Notes on the Troubleshooting and Repair of AC Adapters

Notes on the Troubleshooting and Repair of AC Adapters, Power Supplies, and Battery Packs and Other Related Information Version 1.14 (05-Aug-06)

http://www.repairfaq.org/sam/appfaq.htm Copyright © 1994-2007 Samuel M. Goldwasser --- All Rights Reserved --For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying.

Table of Contents 

Preface

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Author and Copyright DISCLAIMER Introduction o Scope of this Document o SAFETY o o

PART I - AC Wall Adapters and Power Packs 

General AC Adapter Information o AC Adapter Basics o About AC Adapter Ratings

Protect Yourself from "Unknown AC Adapter Syndrome" Why do AC Adapters Usually Use Heavy Transformers? Compact AC Adapters Substituting AC Adapters AC Adapter Troubleshooting and Repair o AC Adapter Testing o Pocket AC Adapter Tester/Polarity Checker o Getting Inside an AC Adapter o AC Adapter Repair o AC Adapter Substitution and Equipment Damage o Determining Voltage and Polarity of AC Adapter Powered Devices o Power Reversal - Better Pray AC Adapter Modifications or Enhancements o Using AC Adapters in Series for Multiple Voltages o Using AC Adapters in Series for Higher or Lower Voltage o Replacing Batteries With an AC Adapter o Converting an AC Output Wall Adapter to DC o Adding an IC Regulator to a Wall Adapter or Battery o o o o

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Part II - Power Supplies in Consumer Electronic Equipment (Including UPSs)  

Equipment Power Supplies o Types of Power Supplies Power Supply Troubleshooting o Totally Dead Power Supply (Non-Switching Type) o Low or Missing Power Supply Outputs (Non-Switching Type) o Uninterruptible Power Supplies (UPSs) and Power Inverters

Part III - Protection Devices in Consumer Electronic Equipment 

Protection Devices o About Fuses, IC Protectors, and Circuit Breakers o Fuse Post Mortems o Fuse or Circuit Breaker Replacement o Comments on Importance of Thermal Fuses and Protectors

Part IV - Power Transformers in Consumer Electronic Equipment 

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Power Transformers o Common Types of Transformers o Testing a Power Transformer o Identifying the Connections on an Unknown Power Transformer o Determining Unknown Connections on International Power Transformer o Determining Power (VA) Ratings of Unknown Transformers o Determining the Ratings of a Fried Power Transformer Transformer Troubleshooting and Repair o Transformer Fault Diagnosis o Rewinding Power Transformers

Part V - Batteries in Consumer Electronic Equipment 

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Batteries and Battery Packs o Battery Technology o Battery Basics o Battery Chargers o Dave's Comments on Building Charger for Small Lead-Acid Batteries o Substituting NiCds for Alkalines o Can a Large Electrolytic Capacitor be Substituted for a NiCd? o Determining the Actual Capacity of a NiCd Battery Pack o NiCd Batteries and the Infamous 'Memory Effect' o Memory Effect in NiMH Batteries? o Care and Feeding of NiCds o Why There Will Never Actually be Closure on This Topic o Nickel Cadmium Versus Nickel-Metal-Hydride in a Nutshell o Randy's Notes on First Aid for NiCd Battery Packs o Identifying Technology of Unmarked Battery Packs o Powering LEDs with Batteries Battery Problem Troubleshooting and Repair o Problems with Battery Operated Equipment o NiCd Battery Pack Will Not Hold a Charge o What is This Thing in my NiCd Battery Pack? o Zapping NiCds to Clear Shorted Cells o Tom's Comments on NiCd Care and Feeding o Battery Juice and Corroded Contacts o Exploding Batteries - These Things Really Happen o Soldering Tabs Onto NiCd Batteries Battery Related Information o Automotive Power o How Do Those On-Battery or On-the-Package Battery Testers Work? o Battery Eliminator for Laptop or Appliance with Dead NiCds

 Back to AC Adapter and Power Supplies Table of Contents.

