOSFETs. Now we still got some noise on the GATE but it works very well (15V, 10A). Greetings MrLeeh nathan - Sunday, 22nd April 2012 5:44pm - #4792

amazing project. cant wait to try it out. I've seen some posts about upping the power with larger caps and higher rated mosfets with larger power supply but is it possible to use more of the same mosfets just hooked in parallel with a larger power supply and caps or would that cause issues with the gates. I've seen it done in certain types of sound amps but I'm a little unfamiliar with high voltage mosfets and want to make sure before trying RMCybernetics - Tuesday, 24th April 2012 12:27pm - #4794

It might be possible to parallel some MOSFETs, but you would need to make short neat connections otherwise they may suffer from interference problems. Deven - Tuesday, 1st May 2012 4:21pm - #4795

which type of FET used in 30v 10A Supply in your second case, and give the dimention copper tube and redious of turn. RMCybernetics - Tuesday, 8th May 2012 12:10pm - #4796

The STP30NF10 is used in both versions. We used 4mm brass tube, with a coil radius of 25mm. Gilles - Wednesday, 6th June 2012 3:34pm - #4804

Hi, Is it possible and what would it take to have a dimmer switch controlling the intensity of heat? RMCybernetics - Tuesday, 12th June 2012 2:39pm - #4805

You would need some type of interrupter circuit and then vary the pulse width of the interrupter. For example; You could use one of our Power Pulse Modulators set to a low frequency to supply the power to the circuit. Adjusting the pulse width setting would proportionally adjust the power in the heater. Hans - Tuesday, 17th July 2012 8:46pm - #4807

Hi Would it be possible to use a welding AC "buzz box" whith a suitable bridge rectifier as a power source ? ( no load voltage 57 V and 25 V at 100 A) Or maybe a inverter welder? I wan't some type of transformer etc since I'm not to fond of hooking up the mains 240 V to a barbed wire !!! Hans whisk - Thursday, 19th July 2012 7:08pm - #4808

I am trying to build a small version of this using 12v power supply but the solder used to connect the coil to the capacitors keeps melting. Any suggestions. I believe the circuit is working but I can't be sure since it only stays on a few seconds before the solder melts. Perhaps something else is going wrong. Should the coil be getting this hot? RMCybernetics - Thursday, 19th July 2012 10:21pm - #4809

Hans, You would also need a very large smoothing capacitor, but it could work. whisk, If your solder is melting so quickly it is probably because your wire is too thin. You need to make the coil from very thick wire or copper pipe. wellington - Sunday, 22nd July 2012 1:22am - #4810

thanks for the info my question is if I can use this circuit to 24 volts from a battery or 110 volts i also used to reduce a source is 24v 0 30v please ci there is a change in the circuit please give me ce can also use more parallel trancistores wellington - Sunday, 22nd July 2012 1:34am - #4811

irf150n MOSFET can be used, or income tax or 54n 64n tanbien the iprf250n for use in parallel bariums RMCybernetics - Friday, 27th July 2012 12:33pm - #4814

wellinton, I do not understand you. If you are using an electronic translator, make sure to use good punctuation and grammar for an accurate translation. Aamer - Friday, 27th July 2012 8:42pm - #4815

I am planning to make this simple Induction Heater for heating Catheter Tipping Molds. Please see the attached picture of the mold for your reference. I plan to heat two molds simultaneously. Mold diameter 1/2" and distance between the molds would be 4". I have 2 questions: a)Is it important to have 1000v capacitors? What minimum voltage rating would be OK? b)The second picture shows 4 designs of work coils. Which of them would be more efficient to heat both the molds. Aamer - Friday, 27th July 2012 8:46pm - #4816

Here are the four designs of work coils. Ah yes, another question: Does a solid copper wire coil would do the job? kevin - Saturday, 28th July 2012 12:20am - #4817

Hey Guys, Ran some calculations on both of your heaters. In the first one, at about 200kHz, the inductor should be about 1 uH. Realize that the peak voltage that is ringing in the tank circuit is going to be pi* supply voltage, in this case about 47v. This yields a peak current of about 39 amps! (27.5 RMS amps) Given that 2mm brass tubing was used, this give the cross sectional area of about 3mm^2, and since it was tubing, most likely about 1mm^2 for current to flow, the approx. size of an 18 AWG wire. Passing 27 amps through 18 gauge wire would heat the wire up to well over 90C if it was pure copper, so being brass would increase the losses and it shows in the photo where unloaded the circuit is dissipating over 40 watts! no wonder the coild was starting to soften. The second circuit is a little better. It is using 1.98 uF of capacitance at 133 kHz. This gives us a coil inductance of about 0.723 uH. This yeilds a current in a tank of about 105 RMS A. Even though the tubing is larger and cooled, the losses before inserting the load was already at 210W. That being said, the current ringing in the work coil needs to be reduced. That can be done by one of two means, increase the inductance or reduce the capacitance or both. i(t)=v(t) sqrt (C/L). This is derived by setting the reactive power formula equal to one another and solving. KVAR = V^2/Xc = I^2*Xl. I would redesign to limit the resonant current in the work coil to be much lower and then see what quiescent losses are. Then put in your load. I bet the time to temperature would be greatly reduce because more power is going into the load and not be wasted in circulating current loses. Kev RMCybernetics - Monday, 30th July 2012 6:40pm - #4818

Aamer, a) The capacitors voltage rating just needs to be larger than the voltage they might be exposed to. Remember to account for the resonant voltage rise. b) A would not be very good due to low coupling, but the others will be ok. Copper wire is only going to be ok at low power levels. Litz wire would be better, but water cooled pipe will be best. RMCybernetics - Monday, 30th July 2012 6:41pm - #4819

Thanks for your comments Kev.

grant - Wednesday, 1st August 2012 3:58am - #4820

I would like a to make one that uses no more than 12V and 10A what size of capacitors and resisters etc. do I need. I need it to vaporize gasoline or diesel fuel in a metal pipe thats about 1/4 in ID max. (Goal temps around 140-200 F) I'm very green at this I need a lot of help with this science project RMCybernetics - Friday, 3rd August 2012 2:23pm - #4821

Grant, You need to work out the values for yourself, we can not do your project for you. john - Sunday, 5th August 2012 7:04am - #4822

Thank you for your time to answer this question...Is it possible to make an induction heater that would heat pieces of iron that are welded to a rotating drum as the individual pieces pass over the induction coil with say, 10 pieces of iron distributed around the circumference of this drum, and be

able to heat the drum by this method quickly to around 460 degrees, with a wattage of in excess of 3000 watts? Again thank you for the time to tell me if I am barking up the wrong tree. John RMCybernetics - Monday, 6th August 2012 1:20pm - #4823

It depends on how fast it is rotating, and how large the metal parts are. Aamer - Thursday, 9th August 2012 10:10pm - #4824

do say about this modification?

Comparing my last proposal of work coils - B, C, and D, what

RMCybernetics - Friday, 10th August 2012 6:40pm - #4825

It looks OK. Is there any reason the tips cant be closer together, or even placed end to end? Separating the two halves of the coil means that the field in each coil will oscillating in strength, rather than alternating polarity as you would have when the coils are together. You would really be better off having two separate circuits and coils as it would be most effective. Hans - Friday, 10th August 2012 8:17pm - #4827

Hi I was thinking about Kevins post. I'm not an EE and can be totaly wrong but : In a self oscillating circuit like this the freqency tend to be where the capacitive and inductive reactance equals out eatch other ie: the voltage and current is in phase and the power factor is close to 1 . If this is true, then the losses must come from resistive load ie the resistance in the coil and copper would probably be a much better material for the coil ? Since it is a self resonant circuit, changing cap or coil would alter freqency : more inductance or less capacitance = higher freqency and less inductance or more capacitance = lower frequency. Now to my question : If I am right, what benefits would there be if I used higher (or lower) freqency? Would I get higher effeciency ? If I am wrong, where did I go wrong? Hans Justin - Saturday, 11th August 2012 7:57pm - #4828

Is there a risk of shock with this circuit? -risk of shock by touching the element that is being heated in the coil with another metal rod? (or is it electrically isolated?) -risk of shock/spark/dangerous short by touching the element to be heated to the coil? -risk of shock by touching the brass coil? -anything else i might not be thinking of? RMCybernetics - Saturday, 11th August 2012 8:45pm - #4829

Hans, Most losses will come from resistance in the components. Mostly in the coil in this case, but losses in the capacitor are not insignificant. It is important to use capacitors with a low ESR. You have things a little mixed regards to altering frequency. Increasing either inductance or capacitance will lower the frequency. At higher frequency there will be more losses in the capacitor dielectric, and also

in the coil due to the skin effect. The skin effect would also mean that at high frequencies the heating would be concentrated in thinner layers on the surface of material sample. Justin, Yes, the voltage rises as described in the article would mean that exposed terminals or the coil could have high voltages present. You should avoid letting the heated object touch the coil as it could short circuit and damage your transistors. wellington - Thursday, 16th August 2012 9:40pm - #4830

thanks for the design I did 12 volt and perfect design I am interested in a dipocitibo de24volts acer 110 volts and 220 volts mofet which I can use or would have k k using thyristors and the condenser capacities thanks for the design is very cool I worked for one time and I'm interested in Majorcan efficient voltage for faster heating Nicollas - Friday, 17th August 2012 11:12pm - #4831

