Can I use a stock SIGNAL GENERATOR to drive an H bridge?

That's a very good app note indeed.

For your application you should perhaps be thinking about a high side auxiliary supply. Prolly a 555 would do the job.

Graham

Ig, I think you would be better served by getting a pile of little transistors, a couple 9v batteries and working on your H bridge design until you can get it generating AC sufficient to light 2 leds in parallel with polarities opposed. Once you have AC, then start increasing power.

Worrying about selection of hi power output drivers at this point isn't getting you any closer to making H bridges. What people are trying to tell you is H bridges are difficult to make, particularly for high power. If you're trying to make a welder, then you are in for a very difficult job. Bridges look great on paper, but once start trying to move kilowatts around with them things get lots more complicate.

Gregm

measure anyway !

Yup. ;-)

Best regards, Spehro Pefhany

Just curious. You want to build something that can be difficult for experienced engineers, for instance the fellow working on the bench opposite mine last week. What electronic test equipment do you have?

jw

You'll need a high-frequency Tek current probe and a shunt to debug power circuits. A big Variac and some load resistors are very helpful.

jw

Graham sez:

"> trouble is.... you really have to have been doing this stuff for ages to pick up

Your best reply so far! Hopefully, Iggy will be wise enough to heed some of your (and other's) advice and pull back his ambition enough to realize there are no instant answers to success in the electronics game.

Bob Swinney

Fair enough. If I get that welder, then I will start with a smaller scale inverter. I hope that I could reuse it for larger scale, in some manner.

Not much, a Tek 475 oscilloscope, a nice multimeter, and also a HP spectrum analyzer. Also wavetek 171 signal generator, hp 204D (which I incorrectly mentioned yesterday as HP 204C), and some frequency counters. A megohm meter also. I use that stuff mostly to test military surplus things that I sell on eBay.

I've been following these threads and I must say I find it kinda scary. I've been "doing" electronics since I was about 12 and I'm now 70. I've done radios, audio, logic, switching power supplies and other things. Of these, I have found the switching power supplies the most difficult to get right. I built my first one in 1960. The higher the power, the more difficult dealing with a myriad of tricky details.

In spite of all that experience, my welder is a Thermal Dynamics (now Thermal Arc) 250 GTSW. I never even considered building my own. For one thing, I'll lay better than even odds it would cost me more.

If you want to get into switching, try a class D audio amp. Then go on to something like a variable voltage, current limited + and - 0 to 50 volt 10amp bench power supply.

Then think about whether you want to do a welder.

This is not intended to put you down or discourage you, rather to try to set you on a path where you can expect reasonable success.

Ted

I would not use the signal generator because it isn't really optimal for the job, and if you get for example a drain to gate short in a blown MOSFET, you will quite likely destroy your Wavetek. Get the National Semiconductor datasheet for a 555 timer and build yourself an oscillator. This is seriously just as good for what you are doing, and when it blows up you will be less upset (or at least I would). If you need to drive MOSFET gates fast, I recommend TC4421 or TC4422 chips connected after the 555, they are supposed to put out 9 Amps, which your Wavetek can't do. The 555 can run off a 5V regulator. Keep the wires from the TC4421 to the MOSFET shorter than 1 inch for low inductance, and put ceramic and electrolytic decoupling caps right next to each 4421. Chris

I would disrecommend an H-bridge - the workpiece has to be grounded to ground, or you run a risk of serious injury.

I'd be very surprised if you could design and build a circuit that would turn your DC welder into an AC one, for less money than you can just go buy an AC welder.

Good Luck! Rich

I testify to that. I have even managed to make a plastic to-92 package explode. Driven a power FET into thermal runaway and destroy it. and fried more IC's with electrostatic damage than i could count.

Actually you might try to make a micro-arc-welder before scaling the power up much. The load properties of an arc-welder includes almost all of the worst case load characteristics, including open circuit, short circuit, very high harmonic contents, wildly variable load impedances, and negative resistance slopes. protect and oversize your components zealously.