Preface Author and Copyright Author: Samuel M. Goldwasser For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page. Copyright © 1994-2007 All Rights Reserved Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

1.This notice is included in its entirety at the beginning. 2.There is no charge except to cover the costs of copying.

DISCLAIMER AC adapters, transformers, and even batteries, are critical safety components. Replacement with an improperly rated or incompatible device can result in damage or destruction of the powered equipment as well as the risk of shock or electrocution in certain cases. We will not be responsible for damage to equipment, your ego, blown parts, county wide power outages, spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury that may result from the use of this material.

 Back to AC Adapters and Power Supplies Table of Contents.

Introduction Scope of this Document This collection of information deals with the troubleshooting, repair, and use (normal or unconventional) of AC (wall) adapters, transformers, equipment power supplies (non-switching type), and batteries used in portable electronic devices and power tools. 

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AC adapters (may also be called: wall adapters, power packs, or wall warts) are those boxes hanging on the end of the power cords of many modern consumer electronic devices. Their output may be a single AC or DC voltage, or several, with or without regulation. Most of those outputting AC are simple transformers and with the addition of a diode or bridge and filter capacitor for DC. (Often, people refer to all types of AC adapters including those outputting DC as 'transformers' but this is not really correct.) Others (those that are compact and weigh almost nothing) may be sophisticated switchmode power supplies. In most cases, the output will be totally isolated from the power line for safety. However, some that are designed for applications like battery chargers may not be line isolated and should not therefore be used for any other purposes. Transformers in this context refer to the actual magnetic components which may be found in AC adapters or equipment power supplies. Equipment power supplies are those portions of the equipment that provide various (usually DC) voltages for its operation. Note that in this document, non-switchmode types are discussed. For switchmode power supply information, see the document: Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies. Batteries are found in all sorts of portable equipment nowadays. The discussions in this document related directly to problems and repair. This is not intended as a comprehensive battery "FAQ".

Note: This document replaces the chapters relating to these topics in the documents "Notes on the Troubleshooting and Repair of Small Household Appliances and Power Tools" and "....Audio Equipment and Other Miscellaneous Stuff". Where another document is referenced, it is assumed to be at this site. If the link doesn't work, find the document of the same name at the Sci.Electronics.Repair FAQ or one of its mirror sites.

SAFETY For the common transformer based AC adapter, there is no danger anywhere inside the device once unplugged. For the switchmode variety, see the document: Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies for information beyond what is covered in this document. Any internal overcurrent fuses or thermal fuses represent essential safety features of an AC adapter. These must not be removed except during testing. Where a fuse is found to be blown, use only an exact replacement. I really don't recommend running a repaired cobbled together AC adapter unattended in any case since even the sealed case provides some additional amount of fire protection. Inexpensive replacements are generally available. For power supplies inside equipment, the same basic precautions apply but access and repair are generally much more easily accomplished. The only real danger from an unplugged heavy iron transformer would be accidentally dropping it on your foot. :(

 Back to AC Adapters and Power Supplies Table of Contents.

General AC Adapter Information AC Adapter Basics It seems that the world now revolves around AC Adapters or 'Wall Warts' as they tend to be called. There are several basic types. Despite the fact that the plugs to the equipment may be identical THESE CAN GENERALLY NOT BE INTERCHANGED. The type (AC or DC), voltage, current capacity, and polarity are all critical to proper operation of the equipment. Use of an improper adapter or even just reverse polarity can permanently damage or destroy the device. Most equipment is protected against stupidity to a greater or lessor degree but don't count on it. The most common problems are due to failure of the output cable due to flexing at either the adapter or output plug end. See section: AC Adapter Testing. 1. AC Transformer. All wall warts are often called transformers. However, only if the output is stated to be 'AC' does the device consist of only a (stepdown) transformer. These adapters typically put out anywhere from 3 to 20 VAC or more at 50 mA to 3 A or more. The most common range from 6 to 15 VAC at less than 1 A. The regulation is typically very poor so that an adapter rated at 12 VAC may put out 15 VAC with no load and drop to less than 12 VAC at rated load. However, some may actually output up to two times the rated voltage or more with a light load. To gain agency approval, the transformer needs to be protected internally so that there is no fire hazard even if the output is shorted. There may be a fuse or thermal fuse internally located (and inaccessible). If the output tested inside the adapter (assuming that you can get it open without total destruction it is secured with screws and is not glued or you are skilled with a hacksaw - measures 0 or very low with no load but plugged into a live outlet, either the transformer has failed or the internal fuse