I set up a circuit like this. but he is not working. one of the mosfets is very warming and the current is very high. put 12V but the current one seems Curco circuit. aumeitei as the number of windings L1 and L2 no more works. nothing changes that. I'm using mosfet IRFP250N what might be happening? my source is 12V/20AMPS. RMCybernetics - Saturday, 18th August 2012 11:30am - #4832

wellington, I have no idea what you are talking about. Nicollas, I do not really understand all you wrote either, but it sounds like your circuit is not oscilating. If you are also using different diodes, it could be because they have too much voltage drop which prevents the transistors from being switched. Mike - Friday, 24th August 2012 4:38am - #4835

Can stainless steel be used for the work coil? RMCybernetics - Friday, 24th August 2012 9:04am - #4836

No. The resistance of the coil would be too high. The losses in the coil would cause it to get very hot and waste a lot of power. Copper is the best material to use. Brass was only used here because we had the pipe in our workshop and it was small enough for making this simple demonstration. Nicollas - Saturday, 25th August 2012 12:13am - #4837

swapped for new 1N4007 diodes and still does not work, the voltage drops to 12 volts to 6 volts, and does not oscillate. 'm using 220ohm resistors, it interfere in the functioning of the circuit? Mike - Monday, 27th August 2012 5:33am - #4838

Ok, great,one more question. I'm using your same circuit and I'm using a 12v 16.7A switching supply is this going to be a problem? I hooked it up and seemed to fry my diodes. I'm using a nte2396 FET with enhancement mode. I ordered the ones your using to see if that's the problem. it also makes my supply go into protection mode, almost like the supply saw the curcuit as a dead short, I'm hoping it's the transistors, or perhaps I need a bigger work coil, any help? RMCybernetics - Monday, 27th August 2012 8:06pm - #4839

Nicollas, Mike; Use the diagram in post 4649 that includes a voltage regulator. Use a bigger choke, and place a large capacitor parallel to the PSU to help it cope with current surges. Mike - Wednesday, 29th August 2012 4:22am - #4840

That's good avice, A: is there a calculation that I can use to determine the inductance for L2 based on the inductance/resistance of L1? Im using 3/8 inch copper pipe. B: do I stll need a 12V regulator in the circuit if I'm using a single 12V supply to drive the whole thing? Andreas - Thursday, 30th August 2012 12:33am - #4841

Hello. In advance, sorry about my poor English! We bougth a inductive heater at work a last week it cost about 1500 dollars, and i said that i was wondering about building one myself, so all the guys i work with laught at me and said i could not do it... So i desided that i had to do it to shut them up by building one! ;) but there is a few problems... I`m really not a expert on electronics but i have a mild understanding of it so i think i understand most of the shematic you have drew, exept on the right of the shematic there is someting called "L2". Can you maybe try to explain it to me? And the second question: I have a 22V power supply, If make the inductor just as you have shown in the first part of your DIY, exept i use My 22V supply instead of 15V, the Diodes that you used on the "phushing it

further" part of the DIY, the thicer brass tubing and maybe 4 capacitors (or do i need 6 on 22V allso?) Would this work??? I thing its great that i can buy all the parts from your DIY on your page! Best regards! Andreas - Thursday, 30th August 2012 10:28pm - #4842

Hello, I am going to use a 22V power supply, will i need the 12V voltage regulator mentioned in the diy? where do i conect it? Are the Mosfet`s marked so i know witch leg on the transistor goes where? wellington - Sunday, 2nd September 2012 10:21pm - #4843

is okey 12 volts 110volts ?mosfet? wat number plees RMCybernetics - Monday, 3rd September 2012 11:55am - #4844

Mike, Andreas, The inductor L2 serves as a choke to prevent high frequencies reaching your power supply, and also as a ballast to limit current. The calulations will depend on your operation frequency and desired current. 22V is fine, but you will need to feed the gates via a regulator as shown in a previous post. You should probably use more than four capacitors. Andreas - Monday, 3rd September 2012 9:50pm - #4845

Hello again. I have ordered 8 capasitors from you, is it best to use 6 like you have on your setup or will it be best to use them all? reg - Friday, 7th September 2012 3:03pm - #4849

i make your schematic and it works very good! thanks just one question, my mosfets are very hot why ? thank you for answer RMCybernetics - Friday, 7th September 2012 3:47pm - #4851

Andreas, 6 or 8 will work OK. Using 8 may share the current a little more and therefore not heat up so much. Reg, the temperature of the MOSFETs will depend on the current flowing and the frequency. If the frequency and current is quite high, it is normal for them to heat up. Try using a larger heatsink, or MOSFETs with a lower drain-source resistance. reg - Friday, 7th September 2012 3:58pm - #4852

thanks for your answer i use mosfet irf 540, use 36V power supply and separate 12V supply for vgs 10 + 10 turns work coil with 680 nF capacitor 1000V , L2 2O turns it is good? RMCybernetics - Friday, 7th September 2012 4:50pm - #4853

See the calculations posted by kevin in post 4817. With a 36V input and IRF540 transistor you are likely to blow the transistors. As shown in the article our 30V input was causing it to push close to the limits of the transistors used. reg - Friday, 7th September 2012 6:06pm - #4854

ok ,i will try 25 volts power supply and greater heatsink , thank you for your fast answer bye... reg - Tuesday, 11th September 2012 5:18pm - #4856

hello , do you know what it happen if the gates becomes less than 12V? Nicollas - Wednesday, 12th September 2012 7:20pm - #4857

I am very pleased with the outcome of my induction heater. after many failed attempts I managed to solve the problem and now it works fine. Thank you all for the help. Waheed - Thursday, 13th September 2012 12:11pm - #4858

Thanks for useful information. My circuit consists of IRF3710, 5x330nF/100V caps, 4 Turns of 3mm OD tubing at 12V. Draws 4 Amps. Tried it on higher voltage using 7812 but didnt work - no oscillation. Removed 7812 but with a large object in coil the current shoots up uncontrollably, burning FETs. What could be the reason. Thanks Waheed - Thursday, 13th September 2012 12:13pm - #4859

Just wanted to show the waveforms. RMCybernetics - Thursday, 13th September 2012 2:00pm - #4860

reg, if the gates are not switched on with a high enough voltage, the transistors will heat up significantly. Waheed, adding a large object in the coil will increase current demand a lot. if you want to limit the current, use a bigger choke (L2). Aamer - Friday, 14th September 2012 6:17pm - #4861

For the choke L2, can a straight ferrite bar be used instead of toroidal core? For the capacitor bank, is it OK to use 28 x 68nF (400V)? Aamer - Monday, 17th September 2012 2:47pm - #4862

Here is the work coil which I have designed for my application. I would like to know how shall I join the two ends encircled in red color for continuous flow of cooling water? Do I need to braze them with brass or simply connect them with PVC tube? RMCybernetics - Tuesday, 18th September 2012 6:00pm - #4864

A straight bar is ok, but will need more turns compared to a toroid. Those caps should be ok as long as they are capable of withstanding large currents and high temperatures. You can just link the ends using some PVC pipe as long as the copper is connected together too. Daniel - Wednesday, 19th September 2012 4:09pm - #4870

Hi I would like to make one that is about 500W Parts I have used: IRFP4768 transistor 1N4007 diodes 240 ohm resistor 12 turn work coil 7x0,15uF (total of 1,07uF) low ESR capacitors. About 2mH choke The powersupply is a 10A 30V lab supply. I use quite a big capacitor bank on the output of the powersupply to help it cope with the current surges. The

problem is that when I crank up the voltage the transistor gets worm and shorts out (drain/source) I suspect it has something to do with the hairy coil since wave (blue trace)? I cannot figure out how to get rid of that noice, so i would really appreciate any help I can get on the subject. I tried mounting 12V zener diodes to protect the gate but haven't dared turning up the voltage again while the hairy sine wave is still there. I am on my last set of transistors. Thank you for sharing sutch a good project with us, and helping us all out. Daniel - Wednesday, 19th September 2012 4:10pm - #4871

Here is a picture of the setup, I did not have my good camera here, so I can take a better picture later if that helps. RMCybernetics - Thursday, 20th September 2012 10:17pm - #4873

Try using a much much bigger choke, and regulating the voltage used to feed the gates. Make sure the supply to the gates is well regulated and filtered. grant - Sunday, 23rd September 2012 10:41pm - #4879

where did you get the radiator from?

