Battery Powered Tig Welder

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Hi guys Im doing a project for school, which is a battery operated tig welder.  The welder is already built and works with 2 car batteries, but we need to do the following:        1: Add a foot pedal to have variable amperage.        2: A way to start the arc.  Right now, to start the arc, we short the tungsten w/ steel woolAny help would be great, im not sure how to accomplish these tasks, Ive heard that I need to add a capacitor in order to start the arc, but im not sure. Thanks
Reply:The variable amp control will be a tough one. For the arc start I suppoe you could make a little curcuit with a 555 timer IC that would switch on/off a power transistor big enough to handle a ignition coil, the coil would have to have a high voltage cap or diode to keep the weld current out of the coil, on second thought a spark gap would work perfect and be cheaper. You can find all kinds of curcuit examples for the 555 on the internet, it's one of the most simple to use IC's around.
Reply:Do you connect the two batteries in series to give 12V and 2x the current?  10 to 12 volts would be about right for argon TIG welding at around 100 amps with an arc gap of about 1/16 to 3/32"I think a DC power welding power supply can be scratch started easily, is because the open circuit voltage (OCV) is up around 80 volts.  In your case, the OCV is 12.  I'm no electronics wiz, but I guess you need high frequency or high voltage to get the arc to initiate.
Reply:To strike the arc, scratching will work. Worked quite well for many years on a lot of critical welds. Sometimes done with a start tab to eliminate the risk of contamination in the weld area, but usually just done right where the business will be.For a high voltage start, you will need to put a moderate inductance in series with the torch, right where the torch connects to the power source, and inject the startiing voltage at that point. The inductor MUST be insultated sufficiently to handle the high voltage, or be protected with a spark gap, to prevent insulation damage, or worse, striking an arc in the inductor. The inductor protects the rest of the source from the high voltage, which will be  short impulses (which effectively are high frequency), and keeps the rest of the source from sinking the HV away from the torch. Goal is as little inductance in the loop from the HV source and the torch as possible.The voltage you will need is of the order of 10000V (which will just draw a spark at about 1 inch) Several ways to get this. As mentioned, and ignition coil will do it. You an also try the coil from a camera flash, but you may need a step up and capacitor for this (the flashes own circuitry might be fine) as these often trigger from the high voltage side, not the low. Ignition coil will allow a continuous spark, the flash unit will allow for impulsing: hit the button, get the spark to strike, hit it again if it doesn't strike. Probably easier this way, as sensing arc ignition to cut off the drive to a continuous source is that much more circuitry. Another option in the impulse catagory is a gas grill striker. You'll need a way to punch the button, but you get  strong HV pulse from the piezo element. You can also try the electronic igniter set from a gas stove or modern gas furnace.
Reply:Originally Posted by enlpckAnother option in the impulse catagory is a gas grill striker. You'll need a way to punch the button, but you get  strong HV pulse from the piezo element. You can also try the electronic igniter set from a gas stove or modern gas furnace.
Reply:enlpck,  I also thought of using a disposable camera flash assy, the problem I think would be if the arc did not start, this being the case you would have to wait for the capacitor to recharge. In some cases this could be up to 20 seconds or so.
Reply:The gas grill lighter will work, I think. I haven't used it for TIG< but have tried using one to strike off stick before, just to see. Contact was at the stinger. I don't know how much lead one will drive, but I do know that it is at least ten feet each way. Had some students using one to fire off a small jet engine.I'm not sure if the camera flash coil will do it, but you will likely need to make a driver for it of your own if it will. Certainly worth a try.
Reply:The camera flash circuit has it's own driver built in, and would make a spark. I would stay away from them though because the capacitor in a camera flash carries an EXTREMELY painful charge (trust me, been there). They are potentially lethal. Much better, and easier idea to use a piezo ignitor.
Reply:On the variable amperage... wonder if you couldn't do a 555 PWM circuit..?  Large IGBT's or Hockey pucks can be had off Ebay for reasonable.The BBQ lighter DOES work for starting TIG.Sounds like an interesting project... keep us posted.
Reply:Quick update.  We are going to try to use the circuitry from a camera flash, it seems to have everything we need.  My only question is how to attach the circuitry of the flash to the existing welding circuit.  I havent opened up a camera to inspect the circuit but i assume i would just connect the output that goes to the flash to the welding circuit in series with possibly a diode to make sure the current doesnt flow back into the welding circuit?
