Making square wave AC from DC? (inverting)

Using Don's mind is a great idea. I made thousands of $$s using his ebay/surplus selling ideas.

i
Reply to
Ignoramus965
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Forgot to say. If I get that welder, and if I decide to proceed with this square wave project, I will first try to make a low current square wave inverter as a test bed, using a small DC power supply. I would just buy some smaller mosfets, make a small H bridge, and try to make it work. It is going to be much safer and cheaper, and I will not have to deal with thick cables, power dissipation, big terminals etc etc etc.

Since I have an oscilloscope, various DC supplies, etc, it will be an easy test project. If it works, I can proceed with doing the same on a larger scale.

i
Reply to
Ignoramus965

Good plan. It is always better (and a whole lot cheaper) to experiment on a smaller scale.

Bob Swinney

PS: Your scope is a handy device to have for experimenting with RPC's also. With it you can tap into individual legs and investigate balancing and power factor considerations.

Reply to
Robert Swinney

that's right... I think, though, that I need to reduce voltage either by using special probes, or by adding transformers...

i

Reply to
Ignoramus965

Not that this really matters, but that would be enough explosive power to destroy the earth.

Interesting trivia: I vaguely remember reading in the Feynman physics lectures that if you took the electrons away from 10% of the atoms in a human body, it would be strong enough to move the earth a few miles...

Reply to
jpolaski

Be careful with how you actuate the 2 sides of the H bridge. If you don't time it properly, there will be short periods as you flip from one side to the other where both legs of the bridge are shorting from supply to ground.

You might try adding a decade counter IC with the first 5 taps operating the left side of the bridge and the next 5 operating the right. Since the taps off the counter are actuated one after the other, the sides of the bridge are never active simultaneously. You could even be clever and put resistors of varying size on the taps to reduce the squareness of the waves. OTOH, getting a clean supply for the IC and isolating it from the transients flying around the bridge is sort of a drag.

A low budget alternative is to feed the center tap of a transformer, then alterate which end of the winding you pull to ground. Then you get AC out of the secondary, but without the pain and agony of getting a H bridge working really well. Efficiency is poor- but the circuit is easier.

It is pretty neat when you start getting AC out. But getting a useful amount of power out is a bit tougher.

Gregm

Reply to
Greg Menke

I was going to do it with regular sinewave AC. So there will be preiods where the absolute value of potential is so low that both sides are deactuated.

I am confused, I can just use one AC source, but I would reverse leads going to certain mosfets. I do not need a center tap.

i
Reply to
Ignoramus965

And hugely more difficult to get it to work, especially without destroying the cost difference in blown up parts.

Reply to
Jim Stewart

You might want some inductance between your power supply and your bridge experiment. A welder is a highly-inductive source.

Reply to
Don Foreman

If all you're doing is making a square wave out of a sine wave, then a couple op-amp comparators seem to me like less work. I guess I misunderstood your problem- I was going from the title of the thread and other comments.

But give your idea a try by all means, just please do start with low current and low voltage.. :)

Greg

Reply to
Greg Menke

I will look into it, it sounds promising.

Absolutely.

I will start with relatively low voltage from a small DC power supply (say 20V), and low current. I will make the H bridge from one mosfet on each leg.

For generating variable frequency AC, I will use a HP 204C oscillator (which I have), and an amplifier circuit that I will build or buy.

If all of that works and inverts DC nicely, I will simply build a big H bridge and will reuse the control system that I would build for the first stage.

i
Reply to
Ignoramus965

exactly.

I have some big ass SCRs, but, unfortunately, cannot see how I would use them.

If you can help me find such a controller chip, that would be greatly appreciated. I just do not know what to search for.

I realized that I want to make this system variable frequency, not 60 Hz. I have an HP 204C oscillator, but need to amplify it to drive the mosfets.

i
Reply to
Ignoramus965

Most offerings are in surfacemount packages, as

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has dead time for shoot-thru prevention.

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available in 14DIP, no shoot-thru dead time incorporated.

Other suppliers include Allegro and STM. Look for half-bridge controllers.

These all work well with resistive or inductive loads and voltage source for power, but dealing with an inductive or constant-current power source (as a welder) will introduce some interesting issues. Where does the current go when all switches are off, and how high does the voltage get while it's trying to do it?

This is not as simple as it might look.

Reply to
Don Foreman

Ignoramus11916 wrote in news:StNTe.17227$ snipped-for-privacy@fe09.usenetserver.com:

Buy "The Art of Electronics"

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This is THE best book on electronics, ever.

Reply to
Dev Null

No. Mosfets, like a lot of other stuff, have a linear region on the graph. When switching power, you don't want to let the device dwell in the linear region, as that is where power is turned into heat. You want to throw them rapidly between off and on states. So, you never would use sine-wave drive in a case like this. You would get pulses with very rounded corners in the above circuit. Also, unless the wiring was done carefully, the transistors might oscillate in the megaHertz range during every on or off transition, which could bring the FCC after you with hooks.

Jon

Reply to
Jon Elson

Ok I will try to give some advice. The AC voltage on the gate has to be low enough to not exceed the spec.'s for the Mosfets. On the other hand the voltage has to be high enough to cut off the current in the Mosfet and to drive the Mosfet into saturation..

The spec.'s for the Mosfet has to be examined to get the requirements for the voltage.

The current through the Mosfet won't be a square wave but will be close

Bill K7NOM

Reply to
Bill Janssen

thanks, I will check it out... maybe I will get it in my local library first...

i
Reply to
Ignoramus965

Be sure to wear goggles.

Steve who has designed several switching power supplies, tested many 1-5kW units, and really appreciates the fact that he wore goggles.

Ignoramus965 wrote:

Reply to
Steve Smith

One of the things that makes switchers difficult is that you have to switch

*fast* to minimize how much the transistors heat. When the transistor is off, it doesn't dissipate any power. When it is fully on, it conducts a lot of current, but has very little voltage across it. For instance, say a transistor is switching 50V at 20A.

Off: 50V, no amps On hard: maybe 0.5V, 20A--10 Watts In between: 25V, 10A--250 Watts

I think it is obvious now why you want to switch the transistor just a fast as you possibly can. Sin wave drive just won't do the trick.

Designing a switching power supply is a wonderful way to blow up lots of transistors. You put a circuit together, fire it up, and the switch transistor fries. If you have a single-shot scope, or a nice digital scope that is fast enough, you check to see if the scope succeeded in saving the waveforms you were interested in. No? try it again. Yes? stare at the circuit for an hour or so trying to figure out exactly what happened. Fix what you figured out, try it again. Bummer, another transistor blew out. Why this time? This can take a

*very* long time, especially given your level of experience.

Steve

Reply to
Steve Smith

One of my favorite quotes was from an engineer that designed and built his own electic van controller "I didn't know that the tops of TO-3 transistors could fly off"

Reply to
Jim Stewart

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