1301694765 3343 FT0 Arc Starter Original Complete W-rev2 in PDF

A HOMEMADE ARC STARTER BY Newton Brawn & 3L3

INTRODUCTION On very simple alternating current (AC) arc welders (buzzbox), the arc is initiated by touching the stick electrode to the workpiece and keeping the electrode at a distance small enough to maintain the arc. This is a difficult task when the welding current is below 60A. Any variation in the small distance between the electrode and the work can interrupt the arc. However, if an arc starter is added to the buzzbox, the welding task at low currents would be much easier. The arc starter initiates the arc, and provides arc regulation during the welding operation. In TIG welding, it is good practice to keep the tungsten electrode from touching the work to prevent electrode contamination. In this case, an arc starter provides a high frequency pulse to the tungsten electrode allowing the main arc to be started without touching the electrode to the work. The welder can manipulate the torch easily, since the arc starter stabilizes the arc over small variations in distance between the torch electrode and the work. When welding aluminum, with the tungsten electrode under the protection of an argon atmosphere, very often the arc will have behavior similar to a rectifier; suppressing one half cycle of the AC current, thus affecting the quality of the weld. To initiate the arc, and break the rectifier action, the HF pulses are supplied by the arc starter synchronous with the main AC current. Different kinds of arc starting devices have been manufactured for providing pulses to start and stabilize the welding arc. A very well known example is the HF15 arc starter from Miller that may be attached to most welding machines. The circuit is identical to a small Tesla coil. It uses a 3600V transformer feeding a capacitor that discharges through a spark gap to a coupling coil. The coupling coil shapes the pulses and provides the proper coupling of the pulses to the electrode or torch. Despite just one pulse being necessary to start the main arc, several HF pulses are imposed on the welding current during each semi cycle of the welding current. The basic circuit of the HF15 is still used as the basis for arc starters in new TIG machines. During the 1960's, the arc starters were manufactured under a new concept. Several solid state pulse circuits, using transistors, SCR, TRIAC or IGBT were developed in order to perform the arc starter function. These are more complex circuits with components that are difficult to find. The purpose of this article is to present a simple arc starter that may be constructed by an electrician or even a welder with some electrical knowledge. The device may be connected to a buzzbox, DC welder, TIG welder, or plasma cutter. The circuit described here is a compound of two Tesla circuits. The first Tesla circuit has a capacitor, a pulse transformer, and an SCR. The SCR acts as a spark gap discharging the capacitor into the primary of the pulse transformer, providing pulses in the range of 370~500V, lasting 0.1~ 0.5 milliseconds. The second Tesla circuit has an HV capacitor, an air spark gap, and a coupling coil. The secondary of the pulse transformer charges the HV capacitor with voltages up to 10kV. When the HV capacitor reaches half of its final voltage, the spark gap triggers and discharges the HV capacitor through the coupling coil. The coupling coil superimposes the pulse onto the main arc AC or DC current. The HF pulses imposed on the electrode or torch are in the range of 15kV, and last 0.5 microseconds or less. One important feature is that the device injects just one HF pulse in each semi cycle of the welding current. The HF pulses are synchronized with the AC voltage of the stick electrode (torch). One positive HF pulse is superimposed to the positive semi cycle and a negative pulse is superimposed to the negative semi cycle. The result is that the device does not waste energy producing unnecessary pulses, the components have reduced stress, the air spark gap has a

long life and operates very cool, requiring infrequent adjustments. The size and rating of all equipment are also reduced. CONSTRUCTION Electronic assembly: The solid state spark gap electronic circuit uses one TRIAC and one DIAC, a potentiometer and a few other components, that are assembled on an L shaped aluminum sheet. It is a typical circuit of a light dimmer control. In the first experiments we used a 240V 6A commercial light dimmer. Despite not having burned the dimmer during the initial tests, we consider it as abusing a 6A TRIAC to commutate the peak current developed in the discharge of the line capacitor. Air spark gap: The classical material for the gaps is tungsten rod; 2 to 6 mm in diameter. In this project we used a tungsten carbide cutting insert, that one would use in milling cutting tools, because they are easily obtained from the scrap box, and perform this function very well. Note that heatsinks are not necessary in this circuit that produces one pulse every 8.3ms. In order to prevent corrosion generated by the spark ozone, it is recommended that the screws, nuts, and metal parts be made of stainless steel or brass. See drawing showing details. HV capacitor: 700 to 1200uuF, 7000~10000V. A metalized polyethylene or polypropylene capacitor is not suitable. Such a capacitor is likely to fail after only 10 to 100 hours. These capacitors have very thin (0.025 to 0.1 micrometer) metalized electrodes which can not carry the high current that circulates during discharges of the HV capacitor. Mica capacitors, or even pulse grade capacitors, with high dV/dt are suitable to perform high current discharges. You may make a serial arrangement of 2 to 6 capacitors in order to get a resultant voltage of 7kV to 10kV, and obtain a resultant dV/dt of 200.000 volts/microsecond. Another option is to construct a glass capacitor, 700~1000uuF x 7000V, using 2mm window glass plates and aluminum foil, that one uses in the kitchen. The thickness of these aluminum foils vary from 0.013 to 0.18mm. We made glass capacitors that have been in service for 3 years in continuous charge/discharge, and no failures have so far been reported. Pulse transformer and coupling coil: A computer monitor or TV found in the street will provide the flyback ferrite core for the pulse transformer and the ferrite beads for the coupling coil. The pulse transformer is wound by hand using a crankcase and a turn counter assembled in a 2x4” wood frame. The picture shows more details. All insulation used in the transformer is polyester or mylar film, 0.1mm thick. Between the winding layers, use 2 layers of mylar, and in the winding form, use 8-10 layers glued with epoxy. See drawing. Line capacitors: The line and discharge capacitors provide the pulses to the pulse transformer, so high dV/dt capacitors are the best for this function. Or you may get the 0.95uF x 2000Vac capacitor from a microwave oven, it is available. Tests: The handmade components are individually tested before the final assembly using an ohm meter, and a capacitance meter if the instrument is available. To measure the high tension insulation capability and the magnitude of HV generated by the assembly, a very simple homemade apparatus may be used. See picture of apparatus for HV measurements. V = d x 1000. V = voltage applied in the electrodes, V d = distance between the electrodes, mm

Adjustments: The distance between the air gaps should be in the range of 0.2 to 0.4 mm when using 2 gaps in series. This means that the voltage in the HV capacitor (and the secondary of pulse transformer) will be limited to 4000 to 8000 peak volts. Increasing the distance of the gaps results in longer sparks at the torch or stick electrode. Greater distances also cause additional stresses in the HV capacitor and pulse transformer and may result in premature failure of the components. Results: The arc starter was attached to a Esab stick welder (a 30 - 250A buzzbox) with the welder adjusted to 60A. We started the weld using a 3.2mm mild steel electrode and a 2.0 mm steel plate. With the arc starter turned off we found some difficulty in starting and maintaining the arc. When the arc starter was turned on, it was very easy to start and maintain the arc. With the regulation at 40A, and the arc starter turned off, it was almost impossible to start and maintain the arc. When the arc starter was turned on, the welding operation could be performed in a very comfortable way. With the regulation at 30A, the only way to start and maintain the arc was with the arc starter on. Conclusion: As a person that have worked more than 30 years with welding machines I may say that the present article is a good arc starter project. Acknowledgments We shall say thanks to Steve McConner for giving directions to post the article.