Reply:Im thinking what they mean by using the camera flash, is to use the trigger for the flash tube.. which is a high voltage pulse.  The voltage across the tube (before firing ) is only a couple hundred volts.  If I remember correctly, the trigger is a couple kV.I think a diode to prevent backfeeding wouldn't work... the diode would have to carry the welding current.  Perhaps a choke would be a better choice?  The coil would carry DC power with little impedance, but would resist flowing the quick firing pulse.The above is just my OPINION... your mileage may vary, standard disclaimer applies.
Reply:I'd advise you to listen to Joe H regarding the danger of photoflash circuits unless you have expert advice to assure safety.  He is correct about the potential lethality - or at least the potential to cause an accident by reflexive reaction if hit with the discharge - of the charge on the flash energy storage capacitor.Your description of how you would apply the photoflash indicates exactly the wrong approach.  There is no need for the flash energy storage capacitor in your application and that is the dangerous component of the flash circuit.If you examine the photoflash circuit you will see that in most, but not all, cases there is a third conductor along the side of the flash tube that is not part of a complete circuit - it just ends single-ended at the outer wall of the flash tube.  That is the trigger electrode that carries the high voltage pulse from the trigger transformer to the tube wall.  It excites the nearby gas molecules just inside the tube wall into ionization by capacitive coupling.  That initiates the ionization of the entire gas column in the tube due to the high DC voltage across the tube from the energy storage capacitor.  That ionization creates a low resistance path for discharge of the energy in the photoflash capacitor through the tube, creating the flash.You only need the trigger circuit which is not dangerous (although you will decide to avoid it after the first shock).  Your assumption that you just take a lead from the energy storage capacitor to the torch is totally incorrect, would not work for your purpose, and is potentially dangerous.You really should study flash circuitry before just taking a unit and playing with it, both to understand what it is doing and to protect yourself and others from injury.  You will see the discrete functions of (1) the oscillator circuit that charges the photoflash capacitor to a few hundred volts, (2) the circuit that triggers the tube into conduction, and (3) the flash tube discharge loop.  You only have use for the trigger circuit, but that might also rely on the charging circuit to charge it's own much smaller capacitor that discharges through the trigger transformer to generate the HV pulse.  Study and understand the circuit!  Don't forget, it is the prime responsibility of a circuit designer to be sure it is safe, not merely that it does some task.awright
Reply:I see where you are coming from, when I was researching the flash circuitry I missed a few things and thought I needed the entire circuit, but in reality the most of the circuit only creates a few hundred volts which is then fed to the trigger ciruit and then the voltage increases correct?  Im not too familiar with this stuff, so your saying all I need is the trigger circuit, but what would the input to the trigger circuit be?Thanks for all your help I really appreciate it.
Reply:you'd want to trigger it when you wanted to start welding.  For example, if you're using a foot control, you could use a microswitch ( also useful in turning on your gas ) to also signal the flash trigger circuit.  At this point, when that trigger fires, the tungsten had better be close enough to the work piece so that the arc will jump there and strike the main arc and not up to your arm, or filler... ( found that out the hard way with my DynastyDX... HF bites )
Reply:Hoopman5000, if you want to adapt and modify a flash circuit you really can't safely avoid actually learning what every component in the circuit is there for and what it does in the circuit.  We may be able to help, but can't do much with your questions in the absence of knowledge of the circuit.  I and others may have some general knowledge of how flash circuits work or how some specific circuit works, but we don't know what you have or are thinking about obtaining.If you already have the flash circuit in hand then please figure out the schematic and post it with all parts and components identified and values given.  With that information, we may be able to help you.  It is also possible that high quality close-up photos of the flash hardware with components described and values listed would allow us to see how your specific circuit works so we could advise you.Based upon your question, I think you still do not understand how your flash circuit works and it is difficult to advise you without knowing what you are working with.  That is not a criticism.  Very few people understand how flash circuits work.  But they are not planning on modifying them and putting them into a project.  Perhaps a search on the web would pull up a tutorial on flash circuit design principles.  Perhaps there is a flash or strobe kit from one of the many electronic kit suppliers that would provide you with both the hardware components and a schematic.  That would greatly facilitate adapting flash to your purpose and would probably not be expensive."...most of the circuit only creates a few hundred volts which is then fed to the trigger ciruit and then the voltage increases correct?"  No, if I understand what you are saying, it is incorrect.  In the simpler flash circuits, the energy storage capacitor is probably directly across the flash tube.  The flash tube acts as it's own switch by being tickled into conduction by the trigger pulse.  The trigger pulse, which contains very little energy, is generated by a much smaller trigger capacitor discharging through the low-voltage primary of the trigger transformer.  The trigger transformer steps the TRIGGER capacitor pulse up to a few thousand volts which is applied to the wall of the flash tube where it initiates ionization of the gas in the tube.  The TRIGGER capacitor may be charged to the full voltage of the energy storage capacitor or to some lower voltage, depending upon the design.  This would be part of what you would have to determine to apply the circuit to your purposes.Gotta go.awright
Reply:I think the best thing for me to do here is to just use a simple piezo ignitor.  I had no idea the camera flash would be so difficult, and the piezo seems much easier and much safer.