had blown. In either case, it is probably easier to just buy a new adapter but sometimes these can be repaired. Occasionally, it will be as simple as a bad connection inside the adapter. Check the fine wires connected to the AC plug as well as the output connections. There may be a thermal fuse buried under the outer layers of the transformer which may have blown. These can be replaced but locating one may prove quite a challenge. Also see the section: Comments on Importance of Thermal Fuses and Protectors. 2. DC Power Pack. In addition to a step down transformer, these include at the very least a rectifier and filter capacitor. There may be additional regulation but most often there is none. Thus, while the output is DC, the powered equipment will almost always include an electronic regulation. As above, you may find bad connections or a blown fuse or thermal fuse inside the adapter but the most common problems are with the cable. 3. Switching Power Supply. These are complete low power AC-DC converters using a high frequency inverter. Most common applications are laptop computers and camcorders. The output(s) will be fairly well regulated and these will often accept universal power - 90-250 V AC or DC. Again, cable problems predominate but failures of the switching power supply components are also possible. If the output is dead and you have eliminated the cable as a possible problem or the output is cycling on and off at approximately a 1 second rate, then some part of the switching power supply may be bad. In the first case, it could be a blown fuse, bad startup resistor, shorted/open semiconductors, bad controller, or other components. If the output is cycling, it could be a shorted diode or capacitor, or a bad controller. See the document: Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies for more info, especially on safety while servicing these units. Also see the chapter on "Equipment Power Supplies" in the document: Notes on the Troubleshooting and Repair of Audio Equipment and Other Miscellaneous Stuff.

About AC Adapter Ratings The following mainly applies to AC adapters using transformers. Those based on switchmode power supplies adapters have tended to be well designed with decent regulation and realistic ratings. Of course, they are generally also much more expensive! There is no standard for rating AC adapters. When a particular adapter is listed as, say, 12 V, 1 A max, there's a good chance the output will average 12 V when outputting 1 A but what it does at lower currents is not known. In fact, lightly loaded, the output voltage may be more than double its nameplate rating! This could be disastrous where a piece of equipment is plugged into it that doesn't expect such a high voltage. The rating also doesn't say anything about the ripple (for DC models) - it could be almost anything. The lifetime of an AC adapter (particularly those outputting DC) when run at or near its nameplate rating may be very short. Why? Because they often use low temperature (cheap!) components that can't take the heat. For AC output models, the transformer itself may fail (or at least the thermal fuse). For DC models, the electrolytic capacitor(s) may go bad very quickly. The likely result will be that the output voltage will disappear entirely (AC models) or drop in value with greatly increased ripple (DC models).

Where the adapter is used with its intended equipment, one can presume the manufacturer did the proper testing to assure compatibility and adequate life (though this isn't always the case!). However, where it is used in some other application, the life of the adapter and the equipment may be much shorter than expected, possibly failing almost immediately.

