Tested my TIG inverter bridge under full power

But this might also mean, that we are equaly ignorant. I'm not a welding-pro.

I just had a look in my welding book (not that much about TIG power sources in there). It says, that you get a diode (when welding Al, but even with steel) due to the oxyde in the puddle. That is the reason for having a DC offset (when welding with AC). It is there to compensate the diode effect. The offset doesn't seem to be very high. From the diagram, I guess about 30%.

I know, that's the reason for the "washing machine"

Seems that pulsing is only used with DC. If you would use it with your H-bridge, you should have to switch polarity in sync with the pulses. Can you adjust the base current and the max current?

LOL! "Digital". You have thumb wheels with digits. But looking at the wire nest in that machine, is is damned analog.

Nick

Reply to
Nick Müller
Loading thread data ...

The schematic very clearly shows a bridge rectifier at the output, and the output terminals are marked "+" and "-".

Jon

Reply to
Jon Elson

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

Reply to
Jon Elson

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

Reply to
Jon Elson

That, too, although this is less of an issue with squarewave.

Yes, I can, and I cannot see why pulsing cannot be used with an inverter.

Yes, I mean digital settings.

i
Reply to
Ignoramus18456

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
Reply to
Ignoramus18456

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
Reply to
Ignoramus18456

Reply to
Glenn

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

Reply to
Lloyd E. Sponenburgh

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.