PWM [or ANY OTHER] chip that can drive an H bridge?

Yes, I did pick it up, it is now in my garage. I already use it to practice arc welding with stick electrodes. I run it off a 1 to 3 phase rotary phase converter that I built.

That's right, in fact I bought them on eBay for $50, which bouht me a heatsink with 4 IGBTs mounted.

i

By adjusting both Ra and Rb. Admittedly, they interact, but for a competent designer, that's trivial to deal with.

The data sheet does corroborate your qualms about the 18V, however:

Cheers! Rich

How do you make frequency and duty cycle independently settable with two potentiometers with a 555? Also I think Vcc=18V is pushing the limit on a 555.

robert

...which you're going to TIG weld into four solid blocks of indeterminedly doped silicon on your first attempt I'm sure.

Not because you're incompetent but because that's what frequently happens on a first try. What do these IGBTs cost new?

At leat you'll be able to re-use the heat sink ;-)

robert

Adjusting frequency and duty cycle independently will be a botch.

robert

That's certainly possible.

Some notes on what I will try to do to prevent it. Comments are welcome.

1. I am a computer programmer and know that things need to be tested before being put to use. I am not going to plug it into the welder right after assembly. There are actually ways to test this setup. For example, I will test it with a regulated power supply that supplies a small amount of electricity. Say, 20V, limited to less than 1A.

1. IR21094 chips have logic in them to turn IGBTs on and off quickly, (they would not work in any other way), thus preventing overheating due to gates being in the intermediate voltage.

2. IR21094 chips have smarts that shut down the gates if logic supply voltage falls below 12V, again helps to prevent overheating due to controlling the gates with insufficient voltage.

1. I can test if there is any internal shorting, by simply measuring current that enters the H bridge, when power is supplied but with no load (I may need to increase switching frequency to get measurable results).

2. This is actually a very forgiving application of IGBTs, since

- switching is done very infrequently, only hundreds of times per second, at most. These IGBTs can easily work in much higher switching frequencies, like 1,000 times higher.

- My welding machine has only 100V maximum peak voltage (note, peak not OCV, according to the manual), and IGBTs are rated to 1,200 V collector to emitter.

- My welder has its current limited to 200 amp, which is well within safety limits of these IGBTs, according to the datasheet.

Looks like $600 apiece (I bought 4 for $60 including shipping, on a heatsink).

I have more heatsinks than I care to admit... :)

i

Yes, you are right, I realized it last night too.

What I think I will do is adjust the frequency by changing capacitance with a 11 position rotary switch (selecting the cap), and adjust the duty cycle by using a pot. It ought to work. I will have, say, 11 frequencies and a good range of duty cycles (cleaning vs. penetration welding)

Is this plan making any sense?

i

Parallel. You'd have the highest frequency position (eg. 1000Hz) with no capacitor connected to the switch and the other positions add various capacitors in parallel with the minimum capacitance.

Best regards, Spehro Pefhany

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I read in sci.electronics.design that Robert Latest wrote (in ) about 'PWM [or ANY OTHER] chip that can drive an H bridge?', on Sat, 8 Oct 2005:

Extreme ingenuity. (;-)

There's been so much work done on the 555 that someone has undoubtedly solved that already. In fact, I think I saw the trick about 25 years ago, and built it to check that it worked. 1000:1 change in duty cycle with only a very small change in frequency.

He only wants 5 V out, so the first part is a 6 V regulator.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
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Think hard about what happens when you switch it.

I did it with a CD4011, as a regulated SMPSU. Well, sort-of-regulated. Having said that, the regulation was better than that in my latest discovery (USB hub PSU with no input smoothing capacitor, and 2 to92 actives)

That's a great point. I think that I should add a little capacitor in series with all other caps. Would that solve the problem?

i

You are right! Thanks...

i

This is an application where an internal short won't hurt the IGBT bridge because the power supply is current-limited. On the other hand, if the bridge OPENS completely (a normal, and even desired, occurence in "normal" PWM applications) your IGBTs will DIE because it is driven by an inductive (constant-current) supply.

The IRF app not mentions this (and I hadn't realized it myself until I read the app note).

robert

Bob, that's an interesting issue indeed. Just a few random points that I wanted to make in response to this. I will appreciate your opinion.

The manual says that the maximum "peak" voltage of the welder is

100V. Note that peak voltaage is a separate term from OCV (open current voltage). My IGBTs are rated for 1,200 volts.

Also, I believe that the issue of voltage rising at turnoff due to inductance could be mitigated by use of a capacitor and a varistor across the supply.

If there are flaws in my reasoning, I will appreciate if you point them out.

Thank you for posting a very intelligent comment about this.

i

On Sun, 09 Oct 2005 11:26:41 GMT, Ignoramus26315 wrote in Msg.

The spec'd "peak" voltage has nothing to do with the voltage build-up when quickly interrupting an inductively loaded circuit. In this case the voltage goes up until something breaks.

I think so, too, but I have no experience in actually building high-power bridge circtuis, and no knowledge whatsoever about welding applications. Your "HF start" thingy, whatever that is, might work against the cap/varistor combo: Either the HF start won't work any more, or it'll fry the cap/varistor.

There aren't just make sure your bridge never open-circuits.

robert

Fair enough...

I will place the module before the HF unit, actually. HF is indeed not compatible with these IGBTs.

I will place it at the reversing switch that reverses DC eectrode negative and DC electrode positive (maybe it is called commutator switch).

When the switch is in the middle position, between DCEN and DCEP, there is no contact. I would label the middle position "AC" and make sure that when the commutator lever is in the middle position, the AC module that I am working on, is activated.

The HF module is right after that commutator.

thanks, will try...

i