Protect Yourself from "Unknown AC Adapter Syndrome" Apparently, manufacturers of equipment powered by AC adapters have discovered that they can improve their bottom line by not printing the AC adapter ratings on the device itself, and possibly not even in the user manual. I don't know whether this is actually done for liability reasons (so you aren't tempted to actually use an AC adapter other than their own exorbitantly priced replacement) or just to same 3 microcents on printing ink but the net result is that the owner has no idea what adapter in that drawer that collects adapters is the correct one. They could at least specify a particular model adapter if they don't think the average consumer has an intelligence greater than a carrot. For example, I own 2 U.S. Robotics modems. One uses a 9 VAC adapter; the other uses a 20 VAC adapter. The power jacks are identical and totally unmarked. Guess what happens if I guess wrong? With too little voltage, the modem may appear to work but be unreliable. With too much voltage, the smoke will very likely be released instantly. :( To save yourself a lot of hassle and possible damaged equipment, put a label on each AC adapter powered device you own with the voltage, current, AC or DC (with polarity), and model number of the adapter (make one up if nothing is obvious and put it on the device and adapter). Then, if you misplace the adapter, you'll know what to look for and if it is nowhere to be found, will have enough information to purchase a replacement.

Why do AC Adapters Usually Use Heavy Transformers? The main reasons are safety and cost. Line isolation is essential for safety with respect to electrical shock - no part accessible to the user must be connected to either side of the power line. A regular transformer provides this automatically. While combinations of passive components can reduce the risk of shock, nothing quite matches the virtually failsafe nature of a simple transformer between the power line and the low voltage circuitry. To achieve similar isolation without a line transformer generally requires a switchmode power supply which actually contains a small high frequency transformer to provide the isolation. Until recently, such systems were much more expensive than a simple iron transformer but that is changing and many modern devices do now use a wall adapter based on this approach. These can be recognized by their light weight, DC (probably regulated) output, and the required warnings NOT to cut them off and replace them with an ordinary plug! I wonder how many people have ignored the warnings when their equipment stopped working and replaced that fat "plug"? What a scenario for disaster!

Compact AC Adapters These use switchmode power supply technology and can therefore be quite small and light weight. In addition to the applications noted below, they are turning up on a variety of other high tech gadgets from shavers to Personal Digital Assistants.

WARNING: DON'T attempt to disassemble or repair one of these unless you are familiar with the safety and troubleshooting information for larger switchmode power supplies - they can be quite deadly. See the document: Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies. (From: Mike Schuster ([email protected]).) For some reason I've been fascinated by tiny wall wart AC adaptors that use switch mode power supplies, since they're light and can supply more current than similar linear power cubes. One type that keeps catching my eye is used a lot for "AC travel charger" accessories for cellular phones. These things connect via a cable to the bottom of a cell phone, much like the cigarette-lighter "charger/saver" accessories, only these are driven by house current. The typical wart is a small rectangular box, about the size of two 9V transistor batteries side by side, manufactured in China or Taiwan. The wall side is distinguished by the fact that the AC prongs line up with the long axis of the box, rather than the other way around as with most wall cubes. This makes it possible to put them side by side on an AC power strip. The opposite face contains a tri-mode LED which may display red, green, or orange under conditions I've yet to figure out. Recently I noticed one of these thingies in K-Mart as part of a modular power system for cell phones. There are several models of cigarette lighter cords, however the actual 12VDC car plug in _interchangeable_ and connected to the cable using a 4-pin modular telephone handset jack. Each model comes with a cable constructed to mate with the phone it's sold for. Next to these on the pegboard is a variant of the wall wart being discussed, also having a 4-pin handset socket, and sold as an accessory to the DC cords. Instead of using the cigarette lighter plug, you connect the cable to the wall wart and create a new device which uses house current. So I picked up the wall wart and started to play. It's marked as being capable of 5-15 VDC at 750 mA. Playing with the 4 output pins; one is ground, two are tied together and supply 14.35 VDC open circuit, and can deliver about 1.5 amps. The other reads about 13 volts between it and the ground. Unpowered there is a small leakage between the ground and the "13 volt" pin. Looking inside, there are two 8-pin DIPs on the PC board; both having identifiers sanded off. One is near the transformer end and the other is near the DC output end. All of the DC side output traces lead, directly or indirectly, to the second IC. My guess is that the "13 volt" pin is really used to program the output voltage between ground and the other two pins that are tied together. The cable sold for any specific phone has some passive components inside that will cause the second IC to produce the required output voltage. Am I warm? I'd like to try programming this myself ... any ideas? Resistors?