Steve - Monday, 24th September 2012 3:05pm - #4880

What voltage should the 2uf capacitor be rated for? 15 V or 90 V? RMCybernetics - Monday, 24th September 2012 10:15pm - #4881

Grant, The radiator is a modified CPU cooler from a computer. I think this one came from an old DELL PC. Steve, The voltage rise in the circuit will be about 3.14 times your input voltage. But your capacitors voltage rating should be significantly larger so that they are not being pushed to their limits. I would suggest using one rated for at least 6 times your input voltage. Andreas - Monday, 24th September 2012 11:14pm - #4882

Hi. I buildt a Induction heater. I used the same components as you, so you know the specs off the transistors, capasitors, voltage regulator, resistors and diodes. My induction heater will not heat any thing and the transistors heat up really fast? They are mountet on heatsinks... what can be wrong here? My coil is maked from brake lines, i have allso tried whit mutch thiner wiers... Would it help if i uploaded a picture? RMCybernetics - Tuesday, 25th September 2012 12:16am - #4883

Are brake lines stainless steel? That has a high resistance, as do thin wires. You need to use thick cable or pipe that is highly conductive. Andreas - Tuesday, 25th September 2012 11:46pm - #4885

The wire i used vas about 1,5 mm2... do you think that`s the problem? Aamer - Friday, 28th September 2012 10:08am - #4886

Just to have everything in perfect order, here is the choke which I made with 2mm, SWG no. 14 copper enameled wire. Is it OK? The 28 x 68nF capacitors which I have mounted at the end of the work coil are MYLAR capacitors rated 400V but they are small size about 12mm x 8mm x 3mm. I wonder if they would do the job or should I replace them. RMCybernetics - Friday, 28th September 2012 1:40pm - #4887

Looks ok. You can try the caps you have but they may overheat quite easily. Andreas - Friday, 28th September 2012 11:21pm - #4888

hi, can anyone see any thing wrong whit this setup? this is my second inductor, but i am having the exact same problem... mosfets are getting real hot real fast? any ideas? RMCybernetics - Saturday, 29th September 2012 1:09pm - #4889

Looks ok, but dont let your heatsinks touch together or you will just be causing a short circuit. Fit a capacitor to the output of your regulator too, that will help smooth out any noisy power line. Something like 330uF or more. Daniel - Saturday, 29th September 2012 2:01pm - #4890

Hi I have tried different size inductors for L2 and it dosn't really seem to do any difference for the high frequency that is superimposed on the sinewawe. The biggest inductor I tried was about 40mH, The voltage in the LC circuit went down, but other than that there was little difference. I also have problems with the voltage spike in the beginning of the sine wawe being about double the voltage of the top of the sine. Andreas - Monday, 1st October 2012 8:59pm - #4891

I cant get this to work... the non of the conections or heatsinks are touching. Here is a picture the wiering, are the diodes the right way, have i solderes them to the right legs on the transistors? RMCybernetics - Monday, 1st October 2012 9:59pm - #4892

Daniel, Andreas, Make sure the supply to the resistors is regulated and smoothed with a capacitor. Check that your supply is capable of providing the peak current demanded by your coil. Decouple the supply to the gates by placing a capacitor from 12V to GND while keeping the connections as short as possible. Place a 12 to 15V zener diode between the gate and source of each MOSFET (cathode to the gate). This can help reduce noise and protects the transistors. Aamer - Thursday, 4th October 2012 7:18pm - #4894

I have assembled the parts listed below as shown in the photo: MOSFET - STP30NF10 - 2 Pcs with Heatsinks DIODES - 1N4007 - 2 Pcs CAPACITORS - 330n x 6 (1250v) RESISTORS - 240 Ohms CHOKE L2 - SWG 14 (2 mm) Copper Wire 8 turns PSU - 12V 30A regulated, smoothed I switched ON the PSU for a few seconds, the MOSFET M1 in the photo started getting hot so, I switched OFF. Need your further assistance, please. One thing, the PSU is connected thru 50cm wires. RMCybernetics - Friday, 5th October 2012 11:34am - #4895

Try replacing your choke with something with huge inductance so that power is limited significantly. You can use an ordinary mains transformer for this. Just use an output winding as the choke. If it still wont oscillate, make sure you don't have a dud transistor or diode somewhere. Aamer - Friday, 5th October 2012 3:52pm - #4896

Before replacing the choke I checked the MOSFETs and found that my all the pins of my M1 are short. For M2, while connecting the negative probe of multimeter (on diode checking mode) to the center pin (DRAIN) and the positive probe to the left pin (GATE) the multimeter beeps for continuity. And touching the positive probe to the right pin (SOURCE) the multimeter shows 490. And if connecting the pins vice versa then no continuity. I can understand that M1 is bad but not sure about about M2. As for replacing the choke with an ordinary transformer, do you mean I can use secondary winding of any 220->3v, 6v, 9v, 12v transformer (less than 1 Ampere)? RMCybernetics - Friday, 5th October 2012 4:39pm - #4897

You should just replace both MOSFETs in your induction heater to be sure. Yes, pretty much any low power transformer will have enough inductance to limit the current to a couple of amps or less. It will be useless for significant heating, but allows you to test if it is oscilating without blowing components. You may also want to add zener protection to the transistor gates. Frank - Saturday, 6th October 2012 12:59am - #4901

Couple questions. 1. Obviously if you push the voltage up, the Mosfet gate must be protected with a reg, but where are you putting that because it appears you are shorting the drain and gate?? 2. You mention using alt caps for this. Would I need the lowest ESR caps possible? IE poly or film/foil caps? 3. There was a question above regarding tubing choice. It would seem that copper might be better for this. Is that accurate? Why did you select brass? RMCybernetics - Saturday, 6th October 2012 9:20am - #4902

See post #4649 for a diagram including a regulator. Yes, the capacitors in your induction heater must be low ESR otherwise resistance losses will soon heat them to destruction. Yes, copper is better due to lower resistance. Brass was used simply becasue we had some available at the time. I've since built one with copper which I will post later, but I did not see any noticable performance difference and the heater coil still needed to be water cooled. Paul - Saturday, 6th October 2012 11:06pm - #4903

Hi, Just an experimenter in electronics but think I will have to try this! Two questions 1) I see people generally heating iron or steel. Are there other common materials which might heat even more efffectively and is there a simple answer to predict which materials will heat well (I am guessing high permeativity, high hysteresis?) 2) Is the heating effect uniform throughout the interior of the coil? Or is it concentrated more towards the center or periphery? Lets say you were trying to heat the water in the copper tubing, would a hollow metal tube fit to the diameter of the coil work? Well I guess that is more than two questions, In any event thanks this looks like a great project. RMCybernetics - Sunday, 7th October 2012 12:25pm - #4904

Iron based metals heat well because of hysteresis losses and conduction losses. Metals like copper and aluminium can be more difficult to heat partly due to higher conductivity. The magnetic field is strongest at the inside edge of the coil. The skin effect also forces heating to concentrate on the surface of a material. Aamer - Monday, 8th October 2012 3:02pm - #4905

I have replaced the MOSFETs. And also have replaced the choke with a low power transformer. I do not have an oscilliscope so how do I know if the circuit is oscillating or not. RMCybernetics - Monday, 8th October 2012 3:19pm - #4906

Place something metal in the coil and see if it gets any warmer. Frank - Monday, 8th October 2012 5:47pm - #4907

RM, Thanks for the reply regarding the voltage reg. I did notice the diagram with the 7812 but thought it was imcomplete because the drains of each mosfet still feed back to the gate of the other after the voltage reg. I was wondering if this should be done with two regs, both right at the gate and after the diodes? Just not sure how this configuration will work? Aamer - Monday, 8th October 2012 9:32pm - #4908

Okay, I have made the test replacing the choke L2 with a low power transformer. Initially, the work coil became warmer and warmer so I circulated water in it. Finally I was able to notice that the iron rod inside the coil has become warm. However, when I replaced the transformer with the choke L2, the MOSFETs became hot quickly. And nothing happened to the iron rod inside the work coil. By the way, I haven't yet introduced zener protection for the transistor gate. RMCybernetics - Tuesday, 9th October 2012 7:46pm - #4909

Frank, The diagram is ok. You should only use one regulator. The way the connections switch on and off the gates is described in the article.

