An update on my TIG inverter project

You remember a while back, I suggested you use a 555 timer instead of your Wavetek? Well I still recommend that. Just imagine that the first time you switch it on, lots of smoke comes out of the Wavetek. That is quite likely to happen. I have let the smoke out of a Wavetek and it upset me. Believe me, it is not hard work at all to build a circuit which will be just as satisfactory as a Wavetek for your use, and much easier to repair. Start with figure 4 in the following document:

look at: Also, you really do need to put at least varistors across the welder output and probably the load terminals too (I suggest the big sort which you connect with nuts and bolts). The inductance within the welder will easily produce more than 1200V when the arc goes out, and I am not talking about HF starting here, just the inductance of the transformer coils.

Good luck, Chris

How cheap? I might be interested ...

Andy

Actually, you really can't turn off HF for the application you are suggesting. You could turn off HF during DC welding, or do without HF altogether. But for AC TIG welding (i.e., for aluminum TIG), you have to have continuous HF to maintain the arc.

Note that you can do some limited aluminum TIG welding using DC, according to Ernie ...

I am not a welding expert, by any means. My understanding is that HF arc stabilization is required for regular 60 Hz sinewave AC, since the period when voltage is low, during transition through zero, is too long and the arc extinguishes itself.

What I am trying to accomplish is to build a square wave inverter, where transition through zero, from high absolute value of voltage to the opposing high absolute value of voltage, happens very quickly.

The difference is that for sinewave, we are talking about a few milliseconds, whereas with square wave and my IGBT, the transition would take about 1 microsecond or less.

Welding books that I read suggest that HF stabilization is not necessary for square wave AC. Miller went as far as suggesting to not use HF even for starting, and created a lift-arc arc starting technology.

i

Actually you can do alot of Al with He. Thick or thin, just change the tungsten and polarity for the purpose. The cleaning and ease of AC just makes it a better choice for most.

Hello Jon and Ignor,

To stop the HF arc starter "frying" electrically delicate parts of the welder, I have seen ordinary capacitors connected across the welders terminals to ground. (metal cabinet) O.1uF 400V. I use three caps just to make sure. Each terminal to ground and one across the output terminals of the welding power source. That is just extra insurance for me to feel better. The heavy input terminals of the arc starter unit will have RF bypass capacitors fitted.

Welding power source------> HF Arc starter ------> Tig Handpiece

This is the usual setup. Why would you rather not do that?

Bill H mentioned he fitted capacitors across all his rectifier blocks. This is normal practise. You can do the same to protect your IGBTs. A resistor and capacitor in series called a snubber circuit can be used to protect your semiconductors against transient voltage spikes.

Nice to see a hobbyist / enthusiast having a go at this project. I take my hat off to you.

Regards, John Crighton Sydney

Makes sense.

That's exactly what I will do. I will make my inverter circuit a part of the welder and place ot before the HF arc start circuit.

I actually did open up my welder's cabinet and looked inside.

It has two positions, DCEN and DCEP, on the switch that commutates lines.

If the DCEN/DCEP switch handle is in the intermediate position, there is no contact in the switch.

I could place my inverter between the 4 terminals (2 inputs and 2 outputs), and turn it on when the switch handle is in the intermediate position. Then the circuit would be switched electronically.

After that commutator switch, there is an arc start circuit.

That's great.Would you have some suggestions regarding capacitance of the cap and resistance of the resistor?

200 a power source, 30-1000 (or so) Hz switching speed, etc. 1,200 volts rating of IGBTs.

Thanks John, it is an interesting project indeed, I would love to get it to completion. As I said earlier, I already have IGBTs.

i

Hello Ignor, sorry mate, I am out of my depth with that question but I did some googling for you and for my own interest. Resistors can be 10 ohm to 0.7 non inductive (not wire wound) Up to a few watts. Capacitors from 2.2 nF to 47nF Those values I just mentioned to you came from here

googled snubber design. If you read that thesis you might get some ideas how the combinations work.

I am reading your post in the welding group. The fellows in sci.electronics.design which I see you have cross posted too (good idea) will point you in the right direction as regards snubber design.

Get hold of some schematics of inverter welders and see what other welders use in that area. Check out the Lincoln and Miller web sites for their manuals. Look up the schematics. No harm in a hobbyist copying as a starting point.

Regards, John Crighton Sydney

It seems to me that I recall a problem with snubbers. They not only have to withstand a certain level of average power, they also have to take a sharp, short power spike. Some types of resistors will exhibit non-passive failure. You need to use a resistor tolerant of spikes. Carbon composition resistors will be much more forgiving than metal film resistors. If you ask which kind they carry at Radio Shack, they will ask you to wait a moment while they check with the boss to see if they carry "resistors" (no help there).

Steve

Mouser.com is a good site - Vishay resistors come in all sorts of wattages and precisions....

Much wider than was provided below - but then I'm a stockholder and know them better. Dale - remember them - owned by Vishay.....

Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder

John Cright> >

John, thanks a lot. Idownloaded that thesis and will review it soon.

i

That has been my experience with Radio Shack as well. They know how to sell cell phones and toys, but would not know anything else.

i