Reply:hows the rest of the circuit coming?
Reply:Im still working on the arc start, but when we try to fire the high voltage, it works when the welder is off, but when its on it just flows back into the welder since there is much less resistance.  Any idea what I could use to stop the high voltage from backfeeding?
Reply:Originally Posted by enlpck For a high voltage start, you will need to put a moderate inductance in series with the torch, right where the torch connects to the power source, and inject the startiing voltage at that point. The inductor MUST be insultated sufficiently to handle the high voltage, or be protected with a spark gap, to prevent insulation damage, or worse, striking an arc in the inductor. The inductor protects the rest of the source from the high voltage, which will be  short impulses (which effectively are high frequency), and keeps the rest of the source from sinking the HV away from the torch. Goal is as little inductance in the loop from the HV source and the torch as possible.
Reply:Did I miss where you told us what your HV source is?  Last I read, yu were going to go with a piezo barbecue starter, but we never got into any discussion of how to apply one to your situation.Did you provide any inductance at the welder output terminals?  That is important not only for forcing the HV pulse to go where you want it (from the tungsten to the work), but to prevent the HV pulse from getting into the welder and causing an insulation or semiconductor breakdown.I wouldn't be surprised to find that a piezo barbecue starter spark was soaked up by the capacitance of long welding leads even if they are just on the ground and draped over the work and table.  Don't forget that, even though the piezo crystal can generated a very high voltage when stressed, there is very little energy behind it.  That is, the crystal can't supply many electrons to flow after the spark jumps from the center electrode of the starter to ground.  That means that the small CHARGE (number of electrons) generated by the crystal can't charge any significant stray capacitance in the welder leads up to a high enough voltage to jump the gap at the tungsten.  (Of course, all of this is just speculation, including my assumption that you are wiring the starter crystal to the welder leads.Don't forget that your little crystal and the energy in your finger squeeze is trying to the same job as the fairly large HF generator in a commercial welder.  I guess the same thing goes for a photoflash trigger circuit except that with some sophistication, a flash trigger generator could be modified to produce a great deal more energy than a finger squeeze.The main issue in the design of a HF blocking inductor at the welder terminal is that it must be able to carry the welding current without overheating within the duty cycle of the welder.  I think your best bet is to find an inductor intended to be used as a smoothing coil on a welder.  Long ago I recall that you could buy exactly that item from the Northern Equipment or similar catalogs.  I presume they are still offered.If you want to wind your own you should search for the thread on this forum by a guy experimenting with coil design.  Sorry, I don't remember the title of the thread.Not sure, but I think a modest value, high voltage capacitor to ground at the input end of the inductor would make the whole filter more effective.  Perhaps 0.01 microfarads at 3KV would work.  I believe it would have too small a value to have any negative effect on the welder.Good luck!awrightLast edited by awright; 03-02-2007 at 06:29 PM.
Reply:The BBQ or flash starter won't do anything. It will simply ground back through the work leads.On a TIG welder, the high voltage/high frequency signal is superimposed over the low voltage/high current work lead in similar way that 18VDC is superimposed over the coax lead on a satellite TV dish system or a tower-top antenna amplifier.Most simple setups use a feedthrough inductor with a very small primary winding that is fed a high frequency/high current signal that will induce a high voltage/high frequency voltage over the work lead. This HV/HF signal travels the work lead using the "skin effect" principal whereas the HV/HF travels as an "RF" (radio frequency) signal over the work lead. On some setups, there is diode isolation to prevent the HV/HF from going back through the welder's transformer/rectifier assembly. With this in place, the welder's electronics need to be protected from the HV/HF signal's RF and EMI emissions. This is accomplished by ferrite loops & beads and bypass capacitors.This isn't as easy as it sounds. You may want to look in to obtaining a schematic of a commercially available HV/HF add-on unit and you'll have the basics of what it will take. This, however, does not mean that it will work on all welding systems or a homebrew system as you are building.