Aamer, Show us your work coil and iron rod. Aamer - Tuesday, 9th October 2012 8:58pm - #4910

You can see the work coil at post no. 4862. You can see the circuit at post no. 4894. The iron rod was not a particular one. I tested on an iron nail, an 8mm iron rod and 5mm rod.....all three got warm. RMCybernetics - Wednesday, 10th October 2012 5:05pm - #4911

I still think you need a bigger choke. The huge inductance of the transformer winding is allowing it to oscillate, so you just need to find the right iinductance value for your setup. When powering up the induction heater, I would suggest that you do so without any metal within the coil, then add the metal after. Aamer - Wednesday, 10th October 2012 6:22pm - #4912

Do you mean I should increase the number of turns in my choke shown in post no. 4886? How many more turns do you suggest? How do we find out the matching coil size other than actual trails? By the way, does every new circuit we build (with same parts) needs different choke setting (inductance)? RMCybernetics - Wednesday, 10th October 2012 8:01pm - #4913

Yes, I think you will need to experiment with what works best with your specific PSU and heating coil. You could also try using a Power Pulse Modulator between your PSU and the induction heater circuit. This would allow you to vary the power to the heater. You woild set the frequency settng to a low value like 20 Hz, then you can adjust the duty setting from 0 to 100% to give you a 0 to 100% adjustment in heating power. paul - Sunday, 14th October 2012 10:12am - #4917

Hi. I did exactly as instructed here and used all the same components but mine doesnt seem to work. Only one mosfet gets really warm. The PSu which was initially set to 15v and 5 amps drops to below 5 volts once the control circuit is connect. Any idea on what seems to be the problem? The PSU is rated at 30V and 10A. Thanks. RMCybernetics - Sunday, 14th October 2012 2:59pm - #4918

Your PSU is not able to meet the peak current demaned by your setup. When you first connect power, there will be a surge of current which is mostly limited by your choke (L2). Use a bigger choke, and add some large capacitors to your PSU. Aamer - Monday, 15th October 2012 7:07pm - #4919

I increased the size of choke L2 from 8 to 9 turns. It increased the amperes and blew off 30A fuse of my PSU. Now, as per your suggestion to introduce Power Pulse Modulator, I have just finished the "SQUARE WAVE SIGNAL GENERATOR WITH PULSE MODULATION" for my Induction Heater. I am confused how would it withstand heavy amperes. Please guide me how should I use it so that I can tune my Induction Heater. RMCybernetics - Tuesday, 16th October 2012 12:05pm - #4921

See the troubleshooting section I've added to the end of the article. You don't tune your induction heater. It automatically works at the resonant frequency. The link I gave you is to a product which will allow you to limit and control the input power so that you do not blow your transistors while testing it. I don't think adding another DIY circuit is going to help you, as it just increases complexity. Adding 1 turn to your choke is going to make little difference. Add something like 20 turns. I really do not think there is anything else left to suggest to you. If you are following the article and all the advice, but it is still not working, then you must have faulty parts, wrong connections, or a PSU that is not up to the job. Allen - Saturday, 20th October 2012 11:42am - #4922

Since you guys have a bit more experience then myself with induction heating devices, I wanted to ask how well might an MTY100N10E N ch. Mosfet work for an induction heating application as far as the drive transistors? This transistor is rated for 100A @ 100v the on-resistance is quite low as well @ 0.011ohms, the only thing I see that is quite different from the recommended transistor pair, is the switching times (rise / fall) are quite a bit higher, but I suspect that to be normal for such high

currents possibly? I'd like to get some feedback from someone with a bit more experience to see if I can use these transistors for my induction heating tests. I'm interested in heating a bit larger objects and a bit faster. Thanks -Allen RMCybernetics - Saturday, 20th October 2012 12:10pm - #4923

While I can't comment on every individual transistor model anyone might use, I can suggest that if you use one with longer rise/fall times, you could compensate by using a larger capacitance to lower the resonant frequency of the system. Allen - Saturday, 20th October 2012 7:41pm - #4924

Is there a ratio of rise / fall times to capacitance values? I'm curious to know the mathematics behind the circuit. RMCybernetics - Saturday, 20th October 2012 8:44pm - #4925

It only becomes an issue if the rise/fall times are a significant value compared to the period of the operating frequency of the circuit. The transistors are switching when the voltage (between source and drain) is almost zero. Just work out what the switching time is as a percentage of the period. By minimising this value, the switching losses are reduced. In this circuit you would probably be more concened with conduction losses due to high currents unless you need it to operate at some very high frequency for some reason. Joe - Monday, 22nd October 2012 9:49am - #4927

Hi...I have been looking for a circuit like this wit a straight forward explanation..Kudos RMCybernetics. Will the caps (pictured) work with this project. .003uF 4Kv (302K4M) I am seeing in my calculations that my project will resonate near 400Khz. And if I am right, not an engineer, would mean I can heat larger objects. RMCybernetics - Monday, 22nd October 2012 1:42pm - #4928

No they wont. That looks like a polystyrene capacitor and judging by the thickness of the wires, it is not meant for high currents. You need to use polypropylene capacitors or an equivalent that is specifically made for high currents. For larger objects you need more power. Joe - Monday, 22nd October 2012 7:02pm - #4930

Thank you, I had guessed they werent going to work..wanted to ask before I released the magic smoke out of them Joe - Tuesday, 23rd October 2012 8:07pm - #4931

Last question: What is, if any, the difference between polypropylene and mylar caps? RMCybernetics - Wednesday, 24th October 2012 4:05pm - #4932

This link should answer that for you. Aamer - Thursday, 25th October 2012 10:07am - #4933

Good day Richard.....Appreciate your devotion to guide others. Just for reference, I posted my Work Coil design at #4862 and the choke at #4886. I had to replace one faulty STP40NF10 with IRFP150. Now my circuit is oscillating with STP40NF10 + IRFP150 but the trial load (4mm iron nail) only gets warm. As per your advice, I re-designed my choke with 20 turns of 2mm wire and then reduced

gradually 18, 16, 14, 12, 11, 10, 9, 8, 7, 6, 5 turns but no significant change. However, at 9-8-7-6 turns the load got warmer + the STP40NF10 dispersed more heat. By the way, I inserted the load for 5-10 seconds. Aamer - Thursday, 25th October 2012 10:16am - #4934

Another question: I want to understand the behavior of this circuit and hence plan to simulate it in LTspice. How should I check if my simulation is correct......I mean to say, at what points I should check for the waveforms and what type of waveforms? RMCybernetics - Thursday, 25th October 2012 1:43pm - #4935

You should allow more time for your load to heat up. I've added a spice simulation to the article just above the troubleshooting section. The STP40NF10 was not in the library so I just chose something else in the library. Run the simulation then check out the collector waveforms and inductor current. The simulation does not include any load, but you can add this if you wish. I'm not here to answer questions on how to use LTSpice though. Aamer - Thursday, 25th October 2012 2:31pm - #4936

As per my results on various turns of the choke, the intensity of heat (as I felt) was almost the same. Of course, keeping the load for more time will make it hot. But, pardon me, my trial load is not thicker than the screwdriver shown in your article, which turned red hot in just 5 seconds. RMCybernetics - Thursday, 25th October 2012 3:34pm - #4937

You are using different components and some kind of DIY power supply. You will of course not get the same results. Aamer - Thursday, 25th October 2012 8:34pm - #4938

Thanks for your swift response. Okay, I shall change the STP40NF10 to IRFP150 and then re-try with my existing PSU which is functioning quite OK. But, I shall definitely not ignore your tip to change the PSU. By the way, what do say about the 12V 30A SMPS, will it be OK?? Your netlist could not be simulated because LTspice is giving an error "Multiple instances of Flag". Would you please check it. Thanks for your attention. RMCybernetics - Thursday, 25th October 2012 11:22pm - #4939

Lol, the spice file is fine, I really don't know what you must be doing. You will need a higher input voltage than 12V to get heating like shown in the video (I used 30V). Have you also considered that your capacitors may be damaged, or just not up to the job? Aamer - Friday, 26th October 2012 12:08pm - #4940

I have limited knowledge/experience of electronics. I am simply following the instructions as well the experience of other fellows. I knew you used 30v 10A. But you also confirmed in post #4698 that the circuit will work with 12v 30A PSU, and Bob was successful (post # 4894). Anyway, I can increase the voltage to 24V at 30A, will it be OK? As far as the capacitors are concerned, frankly, when the circuit is oscillating, I understood that the caps are working fine. I will try again with 24v, if unsuccessful then will check the caps. I renamed the spice file extension to "*.cir" and opened it in LTspice and then clicked RUN icon. If you think I am wrong, can you provide the "*.asc" file??? Lastly, thanks for your guidance. Nicollas - Sunday, 28th October 2012 1:07am - #4941

This is the result obtained by me. was very good and oscillates at frequency of 75kHz. Thanks for the help RMcybernetics and friends. Charles - Sunday, 28th October 2012 2:12am - #4942

> > > > two questions < < < < 1. In a water cooled installation would one still need brass induction coils instead of plain copper tubing? 2. I see where the coil configuration can be modified. So for tempering knives one could have an egg shaped coil allowing 1/4" clearances on all 4 sides. Correct? . . .thank you chas. ps. great site. RMCybernetics - Sunday, 28th October 2012 2:52pm - #4943

Aamer, It is already an asc file. I don't know why you renamed it. Yes 12V will work but of course less powerful than 30V. As mentioned multiple times, the capacitors need to be quality polypropylene or equivalent. It may oscillate when using other caps, but the performance will be poor. If they are damaged by voltage spikes, that will also prevent proper operation. Nicollas, Thanks for sharing. Nice build quality. Charles, please read through the other comments. As mentioned already, brass was only used because it was available at the time. Copper pipe would be better. Yes you can use whatever coil shape you like. However the heating effect may not exactly match the shape as the magnetic interactions and shape of the object will dominate the effect. ad - Tuesday, 30th October 2012 10:20am - #4944