Substituting AC Adapters This relates to replacing a missing or broken adapter for which the specifications are known.

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They must both be the same type with respect to AC or DC. (In rare instances, either will work but this is the exception and there is no way to know without examining the circuit unless this capability is explicitly stated on the device or in the user manual.) If DC, the polarity must be the same. Even if the connectors are identical, it's a coin toss as to whether the center is positive or negative.

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The replacement much have at least the same current rating. Higher current is fine as long as it isn't ridiculously more (like 5 times). The connectors should be identical (including the size of the center hole). If the old adapter is dead, and it's not a cable problem near the connector, cutting and splicing in the old connector on the new adapter is acceptable as long as it's done very carefully and double checked for wiring errors and short circuits. The output voltage with no load must not be much greater than the listed value unless you know for sure that the equipment can take it, or that the other adapter behaved similarly. If the original adapter was regulated, then the new one should be as well.

 Back to AC Adapters and Power Supplies Table of Contents.

AC Adapter Troubleshooting and Repair AC Adapter Testing AC adapters that are not the switching type (1) and (2), above, can easily be tested with a VOM or DMM. The voltage you measure (AC or DC) will probably be 10-25% higher than the label specification. If you get no reading, wiggle, squeeze, squish, and otherwise abuse the cord both at the wall wart end and at the device end. You may be able to get it to make momentary contact and confirm that the adapter itself is functioning. The most common problem is one or both conductors breaking internally at one of the ends due to continuous bending and stretching. Make sure the outlet is live - check with a lamp. Make sure any voltage selector switch is set to the correct position. Move it back and forth a couple of times to make sure the contacts are clean. If the voltage readings check out for now, then wiggle the cord as above in any case to make sure the internal wiring is intact - it may be intermittent. Although it is possible for the adapter to fail in peculiar ways, a satisfactory voltage test should indicate that the adapter is functioning correctly. It's also possible that the power jack on the device itself is damaged from use or abuse. If possible, confirm proper operation with a COMPATIBLE adapter. With battery operated devices, there is usually a set of contacts that should close when the adapter is removed to connect the internal battery to the circuitry. If these don't operate properly, the device may not work off batteries (they may appear to not be

charged), the AC adapter, or both. Check the jack for obvious signs of damage (cracked, loose, etc.). A squirt of contact cleaner into the jack may clear up intermittent contact problems not due to actual damage.

Pocket Wall Adapter Tester/Polarity Checker This handy low cost device can be built into an old ball point pen case or something similar to provide a convenient indication of wall adapter type, operation, and polarity: Probe(+) o-----/\/\-----+----|>|----+---o Probe(-) 1K, 1/2 W | Green LED | +----|).

Power Reversal - Better Pray "That's right, I reversed power and ground on a Sony XR-6000 AM/FM cassette car stereo. (12V negative ground). The little fellow made a stinky smell, so I assume that at least one component is cooked." If it had not been turned on before you discovered your error, the damage may have been limited to the display and some filter caps. Then again...

The problem is that an auto battery has a very high current capacity and any fuses respond too slowly to be of much value in a situation such as this. Any capacitors and solid state components on the 12 V bus at the time power was applied are likely fried - well done. "Is there any hope of my repairing it? (This assumes I show more ability than I did when installing it.) Which part(s) are likely damaged?" (From: Onat Ahmet ([email protected]).) Well, based on that last statement ;-)  

Bad: car batteries can provide amps and amps of current (much worse than reverse connecting a wall adapter for example.) Good: The stinking might be due to a component getting too hot and vaporizing the solder paste/preserver/dust on it, but not actually giving up the ghost. I would find and check any fuses, or components directly in-line with or parallel to the power lines (the latter might include the IC's unfortunately...)

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Good: There might have been a protecting diode somewhere (but why did it stink then (^_^) Neutral: Did you disassemble it to see if there were any blackened areas/components? Smell from a close distance; I can often locate a burnt component that way even after a long time.

