Tested my TIG inverter bridge under full power

Just wanted to say that after slight snubber circuit modifications (more below), I was able to test my bridge under full welding power of
199 amps.
http://igor.chudov.com/projects/Homemade-TIG-DC-to-AC-Inverter/05-Prototype-3/
I also added a cooling fan to cool the main heatsink with IGBTs, as well as a heatsink for the big ass diode in the snubber circuit. I drilled a hole in that second heatsink and tapped it for 3/8-20 NF thread, and screwed the diode in.
It is cold in my garage, perhaps 35F. Given that, and the fan action, all parts of the bridge stayed cool, except for the main heatsink, which became warm. Under full power, the IGBTs are expected to produce approximately 1.2 kW of heat.
Snubber modifications: I added a little Christmas tree of 150 V rated varistors (total of 5 now), as well as a small 0.33 uF capacitor across the DC rail. I also remounted the snubber diode by drilling a tapping a heatsink that I had, and screwing the diode in.
The test (among many tests at smaller amps) went on for a couple of minutes, until the breakers blew. Like I said, nothing warmed up (I checked caps, diode, and varistors).
i
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On Sat, 26 Nov 2005 08:19:57 GMT, Ignoramus29530

Congratulations! Was that 199 A into a short or into an arc?
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Thanks... It was into a short. To have a welding arc, I would need to put the bridge to the proper place on the commutator.
i
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Congrat's. I'm wondering if you came across any schematics of commercial tig inverters during your hunt? Or are they drawn as black boxes? Thanks, Jim.
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Thanks. I may try to weld aluminum this weekend.

I have not found anything.
The only thing that my inverter does not have, is control of power when electrode is negative, and separate control when electrode is positive (I think that it is called AC balance). For my inverter, the absolute value of the voltageand current is the same for positive vs negative half cycle.
Basically, I think, commercial inverters are a lot like mine, they have a timing circuit, drive circuit, full bridge driven by FETs (MOSFET or IGBT), a snubber (maybe) and controls.
I am just a chump who put together my first electronic contraption, so I may not know as much as more learned members of these newsgroups.
Take my answer for what it is worth.
i

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I have just scanned the schematic for a Hitachi inverter TIG and added it to the dropbox. The jpg is about 1mb.
http://metalworking.com/DropBox/HitachiInvTIG.jpg
http://metalworking.com/DropBox/HitachiInvTIG.txt
Ignoramus12834 wrote:

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David Billington wrote:

This isn't much of a schematic. All the circuit boards are just rectangles. Also, this is a DC only welder.
Jon
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I thought it was worthwile posting it as it does show some of the details of the power side, if not the control system. It was mentioned earlier in the thread that no schematics had been found so it may be of some use. The welder is definitely AC and DC, if the schematic can't do that then I guess the manual is wrong, but it is entitled "power transistor controlled, AC/DC TIG arc welding power source".
Jon Elson wrote:

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It is a AC/DC welder. Look at FETs T21 and T22, you can see that they can be used to supply either +, or -, or +/- to the line labeled +. Whereas line marked - is the center tapped point of the secondary.
in AC mode, I think, only one half of the secondary winding is working. I may be wrong, I slept only 3-4 hours for the last 2 days and am barely thinking, .
You can also see RCD snubbers near T21 and T22.
i
On Sat, 26 Nov 2005 23:49:48 +0000, David Billington

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On Sun, 27 Nov 2005 00:57:12 GMT, Ignoramus12834

I agree, except they appear to be bipolars, not FETs.

I think you are wrong. The tapped secondary is connected into two full-wave rectifiers for the two supplies (+ and -). Both halves of the secondary are always working, but only one supply (+ or -) will be connected at a time and supplying current.

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Could be. I thought that bipolar transistors are also field effect transistors, that's why I said so. I am an amateur.