Hello i build you heater, and it works ok....Now i want more power....but i hav one question... If i use bigger FET's / IGBT,up to 600v and 150A (MG150Q2YS40).. Can is use the same diodes to the gate, i think the power over the gates will be more than 20 volts.... RMCybernetics - Tuesday, 30th October 2012 11:13am - #4945

You can not just replace the transistors with power IGBTs. The saturation voltage of the transistors would be too high, preventing proper switching as explained in the article. A different and more complex circuit would be needed. Aamer - Tuesday, 30th October 2012 2:14pm - #4946

Hi Richard. (1) The link for simulation opens NETLIST in another window which deceived me that you are providing netlist instead of asc file. Anyway, I have opened your asc file and compared with mine which wasn't simulating. The main difference was that you used 2 power supplies. Now my simulation results are same as yours. (2) In the simulation, I noticed that the circuit (except the capacitors and inductors) is drawing less than 1 Amp. So, I physically connected 12v-2A supply to the MOSFETs and 12V-30A to the choke. No oscillation, one MOSFET burnt. Maybe no oscillation because MOSFET burnt. Am I wrong somewhere? I plan to use 24v-30A for the choke and 12v-2A for the MOSFETs. (3) Mosfets IRFP150 + 6 x 330n Polypropylene Caps still oscillating at 12v-30A (but not red hot). Shall I use 24v-30A supply and add 7812 for the MOSFETs? RMCybernetics - Tuesday, 30th October 2012 2:46pm - #4947

The link is directly to an asc file. You can click to view it, or right click and save it to your computer. The simulation does not include any load and is not accounting for the losses in the system which is why it only draws a low current. You should observe the voltage on the 30V supply. During the first moments it will drop very low which could prevent oscilation in a single supply system if not accounted for. You should also consider the initial voltage spike as this could blow any of your components. Try this simulation. It will not oscilate becasue the PSU has too much internal resistance. Run it, you will only see DC, then alter the resistance to something lower and you will see it begins to oscilate. Yes, add the regulator and zeners as suggested. Aamer - Wednesday, 31st October 2012 7:30am - #4951

Thanks for your swift response. (1) Okay, I shall change to 24V 30A + 7812 but for the sake of learning, I want to understand why my circuit did not worked with 2 power supplies? (2) The simulation helped me a lot. Thanks for the newer simulation with single supply. For load, I coupled another inductor L3 (1uH) to L1/L1b and connected a load in parallel to L3. The maximum readings (during the initial spike) I got with 200 Ohms......V at drain 200V, I at drain 24A, V at gate 14V, I at gate < 1A. So how about changing the mosfets to 300V/35A to ensure protection against the initial spike? RMCybernetics - Wednesday, 31st October 2012 9:34am - #4953

Yes, use a single 24V supply and regulator for the gates. When using two supplies, did you create a common ground? You could change the MOSFETs if you like, just choose good ones with low resistance and high speed. ad - Wednesday, 31st October 2012 9:47am - #4954

Okay thank for the reply..

ad - Wednesday, 31st October 2012 9:50am - #4955

If i use a fets who has a voltage of 600 and 50 Amps , can i use it from the net after reverse it from 220 volt to DC, the IN4007 and caps are up to 1000 volt...BUT what wil happen to the gate power of the fets...? Aamer - Wednesday, 31st October 2012 11:23am - #4956

Thanks for your usual prompt response. (1) Yes, I had a common ground when I used 2 supplies. (2) So changing the MOSFETs to higher rated voltage can withstand the initial spike? RMCybernetics - Wednesday, 31st October 2012 1:48pm - #4957

Well the STP30NF10 used here does fine even with 30V input if you have a regulator and zener protection on the gates, but you can try other transistors if you want. Aamer - Wednesday, 31st October 2012 3:05pm - #4958

(1) Noted. (2) Any comments --- why didn't my circuit worked with two supplies? RMCybernetics - Wednesday, 31st October 2012 3:42pm - #4959

There's nothing else I can suggest. ad - Wednesday, 31st October 2012 7:09pm - #4960

I used HFR3205, and it works good.....what kind of regulator and zener you maen or think is possible.....can you make a schematic.... thanks ad - Thursday, 1st November 2012 11:03am - #4961

this is want more power !! RMCybernetics - Thursday, 1st November 2012 11:10am - #4962

Looks nice. DId you etch your own PCB? You may want to reinforce the PCB traces from the caps though as they will be carrying a lot of current. You can see the regulator diagram in post 4649. The zeners would just go between the gate and source with the cathode to the gate Charles - Thursday, 1st November 2012 11:20am - #4963

I own a 12 volt, 5.5 amp dc power supply. I would like an induction furnace that could heat 25mm round or square stock. Is this power supply sufficient to accomplish this. . . 62 years ago I fell off the 'Wheatstone Bridge' in college..hence my need for assistance. thank you. RMCybernetics - Thursday, 1st November 2012 11:30am - #4964

It would heat it, but I would not call it a furnace. You may even struggle to get it red hot. Once such a large load is placed in the coil, the current draw will rise significantly, and quite probably need more than 5.5A. We will however post a new project in future which allows for higher power levels. ad - Thursday, 1st November 2012 12:44pm - #4965

the other side.....used a lot of solder tin :-)

ad - Thursday, 1st November 2012 1:03pm - #4966

one problem if i rectify 230 volt straight from the net (its 230 x 1,44)it will be 330 volt DC a simple regulator (7812) will go to max 30 volts...and how much volt wil there be on the diodes...? RMCybernetics - Thursday, 1st November 2012 1:11pm - #4967

You can't use this circuit at those voltages. You would need a different, more complex setup which is beyond the scope of this article. Mike - Friday, 2nd November 2012 11:32am - #4968

@#4966: Why do you want to use 230Vac? It is all about amperes for induction heating and high frequencies to stimulate hysteresis in the workpiece. When Xc and Xl (reactance of capacitance and solenoid) are the same you are in resonance and the circuit acts as a short (for this parallel circuit it does). So if you would have 0.4 Ohm resistance on the copper wires, you would have 330 / 0.4 = 825 Amperes with 250Vac rectified. Why not taking a 12 volt power supply instead? Then you would have 30 amps. That is quiete alot. Or take a 24v power supply and you would have ~60 Amps!! Charles - Friday, 2nd November 2012 12:24pm - #4969

Just obtained 24vdc, 15 amp power supply. . . .Please furnish me with the type and number of torroid coils (L2)you recommend and sell to max this power supply. And the size, number, voltage and type of caps I will need, and the two transistors (T1 & T2), you sell. . . .I will want 4 Zener Diodes, plus 6 resistors (R-1 & R-2). . . .I have the heat sinks for the transistors and for the caps. I assume the Voltage Reg (7802?) is readily available locally. . . .I would appreciate the material costs and the shipping to zip code 59711, USA. I would best like an email in responce to any necessary questions regarding this request. . . .Thank you, charlie. RMCybernetics - Saturday, 3rd November 2012 6:30pm - #4970

Links to all the available parts are in the article. If you add them to your basket, you can get a shipping price calculated automatically. Nicollas - Saturday, 3rd November 2012 7:29pm - #4971

I wonder if this circuit can operate with a voltage of 220 volts? I know I'll have to change the components, but I wonder if the circuit is capable switching at higher voltages. I need heat a billet steel 15mm in diameter. Aamer - Monday, 12th November 2012 6:18pm - #4975

Good day! (1) After many failures, I decided to start all over again. I made a new work coil with 2mm solid copper wire 5+5 turns. For choke I used 1mm wire, 16-18 turns. For MOSFETs 12v-2A supply and for CHOKE 19v-4A (laptop) supply. The combination worked nicely, the sample got very hot in just 3-5 seconds (not red hot because of only 4A supply). (2) The results encouraged me, I made another W/coil from 4.7mm outer dia/2.7mm inner dia copper tubing. I started with 3+3 turns and coil shape round unlike the one shown in post no. 4862. Power supplies remained as above. The load remained cool and the w/coil became warm without water circulation. Modified the turns of 1mm wire choke --- No improvements. Changed the choke to 2mm wire 7-8 turns ---- No improvements, additionally the MOSFETS are becoming pretty warm. Aamer - Monday, 12th November 2012 9:00pm - #4976

(1) Failure of my oval shaped work coil leads me to make 2 circuits which I have to use simultaneously. Do I have to arrange separate PSUs for chokes in both circuit? (2) Will there be some

sort of electrolysis effect if the cooling water from one work coil flows into the other? (3) Is there any way which can indicate resonance.... something like an LED or a neon bulb? Mii Yeti - Saturday, 17th November 2012 10:15pm - #4978

Hi I would like to know if it is possible to use two 2n3055 transistors Thanks Mii Yeti RMCybernetics - Monday, 19th November 2012 10:40am - #4980

As long as the water is not contaminated with significant amounts of electrolyte (like in salt water), then the length of tubing should make the total resistance so high that any current flowing in the water would be neglegable. 2n3055 is not suitable. Use the MOSFETs mentioned in the article. Mii - Tuesday, 20th November 2012 4:23am - #4981

Thank you for your help and time.