If not, join the happy crowd, and gut the good old stereo for parts!

Determining Voltage and Polarity of AC Adapter Powered Devices This is often required when the original adapter is lost or misplaced or isn't labeled so you are not sure if it is the correct one for your device. It's amazing how many things like modems and phone answering machines don't list the voltage and polarity on the case - it's not like the extra printing would cost anything! While I would stop short of calling this a conspiracy, there does appear to be an industry-wide practice of leaving out key information to encourage replacement of the equipment rather than the much less costly and much less profitable repair or replacement of only the wall adapter. Information on voltage, current, and AC or DC polarity, is often missing on the equipment itself. And, absolutely totally incompatible wall adapters having similar plugs can be attached with the possible result being instant destruction of the device. This even applies to equipment from the same manufacturer! At least wall sockets are standardized - wall adapters are not. If you are simply replacing a broken adapter with a universal type, check the label on the old one - they almost always provide this information. There are three issues: AC versus DC, the voltage, and polarity. Unfortunately, fully determining these requirements experimentally can be non-trivial. While many devices have built in protection for reverse polarity (which would probably also include putting AC into a device requiring DC), others do not and may be damaged or may at least blow an internal fuse. Few devices protect against extreme overvoltage. If you have a multimeter, there are also some tests you can perform without opening the device but they are not foolproof. Here are some general guidelines. The more of these you can confirm, the greater the confidence of avoiding disaster. 1. The best way would be to find the information without serious testing. It may be readily available:

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Examine the device for labels, either embossed near the power jack or on the rear or underneath such as: DC 5V ---____

AC 12 V ~ _

If there is a voltage listed but no indication of AC/DC, 6 V or less is likely to be DC (and may require decent regulation; higher voltages could be either AC or DC (probably filtered but unregulated though not always). o A symmetric (non-polarized) jack means it is supposed to operate on AC. o If the device has a metal case or you can get to the metal shields on connectors, check for continuity to the power jack. This probably is the negative input (though no guarantee some manufacturers do really strange things!). o Check your user manual! o Contact the manufacturer or their Web site. 2. The next best way would be to open it up and trace enough of the power circuitry to identify components which have obvious voltage ratings and polarities like electrolytic capacitors. There may even be labeling on the circuit board. o There will almost always be at least one electrolytic cap very near the power input. o If there is nothing between it and the power jack, then polarity will be that of the cap and you will have an upper bound on voltage (but the actual safe operating voltage will probably be considerably less). o If there is a diode in series with the cap, then the voltage and polarity will be as above (except for the 0.7 or so V diode drop) and the device is probably designed to operate on DC (and possibly AC but there may not be enough filtering). o If there is a bridge rectifier or multiple rectifier diodes between the input and any DC loads, it is probably designed to operate on AC. o If the device also has a battery compartment and the battery powers the device the same way as the adapter (possibly with one connection going through a diode or an interlock on the power jack), then the AC adapter polarity and voltage will be the same (+/- 0.7 V or so) as the battery. However, some devices use totally different means of powering themselves with battery and AC operation! 3. If you have a multimeter for which you know the polarity of its output on the ohms ranges (VOMs may be reversed from the probes; DMMs are often the same - this can be determined by testing a diode or with another meter), then test on the low ohms range first in one direction, than the other. This is like applying a very low safe voltage to the device: o Open in one direction and a charging cap (resistance starts low and increases relatively slowly) indicates a series diode (protection or a rectifier). The probe polarity where the cap is charging is the correct one. (Note: once the internal caps charge up, reversing the leads again may result in an apparent open reading.) o A diode drop in one direction and charging cap in the other indicates a parallel protection diode. Again, the slowly charging direction is correct. o Symmetric behavior may indicate it is supposed to use AC. However, this could just mean that a filter cap is directly across the input and DC is required. o

Anything else will probably require you do (1) or (2). And, except for manufacturer supplied information, even these are no guarantee of anything!

