Making square wave AC from DC? (inverting)

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Let's say that I have a source of high current 300A 40V DC. Let's say that I want to make square wave AC from it. I am thinking about building a device on the cheap that could do it.

I could buy 40 power transistors like these:

Item 7543195186

Say I have DC inputs A and B. Say I want square wave AC on outputs C and D. All I need is connect A to C through 10 transistors, A to D through 10 transistors, B to C through 10 transistors, and B to D through 10 transistors.

I would then make appropriate AC input to signal input of transistors so that A to C and B to D are turned on when AC signal input is positive, and A to D and B to C are turned on when AC signal input is the opposite.

That would generate a square wave AC between C and D.

The cost of this project, not counting time expense, would be around \$60 or so.

Are there any fatal flaws with it?

Would this be sufficient to use it in welding applications?

i
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How many MORE transistors do you need for the TIME MACHINE? (I'll go with you... we could go back and slap Cliff's mother and tell Gunner to quit smoking.)

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We could just strip Cliff of electrons and watch him explode from positively charged particles pushing away from one another...

i
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So you plan to connect 10 transistors in parallel to drive one leg of your output waveform? There may well be a problem with thermal runaway. No two physical devices are actually 100% identical. If you have two transistors fully "on" then one's conductance from collector to emitter will be slightly different from the other. In other words, one transistor will conduct slightly more current than the other. All solid state devices will change conductivity as they heat up -- some will become more conductive, some less. If your devices become more conductive as they heat up, then the one that starts out carrying an infinitesimal amount more current can wind up trying to carry it all, and blowing, and then they can all fail zipper-fashion. Google on "thermal runaway".

That's the only thing I can think of. You will have to be a little careful of your triggering circuit.

GWE (> Let's say that I have a source of high current 300A 40V DC. Let's say

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Well, maybe a couple of little problems.....

What you are describing is an H-bridge. Bipolar power xstrs like 2N5302 are kinda slow. You could get "shoot thru" if transistors say, from A to C don't turn off before transistors from B to C turn on, etc. POW. (They really do go "POW", sound like a pistol shot)

You're gonna need about 60 amps of base drive (3 amps per transistor) to saturate your transistors.

Vce(sat) on 2N5302 at 30 amps is 3 volts -- which will dissipate 90 watts per transistor, or 1800 watts with 20 transistors conducting. That's just on-state dissipation, doesn't count switching losses. You are gonna need either water cooling or a BIG heatsink and fan.

You can buy IRFZ48V 60-volt 72-amp MOSFET's for \$0.84 each (10 or more) from Digi-Key. If you ran these at 30 amps they'd only dissipate about 11 watts per device -- and they are a LOT faster than bipolars so switching losses are considerably lower. They also need much less drive power. You would still need to think your drive scheme thru to be sure you avoid shootthru. There are H-bridge drive chips available that handle hi-side drive and contain anti-shootthru cctry. You could probably run them at 50 amps each, needing only 6 per leg. Dissipation would then be 30 watts per part or 360 watts total.

Not unless your power supply is capable of constant current operation. Even then, 40 volts open circuit would be marginal for stick (SMAW) or TIG (GTAW), which is where you might want squarewave.

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A bit of resistance in series with each emitter will make them share nicely. MOSFET's don't need that; they're routinely used in parallel.

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OK.

okay, so they are wrong for my application.

That's nice. I am thinking, regular sine wave AC should, according to my uneducated logic, be able to control that nicely. The times when the voltage is below actuating voltage in absolute value, should hopefully allow both sides to become turned off without "shoootthrough".

I am thinking about buying one that should be capable of CC.

40 volts was an example... I am not sure of the capabilities of that welder... i
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Thanks Grant, some deep stuff for me to digest...

i

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The transistor, 2N5302, is only good for 60 volts. No more. Your DC levels plus your signal level x 2 will far exceed that. So unless you are going all the way with circuit protection your circuit will go up in smoke.

I am not sure where you are getting your transistors. Unless they are a documented source they will be a large part of your problem simply because the chance they are rejects from a quality source.

There are better designs for what you want to do. Use some 600V Triacs or the like. And there are switchers that do this also.

Bob AZ

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Bob, can you give me some detail on that?

i
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Thanks. Can you go into a little detail, please? That's interesting.

i

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Can you make that Time Machine a three-seater? This is one ride I'd really like to go on..

John

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I have been designing switching power supplies for 25 years for a living. The 25 years before that, I made electronics for fun.

The project you have picked is not a good starting point for learning to design electronics. I started at age 6 by making a buzzer. It buzzed, but at a higher frequency than I wanted. Try making a buzzer first. I had a 13VAC electric train tranformer as the supply, but you can use a 40 Volt DC supply. You may be generating AC sooner than you think.

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Clark sez: "You may be generating AC sooner than you think. . . . The project you have picked is not a good starting point for learning."

Right on, Clark. It is the absolute worst starting point for learning. In spite of the good advice from learned members of RCM, I'm afraid Iggy isn't likely to soon make the leap from hack to circuit designer. No offense Iggy. Far be it from me to discourage anyone, but switching power supplies is not a great "learning place". Why'nt you study a little and get a better grasp of the fundamentals before undertaking more daunting and, well, errr, expensive projects? Nothing will cool your enthusiasm more than having an expensive (experimental) electronic investment go up in smoke.

Bob Swinney

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Thanks Bob. I am not in any way offended. There is a huge leap from making, say, a phase converter with some solid state switches, to making a switching power supply. I do lack a lot of knowledge. On the other hand, I am known for being able to learn some things and would not mind reading.

If a \$50 investment goes up in smoke, I would be upset, but not hugely.

i

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Bob Swinney

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Thanks. I appreciate the compliment as well as your input.

So, back to the original issue. I want to understand how mosfets work, a little better.

Let me give a simple example.

Forget about H bridges etc, just think about a single mosfet, controlled by 60 Hz AC, and a resistive load.

Let's say that I have a mosfet. There is voltage and resistive load applied between source and drain, with the mosfet in series in that power circuit. I apply 60 Hz Ac to the gate (control input).

Would it be correct to say that the resulting current on a load would be a sequence of almost square pulses that would be a little less than

1/2 of AC period, in length? i

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Just to slap Cliff's mother!

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Iggy sez: "So, back to the original issue. I want to understand how mosfets work, a little better."

I'm not qualified to counsel much on mosfet's. There are others on RCM much better than I on power switching, in general. I suggest you get a handbook on power solid state devices. The handbooks are pretty good tutorials and have a lot of sample circuits, as well. Also you might do a google on Marcus's books and see if he has a (current; no pun intended) one with power switching circuits. Don Lancaster is a great teacher - you might try him as well.

Bob Swinney

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