Mike - Sunday, 25th November 2012 10:11am - #4983

How is it possible that the applied 12 Vgs on the MOSFETs gates cause a 90 Vpk-pk on the scope? I mean Vds will never be higher than Vgs, right? But (90Vpk-pk) / 2 = 45Vmax. 45 Vmax / SQRT(2) = 31.8 Vrms. Where 31.8 Vrms equals ~ the input voltage of 30V from the PSU. How can you ever reach a Vrms higher than the applied Vgs on the MOSFETs. Please do explain as I wanted to use a power supply of 45Vrms to crank up the power! RMCybernetics - Sunday, 25th November 2012 11:57am - #4985

Vgs does not determine Vds when the gate is saturated (such as above 10V). The voltage applied to the gate here is 12V, but the voltage from the supply for the rest of the circuit is 30V. When a 'Tank Circuit' (inductor and capacitor in parallel) is pulsed, it will resonate at its fundamental frequency. If the Q factor of the circuit is good enough (there are not significant losses through resistance etc), then there will be a resonant voltage rise in the circuit. The peak voltage here will be about pi x input voltage which is why we get about 90V from 30V input. The power actually builds up in the LC tank circuit. Once running, the current drawn from the supply will be less than what is actually flowing in the induction heater circuit. Try the simulation in the article to see this. farhad - Saturday, 1st December 2012 9:20pm - #4986

hi,I build it by irf3205 VDSS=55V and it work good at 10v power my coil diameter is 6cm and 330n capacitor it resonant at 150KHz but not hot I ask you to explan why we use choke what is the function of choke exactly ? and why in simulation in voltage 4V diod and mosfet switch? i eliminate L2 and power2 in simulation and see it resonant and voltage being up per second why? what is the function of resistanse (its effect on speed of starting resonant)? why frefuensy is not egual to 1/2*pi(L*C)^0.5? Joseph - Sunday, 2nd December 2012 1:23pm - #4987

I have ordered the parts to spec's in the artical, and while waiting read the artical and comments several times over. I have a much better understanding of the process now, but a few questions

remains. With the old cps I am using (310w) I have a 12+ output rated to max 18amps. I plan to connect this to both the choke and mosfet inputs. Will this voltage severly limit the work coils heating? Also, since it is a cps and likly already regulater, will I still need to consider the lm4812 mentioned in the comments section? Past and future thanks! RMCybernetics - Sunday, 2nd December 2012 3:21pm - #4988

Farhad, The choke in the induction heater is to separate the high frequency oscillations from the PSU, and to limit the current flow. This also helps ensure you get a good sine wave output. When oscillating well, the frequency should be pretty close to 1/2pi*sqrt(LC) where L is the combined inductance of the two halves of the coil. Adding a load will also alter this. The simulation does not take in to account many factors such as the specifics of your PSU, or parasitic components. Joseph, Yes 12V will limit the power. You may not need the additional regulator, but I would recommend it as it helps protect your transistor gates and ensure smooth operation. Joseph - Sunday, 2nd December 2012 4:20pm - #4989

Power limitation, bummer... Considering the psu has both 12+/- outputs each rated to the 18 amp max, could + go to the choke, and - to the mosfets? Thus possibly resolving some of the limitation of dividing only one 12v source? (fyi-i am a novice if the questions dont do enough to signal such) :) trevorb - Monday, 3rd December 2012 1:47am - #4990

RMC-Trying to make a blade heater using pancake coils. I simulated with LT Spice and this calculator. I have little electronics experience. I have a 15v 10A Power Supply and it is going low at 1.5V when turning on the circuit. I am using 100V 35A TO220 n channel mosfets, 1N4007 Diodes, 2.8 micro Henry choke, 240 ohm resistor. The pancake coils are 11.2 microhenry per the calculator. I have tried to add several chokes in series and have tried to add .6 microfarad capacitor at the 15 volt supply. I would appreciate any advice to get this to oscillate. RMCybernetics - Monday, 3rd December 2012 9:36am - #4991

Joseph, If it has +12V and -12V then just use the -12V as your GND connection. This gives you a 24V supply. Trevorb, Bigger choke. I've used 500uH in the simulation. 2.8uH is way too small. Please read through the other comments and troubleshooting section carefully. I have mentioned this multiple times. Joseph - Tuesday, 4th December 2012 2:18am - #4993

your right, because the difference of 24v... but will that be evenly distributed between the two +v inputs if I just use a y-splitter? Or in this case, post of the y-split, should there be two lm4812's, regulating each input (choke and mosfets)? Also, any clever ideas on where one might scrounge up a proper toridal core? I found no blanks cores in the shop I sent for the projects other parts. As you mentioned, a bolt with many more winds would work, but im attracted to the elegant option of the torid. Form and function. Thanks! RMCybernetics - Tuesday, 4th December 2012 9:12am - #4994

Just use the -12V in the same way as GND is used in the diagrams. It will work just fine. You don't need to treat it any differently than an ordinary 24V supply. You can find torroidal cores inside old amplifiers or computer power supplys. Joseph - Tuesday, 4th December 2012 5:41pm - #4995

Sound advice RMC - will be building soon. For the core I spent on 3 compressed high carbon steel machine washers (5/8") which were very ferrous. Will combine these, then coat with insulation of some form. Should be a nice substitute for not having any devices to scavenge. Hope to share results in a few weeks. Kudos on the article and comment maintenance! Joseph - Saturday, 8th December 2012 2:50pm - #4996

Couldn't sleep last night - so I cold forged the work coil from 3/8" (9.54mm) solid core copper wire. Should there be concern on the proximity of the choke and work coils causing interference with each other? This photo is a mock setup with the choke core beneath the rails of the work coil. Trevorb - Tuesday, 11th December 2012 5:59pm - #4997

BLADE HEATER PANCAKE COIL UPDATE - Tried bigger chokes including 500uH, 1mhenry, and both in series, also wound my own with 40 turn thick magnet wire on a really large (est 4 inch od x .5 inch thick ferite torroid). I have tried all combinations of the above inductors in series before branching to the work coils. Also tried each inductor individually. I have added capacitors on the power supply and am placing the choke coil on the +15V supply to abruptly switch it on. I disassembled diodes and they tested ok and identical to unused ones. Changed out and checked mosfet by charging the gate with a multimeter in diode mode and watching the voltage change slightly when discharging gate to ground. Voltage across power supply terminals was 1.5 volt with the lower resistance chokes, but rose to about 3 volts with the 500uH choke with the thin wire (rated for only a few amps). Still no oscilation the pancake work coils (I am not reading an rms voltage from coils to ground when L2 touches +15V). Power supply rated at 15V 10 amps. FETS and smaller diameter L2 wire heating up - I think I am getting 10 amps. Not sure if MOSFETS are off. I am measuring about 1.4 volts at gates regardless of whether L2 is in contact with +15V for not with power supply on. No AC voltage measured after a split second after touching L2 to +15V, but lots of activity on the lcd display of the multimeter for a split second. I have a good spark made when touching L2 to +15V. Any other advice to try and get this to oscillate would be greatly appreciated. Trevorb - Wednesday, 12th December 2012 2:14pm - #4998

Added picture of setup

mahar - Thursday, 13th December 2012 5:54pm - #4999

hi,can you please explain how the voltage of capasitor go up to 3*Vin ? Joseph - Friday, 14th December 2012 4:01am - #5001

In light of blowing up my only power supply (because my choke was too small?) I decided to see if anyone here wants to take on what I have built. I would charge basically half of what I paid for parts plus shipping. I would go further on this kit, but in the process of the PS burning I got an extremely high voltage shock. A bit deterred to say the least. I can almost guarante all parts are sound. Nothing had time to even heat up, as the PS fried immediately after connecting power. Sure, I could ask question after question on here to figure the problem, but no need for my novice to be a nucence. I would rather share. Joseph - Monday, 24th December 2012 5:17am - #5002

[consider removing posting #5001 due to having re delved into the project] Eureka! Finally discovered a power supply that works. I wonder which would be best for the post psu elect. capacitors to smooth adding the load; 1 680uf 200v, or 2 2200uf 50v in parralel, or 6 2.4F 4.7v super caps in series=2.5F 28.2v...? Joseph - Monday, 31st December 2012 5:55pm - #5005

Thanks to RMCyber. for sharing this tutorial. After many troubles - problems were solved. The housing is for a shot nichrome AC wax carver. The Transformer remained, and the new components were swapped for the old. Much more versatile. For a 25mm long x 9mm wide x 2.5mm carbon steel carving spatula to get cherry red all the way through (confirmed non-surface heating by lengthy cool down time) it takes about 9 sec. As good as needed for wax carving! If someone sees major errors in diagram please correct! Cheers. Dylan - Wednesday, 9th January 2013 11:09am - #5007

Here's mine. The work coil is only temporary until a some proper copper pipe arrives. It's working quite well so far, oscillating at around 110 kHz. Thanks for the tutorial.