Once AC versus DC and polarity (if relevant) are determined, start low on voltage to see at what point the device behaves normally. Depending on design, this may be quite low compared to the recommended input voltage or very near it - no way to really know. Devices with motors and solenoids may appear to operate at relatively low voltage but fail to do the proper mechanical things reliably if at all. RF devices capable of transmitting may behave similarly when asked to transmit. Devices with more constant power requirements may operate happily at these reduced voltages. However, depending on the type of power supplies they use, running at a low voltage may also be stressful (e.g., where DC-DC converters are involved). NOTE: Some devices with microcontrollers and/or logic will require a fast power turn-on so it may be necessary to switch off and then on for each input voltage you try for proper reset. Again, determining the requirements from the manufacturer is best!

 Back to AC Adapters and Power Supplies Table of Contents.

AC Adapter Modifications or Enhancements Using AC Adapters in Series for Multiple Voltages Where a bipolar DC power supply is needed, it is possible to create this with a pair of DC output adapters in series. Each adapter must have voltage and current ratings adequate for your application. They can be used with or without external regulators (see the section: Adding an IC Regulator to a Wall Adapter or Battery. Since they are fully isolated from the AC line and each other, they can be tied together with any desired polarity and common point. The only cautions are that if one of them is unpowered for any reason (it falls out of the AC outlet!) or the current rating of one of the adapters is exceeded, then current may be forced through the other one in the wrong direction possibly damaging its electrolytic capacitors or other components. To prevent this possibility, place a rectifier like a 1N4002 (this is 1 A, use a larger one if your adapters are really huge) in REVERSE across each output. This will bypass current safely around the internal circuitry. The idea of using multiple adapters can be extended to even more outputs but this is left as an exercise for the student.

Using AC Adapters in Series for Higher or Lower Voltage Wall adapters are totally isolated from everything (except possibly for a very high value resistor to one side of the AC line which for this purpose can be ignored) so using one set of wires as a common for the series connection won't blow anything. However, obtaining an AC adapter with the proper ratings for long term use would be a good idea. There are two cases: 1. DC output. It's probably not recommended and only the sum case (higher voltage output) is generally possible but will work (at least for testing purposes) as long as neither current rating is

exceeded and both units are powered. (Else, you could end up with the wrong polarity on one of the unit's electrolytic capacitors.) 2. AC output. There should be no problem as long as the current rating of neither adapter is exceeded. Unless they are identical units, you will probably have to experiment with the phasing to get the sum or difference of the voltages WITHOUT the equipment attached! WARNING: If one of the adapters is not plugged in, high voltage (possibly even more than the normal line voltage) may appear on its exposed prongs due to the AC from the other adapters present on its output (being stepped up going the wrong way through the transformer). The voltage and available current may be enough to be dangerous in some cases. CAUTION: For the difference case, if one of the units isn't powered, you may get a HIGHER voltage than expected at the output of the series combination which may let the smoke out of your equipment. :(

Replacing Batteries With an AC Adapter While most appliances that run off of internal batteries also include a socket for an wall adapter, this is not always the case. Just because there is no hole to plug one in doesn't necessarily mean that you cannot use one. The type we are considering in this discussion are plug-in wall adapter that output a DC voltage (not AC transformers). This would be stated on the nameplate. The first major consideration is voltage. This needs to be matched to the needs of the equipment. However, what you provide may also need to be well regulated for several reasons as the manufacturer may have saved on the cost of the circuitry by assuming the use of batteries: 

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The maximum voltage supplied by a battery is well defined. For example, 4 AA cells provide just over 6 V when new. The design of the device may assume that this voltage is never exceeded and include no internal regulator. Overheating or failure may result immediately or down the road with a wall adapter which supplies more voltage than its nameplate rating (as most do especially when lightly loaded). Most wall adapters do not include much filtering. With audio equipment, this may mean that there will be unacceptable levels of hum if used direct. There are exceptions. However, there is no way of telling without actually testing the adapter under load. The load on the power source (batteries or adapter) may vary quite a bit depending on what the device is doing. Fresh batteries can provide quite a bit of current without their voltage drooping that much. This is not always the case with wall adapters and the performance of the equipment may suffer.