Exactly what I was trying to say, not so well.
i
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Ignoramus12834 wrote:

Bipolar Junction Transistors come in NPN and PNP types, and do not use the field effect. They use base current to modulate (and amplify) collector current.
Field effect transistors use the electric field between the gate and source to modulate the resistance in the channel. Gate voltage essentially controls drain current through this method. Generally, gate current is zero.
Insulated Gate Bipolar Transistors are a combination of the two. They really are a bipolar junction transistor with a FET connected between the collector and base. They give many of the advantages of both types. With the FET, the voltage drop across the drain-source increases linearly with current. (It is a resistance, after all.) With the Bipolar and IGBT, the voltage drop is almost flat with changes in current, usually around 1.5 V. Regular bipolar junction power transistors require a base current about 1/10th of the collector current. With 100 A transistors, that amounts to 10 amps the control circuit needs to deliver to the base. Darlington transistors have at least 10 x higher gain, but the voltage drop will be double, a major disadvantage. The IGBT solves that problem. The gate current required is zero, once the gate capacitance has been charged.
One other pro/con thing about IGBTs is that, like other bipolars, the forward voltage drop goes DOWN as they get hot. This effectively prevents thermal runaway with single devices. But, it makes it a lot harder to balance current in paralleled devices, as the hottest one will have the lowest voltage drop, and thus get all the current. (By comparison, the resistance of FETs goes UP with temperature, making thermal runaway possible. But, it makes paralleling devices a breeze, as they will self-equalize their current.)
Jon
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Thanks. After reading your post, I finally understood what IGBT was about. And I tried to understand before, but your post explained it very nicely.

As far as I understand, homogeneous (equal in all respects) IGBTs can actually be paralleled. Mine are homogeneous and mounted on same heatsink, etc. At low levels of current (say under 50%) they could be unbalanced, but that's not a reason to worry. They do balance out at higher currents. At least that's how I understood things.
i
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Thanks for the very nice explanation Jon! I guess I have been a tech for too long .. I just can't get into staying current unless it is directly related to the problem I am trying to fix :) If Iggy keeps this up I may just have to dust off all the test equip on my bench and start playing again LOL. It is true what they say about never going to work in a field that is a hobby. It really takes the joy out of it in 30 years or so. I just got back from driving about 500 miles .. the last 20 in a snowcat .. just to turn a pot a couple of turns. Appreciate y'all bringing some fun into the mix :) Glenn
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Nah... that isn't necessarily so. But it can be. Like Cindy Crawford says on that furniture commercial, "I only want to do what I'm passionate about." (Why, oh, WHY couldn't she be passionate about my body????)
If you truly LOVE what you're doing as a hobby, making it a vocation isn't all that bad. I did fireworks for a hobby for years, then decided to make a living doing it. It was the best career decision I ever made!
'Pends on what you expect of it.
LLoyd
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Ignoramus12834 wrote:

OH, heck, you are absolutely right! I was viewing the thing in Mozilla, which just gives me a peephole view of the file. Yes, T21 and T22 are between the bridge rectifier and the output terminals. (Dumb labeling of those, they should be electrode and work, not + and -.)

No, only one half of the bridge rectifier is working. Presumably the work terminal is connected to the center tap, and the bridge always produces a + and a - voltage referenced to that. The two transistors select from the two polarities. Certainly a minimum solution to the problem.
Jon
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I am glad that I understood it correctly.

Try left clicking on the image. It works for my firefox and also mozilla, I think.

Agreed.
Yes, but also at any given moment only one half of the secondary is supplying current, although in AC mode these halves alternate.
i
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On Sat, 26 Nov 2005 23:49:48 +0000, David Billington

Certainly looks like an AC/DC welder. One good hint is the AC/DC selection switch in the lower-right corner (after rotating the page so it is readable).
The right-hand side shows two DC supplies, one + and one -, that are selectively connected to the "+" output via the transistors T21 or T22. They could be controled to give DC+, DC-, or AC.
It would be interesting to know what parts they are using for the IGBTs and the T21, T22 transistors.
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David Billington wrote:

The schematic very clearly shows a bridge rectifier at the output, and the output terminals are marked "+" and "-".
Jon
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What is wrong behind my idea to switch the bridge asymetric? For example 70% one direction, 30% the other. This way, you get a (averaged) DC-offset.
Nick
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