Frank - Saturday, 26th January 2013 3:28am - #5008

I have a couple of questions. 1. Regarding the simplified self oscillating circuit, are there any cons to using this type of circuit. 2. It seems the capacitors are a big issue in terms of heat gain and I am curious what one might select for a more continuous duty system? Not sure if you must have some liquid cooling for even smaller units or if using high end low ESR caps will solve that issue? dimitris - Sunday, 27th January 2013 7:52pm - #5009

first thanks to rmcyber for this very usufull project..and sorry for my english after... i build mine and working very well,but i have a question about a upgrade on this.. if instead of two biggest mosfet were used more than one on parallel would that work or not? thanks Rajesh - Monday, 28th January 2013 10:13am - #5010

Thanks RMC for sharing your experience. I built one circuit and was able to run it at 100 KHz. I have a question. Can we split capacitor instead of coil of tank circuit??? RMCybernetics - Monday, 28th January 2013 10:23am - #5011

Frank, Cons are limited power and needing a center tapped coil. You would need capacitors that are specifically designed for very high current, high frequency use. Dimitris, You can add parallel MOSFETs if you need more current handling. Rajesh, No, it must be the coil. Rajesh - Monday, 28th January 2013 10:50am - #5012

Thanks for reply RMC. I had one situation when i placed an MS Bolt for heating. It got red hot in one minute but current taken from DC kept increasing. Can you suggest any form of power control to this circuit. Since it is a self tuned circuit frequency variation is not possible. Is DC voltage variation the only way ? RMCybernetics - Monday, 28th January 2013 11:05am - #5013

You can use pulse width modulation to control the power. I found that our PWM-OCm works very well. To use it, I connected a diode between the gate terminal of each MOSFET, and the L- connector on the Power Pulse Modulator. The cathode side of the diodes connects to L-. You also need to power the OCm from the same power source, or create a common ground if using a seperate one. When the OCm switches on, it pulls the MOSFETs low, thus disabling the induction heater circuit. You can now adjust the modulation frequency and duty very easily. Note that the duty control is inverted when doing this.. i.e. turning up the OCm to 100% will set your induction heater to 0%. Frank - Tuesday, 29th January 2013 5:01am - #5014

I have read where several people have toasted their FETs testing this circuit and curious if anyone would advise using a gate resistor for FET protection or if that would cause ill effects? I was looking at maybe a 10k in series just before the gate? Rajesh - Wednesday, 30th January 2013 5:33am - #5016

Thanks RMC for reply. You mean we can connect the gate pin of MOSFETs to PWM Supply instead of 15V DC supply. This way we will be able to Modulate Gate Pulses and control power by varying duty cycle of PWM Supply. I am sure the modulation frequency needs to be much smaller than operating frequency of Tank Circuit. sachin - Wednesday, 30th January 2013 7:24pm - #5017

Dear sir RMCybernetics, I am new in the induction circuit . i have the following circuit diagram of induction circuit.Will it work? Because u have used a choke L2 and i am using transformer of 18:1 or 11:1 of ferrite core only.Will it work to heat a job in coil if circuit in resonant? RMCybernetics - Wednesday, 30th January 2013 9:36pm - #5018

Frank, there is allready a gate resistor. Increasing this will slow down the switching and increase losses. To protect the gate, use a zener diode between gate and source. Rajesh, No, just do as described. Sachin, It could work in theory, but that is not a practical circuit. I can only offer support for the circuit in this article. dimitris - Thursday, 31st January 2013 7:39pm - #5020

thanks for the reply. i used two irfz44n and i'll go to 4+4..i'll post the result after.. Sam - Thursday, 31st January 2013 10:16pm - #5021

Hi, thanks for the great project and fast shipping, worked great first try! Trevorb, any luck on that pancake coil? I'm hoping to build one too but have started out with a standard coil to kick things off. Have you tried applying your circuit to a standard coil? I wonder if your problem could be coil side? I've made pancake coils for an industrial induction heater which I run in a lab using a coiling pattern similar to 1) in the picture below. I wonder whether the centre tap requires coiling similar to 2) to avoid cancellation in the coil? If anyone is interested here is an awesome summary on coil design: It's for standard non-centre-tapped coils but gives a great overview of key considerations when designing a coil for a specific purpose. Rajesh - Friday, 1st February 2013 5:32am - #5022

Thanks RMC, for your valuable suggestions. i will post my circuit pics soon. Adithya - Sunday, 10th February 2013 10:47am - #5023

can i use STP30NF20 ...... tnx in advance

yee - Thursday, 14th February 2013 1:46am - #5037

Hi i have a induction cooker which whenever i power on for sure its will power trip,what could be the problem? Could it be the rectifier spoilt? Please advise,thanks RMCybernetics - Thursday, 14th February 2013 12:58pm - #5038

This is not the place for getting support for your cooker. Contact the manufacturer. This is only for information and support for the DIY Induction Heater project above.

Sam - Wednesday, 20th February 2013 4:51am - #5039

Hi RMC, I've now upgraded my setup to the six cap configuration and bulked up my wires and connectors (managed to start melting the insulation off my wires with a 2 cap breadboard setup). Unfortunately my power supply is current limiting and I'm unable to get power up high enough. Any thoughts on hooking up directly to a 12V car battery with a 10 - 20A fuse in line? I've also got access to a 24V truck battery if you think higher voltage would make a difference. Cheers Sam RMCybernetics - Friday, 22nd February 2013 9:31pm - #5040

A car battery has a huge current capacity. The problem with this is that the sudden rush of current when connecting the circuit could blow the transistors before the oscilation even gets started. I would suggest using a PWM so that you can slowly turn up the power. See post #5013 sachin - Friday, 8th March 2013 4:10pm - #5050

thank you RMC for your suggestion . sir can i switch the transistor externally? and all the circuit is same as your.switching frequency adjusted according to resonant frequency f=(1/2*3.14*sqrt(L*c)).and that switching frequency given to both transistor using square wave generator circuit then will that circuit works for induction heating for bolt? Robert - Monday, 11th March 2013 4:53am - #5052

I am new to Induction Heaters and currently trying to build one on my own, so I am attempting to mimic yours exactly. For your DIY induction heater design, I cannot find what type or size of inductors you used for L1 and L2 (choke) and in the pictures you provide I cannot even see L1 to get a better idea of where it plays into the system. Secondly, I notice yours has 6 turns and the diameter of the coil is only about 2 inches or so. Would I have to change anything if I wanted to increase the diameter to 4 inches? RMCybernetics - Tuesday, 12th March 2013 1:16am - #5054

The inductors are described in the article and comments. Nothing needs to change for a wider coil, but you may find that it will not heat large items so well. Robert - Thursday, 14th March 2013 10:59pm - #5056

There is nothing about specifics mentioned about L1 in the article or comments. Regarding L2, all I see in the article is this: "The one shown here was made by winding about 8 turns of 2mm thick magnet wire on a toroidal ferrite core." But it doesn't give me any specifics on the toroidal ferrite core. Robert - Thursday, 14th March 2013 11:55pm - #5057

Regarding my previous comment: I found somewhere in the comment section that you used a 500 micro Henry ferrite core for the choke, is that correct? And I understand now that L1 is simply the wound work coil. If I wish to flow water through my copper pipes to cool them, how do I go about soldering or attaching the copper wire from the choke to the center tap of the copper pipes? sam - Monday, 25th March 2013 3:10pm - #5059

I am about to start building the induction heater into an (almost) instantaneous smoke generator, my question is how to modulate the power in the work coils so as to control the temperature of the work piece (10mm OD steel tube with 1mm bore). If is use PWM will the pulses interfere with the oscillation in the circuit? Is there a better way to allow for thermostatic temperature control? I will be using noncontact IR sensor to monitor the temperature or the work piece, interfaced through Arduino. Any advise would be extremely welcome. Thank you.