Thus, the typical universal adapter found at Radio Shack and others may not work satisfactorily. No-load voltage can be much higher than the voltage at full load - which in itself may be greater than the marked voltage. Adding an external regulator to a somewhat higher voltage wall adapter is best. See the section: Adding an IC Regulator to a Wall Adapter or Battery. The other major consideration is current. The rating of the was adapter must be at least equal to the *maximum* current - mA or A - drawn by the device in any mode which lasts more than a fraction of a

second. The best way to determine this is to measure it using fresh batteries and checking all modes. Add a safety factor of 10 to 25 percent to your maximum reading and use this when selecting an adapter. For shock and fire safety, any wall adapter you use should be isolated and have UL approval. 

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Isolation means that there is a transformer in the adapter to protect you and your equipment from direct connection to the power line. Most of the inexpensive types consist of nothing more than a transformer (and for DC types), rectifier and filter capacitor. However, if what you have weighs almost nothing and is in a tiny case, it may be meant for a specific purpose like a battery chargers or rechargeable device where human contact is not possible and may not include line isolation. But, if there is a low voltage plug with exposed contacts and/or the powered equipment has exposed shields or other parts, the compact light-weight types are actually miniaturized switchmode power supplies which are functionally equivalent to the heavier, bulkier adapters and do provide line isolation. UL (Underwriters Lab) approval means that the adapter has been tested to destruction and it is unlikely that a fire would result from any reasonable internal fault like a short circuit or external fault like a prolonged overload condition.

To wire it in, you can obtain a socket like those used on appliances with external adapter inputs - from something that is lying in your junk-box or a distributor like MCM Electronics. Use one with an automatic disconnect (3 terminals) if possible. Then, you can retain the optional use of the battery. Cut the wire to the battery for the side that will be the outer ring of the adapter plug and wire it in series with the disconnect (make sure the disconnected terminal goes to the battery and the other terminal goes to the equipment). The common (center) terminal goes to other side of the battery, adapter, and equipment as shown in the example below. In this wiring diagram, it is assumed that the ring is + and the center is -. Your adapter could be wired either way. Don't get it backwards! +--+ X V | (Inserting plug breaks connection at X) Battery (+) o------- | Adapter (+) o---------+------------------o Equipment (Ring, +) \______ o===+ Battery/ | Adapter (-) o-----------------------+----o Equipment (Center, -)

WARNING: if you do not use an automatic disconnect socket, remove the battery holder or otherwise disable it - accidentally using the wall adapter with the batteries installed could result in leakage or even an explosion! A possibly simpler alternative is to fashion a 'module' the size and shape of the battery or battery pack with screw contacts at the same locations and connect your external power supply to it. For example, a couple of pieces of wooden dowel rod about 2-1/4" long taped together with wood screws in the appropriate ends would substitute for a pair of side-by-side AA batteries. Then, you don't need to modify the Walkman or whatever at all (or at most just file a slot for the wire to exit the battery door).

Converting an AC Output Wall Adapter to DC Where a modest source of DC is required for an appliance or other device, it may be possible to add a rectifier and filter capacitor (and possibly a regulator as well) to a wall adapter with an AC output. While

many wall adapter output DC, some - modems and some phone answering machines, for example - are just transformers and output low voltage AC. To convert such an adapter to DC requires the use of:   

Bridge rectifier - turns AC into pulsating DC. Filter capacitor - smooths the output reducing its ripple. Regulator - produces a nearly constant output voltage.

Depending on your needs, you may find a suitable wall adapter in your junk box (maybe from that 2400 baud modem that was all the rage a couple of years ago!). The basic circuit is shown below: Bridge Rectifier

Filter Capacitor

AC o-----+----|>|-------+---------+-----o DC (+) ~| |+ | In from +----||----|--+ - | | | | AC o-----+----|