RMCybernetics - Tuesday, 26th March 2013 5:06pm - #5063

Robert, The specifics of the choke and work coil do not need to be exact. We soldered the wire directly to the side of the pipes, but leaving enough space from the end so that water cooling pipe can pe pushed over. Sam, You will need to use a PWM as suggested in post #5013. If you set the pulse frequeny to something much lower (50Hz is fine) than the resonant frequency, then it will not cause any oscilation problems. Your arduino can be programmed to disable the PWM when it detects the temperature i s too high, then re set it agoun when it has cooled a little. SARA - Thursday, 28th March 2013 10:33am - #5068

Hi RMC I am about to start building the induction heater I have started with creating my own PWM command Circuit I used optoelectronics 6n139 for the connection between the PIC and the MOSFET I wonder if it works with your power circuit and what do you think about using a full bridge??? please help me I need it for my study project kevin - Thursday, 28th March 2013 11:21pm - #5069

Trevorb, are you still out there? If so, from your pictures, your work coil is wound incorrectly! You must keep turning in the same direction all the time or else the magnetic fields will cancel out nullifying the inductance, hence, not oscillation. If you start winding a flat coil clockwise, once you get to the center, you must start the second coil out clockwise and move to the outside. I looks like you made two coils at the same time winding the same direction, flipped one over (making it now wound COUNTERCLOCKWISE) connecting the centers together. A good way of checking if you did it right is to start tracing out your coil with your finger to the center and then back out. If you change direction, that is change from CW to CCW , you made a mistake and need to fix your coil. Also, try to get it to oscillate without the load first. If you could post your control circuit up so we can take a look at it as well, with the part number of your components,there might be an error there too. Good Luck! Kev kevin - Monday, 1st April 2013 8:17pm - #5070

Trevorb, give us more details. Size of capacitor on the work coils, the P/N of the transistors you are using, etc. From your readings, something is not connected properly. With your work coil disconnected, you should not see the readings you are reporting. Double check your set up. Also, the MOSFETS both seem to be in conduction. The gate voltage is below the threshold range of a typical device, so I am thinking that you may have depletion mode MOSFETs instead of standard (or enhancement mode) MOSFETs? The readings indicate that both are fully on. RMCybernetics - Tuesday, 2nd April 2013 10:17am - #5071

Sara, a full bridge would be a different system altogether. I can not offer any help for that as it too far outside the scope of the article. duc tai nguyen - Thursday, 4th April 2013 6:10pm - #5072

hi i want to build my first induction heater by rely on your post your schemetic your part list any way i can buy one of your induction heater like your post with 100 % assembly and test work already and i need one as example so i can look at it and do the second one how much one already assemble test good cost plus shipping i want to pay for it than make it as sample to assembly my own the second one ... tell me will you sell me one you part very cheap.tell me the price i have to pay for .. thank you RMCybernetics - Thursday, 4th April 2013 11:56pm - #5073

We do not currently sell an assembled unit. We will have some new products available soon for making induction heaters. Check our Facebook page or newsletter for updates. Robert - Thursday, 11th April 2013 5:49pm - #5075

RMC, I have ordered all my parts, and they have all come in. I am beginning to assemble them, and I have a couple questions. (1) Why is it necessary to have the center tap? Why couldn't you just solder the choke straight onto one continuous solenoid coil? I only ask this because I'm having difficulty soldering the copper pipe to each other at the tap. Suggestions? (2) Is there a formula that relates the magnetic field strength or flux density to amount of heat applied (or something of that sort)? Thanks!! RMCybernetics - Sunday, 14th April 2013 11:39am - #5077

1) Because the current is delivered to the center of the coil, then each transistor switches alternately so that current flows in both directions. 2) Probably, but it will not be simple. The material, size, shape, and position in the coil will all effect the heating. There is also the frequency, current, and coil dimentions to consider. Robert - Wednesday, 17th April 2013 9:40am - #5078

1) For the center tap, could I not just make a solenoid and solder the choke to the center of the solenoid? Wouldn't that perform the same function as what you did since in yours both parts of the copper/brass coil are all connected and touching anyway? 2) I notice in some cases you have 2-330 nF capacitors and in some you have 6. I understand it allows the system to run at a lower frequency, but would you recommend 6 over 2? What effects will it have on the end result? Thanks for your help! RMCybernetics - Friday, 19th April 2013 9:28am - #5079

Yes you can, allthough you may find the solder melts if not kept cool. More capacitors is better as it helps share the current and reduce frequency. Robert - Saturday, 20th April 2013 12:26am - #5080

I have my circuit completely wired up with 15 V, 5 A power supply, 500uH choke, and a 10 turn copper coil, but I can't get it to heat up anything. I've noticed that only one MOSFET heats up but no wires cross or touch on the other. It is wired the same way as the one that heats up but doesn't seem to be working properly. Any suggestions? RMCybernetics - Saturday, 20th April 2013 11:34am - #5081

I am not sure there is anything else to suggest. There are a lot of tips in the article and comments. You will have to work through those :) Robert - Sunday, 21st April 2013 3:19am - #5082

Some additional information on my setup, regarding my previous question about only 1 transistor heating up: -I have verified that the 3 prongs of the transistors are wired correctly (source, gate, and drain), and the transistors themselves are able to handle 35 A and 100 V as you specified. -I am using the 1N4007 diodes you specified in the improvement section, instead of the 1N5819 diodes as originally stated. I don't know if that could be the source of the issue. -My copper coil has 10 turns (no turns touching); it has a wound length of 5 inches and an inner diameter of 2 inches. The pipe itself has a 2mm inner diameter and a 4mm outer diameter. I hope I didn't go too far with anything. Sam DT - Tuesday, 23rd April 2013 8:57am - #5083

Hi, Just added a PWM to my system - works really well even with my old power supply. I assume this is because when it's turned down it decreases the amount of power required to kick off oscillation? I've also successfully built a pancake coil which works nicely although it seems to require a bit more power than a standard solenoid coil. I've just added the centre tapp by looping 10A wire over the coil approximately half way around the coil (Robert I think this was your plan for the solenoid coil worked ok for me but did get hot). Picture below. Robert regarding your question above the frequency can change the depth of heating - higher frequency heats closest to the surface and then conducts inwards, lower frequencies heat deeper into the component. Probably not an issue unless you're heating something really large. RMC couple of questions; My next task is to add water cooling (I'll make a new coil without all those pinch points). Is it also necessary to cool the centre tapp wire going to the choke? Mine seems to warm up a bit (not as much as the coil itself but still reasonably warm).

Secondly, if I run the system on low power (controlled by the PWM)and I have fan and water cooling is it likely that I'll be able to run the system for longer than your suggested 5min? I'm only heating something the size of a medium sized coin and I'm going to try and put it in a vacuum which should significantly reduce the power required to maintain a higher temperature. It seems like it should work but I wonder if there's anything on the PWM or even in the main circuit which will heat up regardless of how low I drop the power? Thanks again for the great project, help and advice. Sam RMCybernetics - Tuesday, 23rd April 2013 10:35am - #5084

The center tap does not need extra cooling. The large currents circulate between the coil and capacitors. For extended high power use, you might need to cool the capacitors too. The PWM will not heat up as it is only controling the transistors and not switching any significant power directly. Robert - Tuesday, 23rd April 2013 9:02pm - #5085

I know the diode side with the band goes on the more negative side; I've tried several configurations but are both your bands on the transistor side? Robert - Tuesday, 23rd April 2013 10:15pm - #5086

Thank you RMC!! I finally got my induction heater to work. I simple had a diode in the wrong direction. Sam - Tuesday, 23rd April 2013 11:25pm - #5087

Great, thanks RMC! Pancake coil pic below: amirrul - Wednesday, 1st May 2013 3:40pm - #5088

hi rmc i have completed the connection of all the components based on the circuit you given above but i dont get any frequency at all,what is the problem? RMCybernetics - Saturday, 4th May 2013 11:21am - #5091

Facepalm! Please read and follow the troubleshooting tips and the other suggestions throughout the comments. Donald - Sunday, 12th May 2013 2:36am - #5094

Curiously, the first thing I did before finishing the article was fire up LTSpice and enter a few components. Got 27kHz on my first oscillations, so a little low, but now I see you have a .asc right here, I'll pick it up and have another go. Thanks. Dudi - Wednesday, 15th May 2013 6:59am - #5095

Hi rmc, It is posible if I try to made a simple heater to warming n boiling water in a coffee cup? I have no idea about electronic. But I want to make an induction heater for my coffee cup. I wish it can be 9volt battery, size about 70-80cm in diameter. Can you help me please? RMCybernetics - Wednesday, 15th May 2013 12:54pm - #5096

A 9V (PP3) battery does not have enough power to heat a cup significantly. There is no point using induction heating for this anyway. Just a standard resistance heating element would be best. mister - Wednesday, 15th May 2013 7:56pm - #5097

I have an old yaskawa V1000 drive for induction and pm motors would i be able to use this as an induction heater or salvage parts from it RMCybernetics - Wednesday, 15th May 2013 10:22pm - #5098

No, it may contain some useful transistors, but you would be better off just buying the few parts that this project requires. bob800 - Saturday, 18th May 2013 6:24pm - #5099

Hi, I've been trying to find an affordable 0.1uf capacitor to use in a chain of 40, for a total capacitance of 4uf. I found two such models: The first is significantly cheaper, but it is described as "General Purpose" vs the more expensive "AC Pulse, Suppression" model. Also, the second model is "Double Metallized". Are these features necessary for a ~600W heater, or can I go with the cheaper model? Thanks! Michael - Sunday, 19th May 2013 3:18am - #5100

Is it possible to use a 200amp ac arc welder for the power supply? by up sizing ea component ? For example I have a couple of high current igbts 1000v / 200amp to replace the mosfets? Christopher - Monday, 20th May 2013 2:12am - #5101

Hi RCM, I put together the circuit using the components you have suggested; however, the power source I'm currently working with is a 25V 1A power source. The problem is that there is no voltage across the mosfet and there is very little current
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