variable amplitude ac to ac converter

Depending on how good the output sine wave needs to be, you may not be able to do any better than a Variac.

What is the load? How sensitive is it to waveform?

maybe you dont need to go ac-dc-ac, maybe you could just put a bidirectional switcher, ie 2 mosfets back to back or inside a bridge rectifier and feed them with a fixed duty cycle.

this would give you a fairly close fraction of the AC input.

I havnt given this much thought yet, coffee not working at all today, you would also need a second bidirectional switch driven in antiphase to clamp the voltage, after the first fet before putting it into an LC filter. this would be like a flywheel diode wich ofc you cant use a diode becuase its AC, so its like a synchronous diode.

usual bootstrapped mosfet drivers might be a challenge, or you go with opto coupled with seperate power supplies.

Colin =^.^=

"colin" wrote in message news:_%Rdi.1325$ snipped-for-privacy@newsfe4-win.ntli.net...

I have done some experimenting and simulation for this type of application. I used two IGBTs or MOSFETs in series, with the emitters or sources connected together to a common rail. The same PWM signal can drive both at the same time, but it might be better to alternate the drives at the zero crossings. I found that a parallel LC circuit at multiples of the PWM frequency helped reduce the chopped waveform to be fairly smooth.

It is also necessary to provide a current path during the off period of the PWM. This complicates matters, but I have simulated it by using an optoisolator, a bridge rectifier, and a MOSFET. The LTSpice ASC file is at the end of this post. I was simulating a high current AC source, so I have a transformer with a high ratio and a low impedance output.

There's probably a better way, but this proves the concept.

Good luck,

Paul

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yes the second switch I mentioned provides the current during the off period, if it uses two mosfets back to bak this can be driven with a squarewave at zero crossing thus providing diode action in the right direction on each half cycle. although it still requires a level shifted driver.

Colin =^.^=

When I think *signal*, I don't think 10A. 8-)

A motor, lamps or what ?

....... Phil

DC

Unless you have considerable experience and plenty of money, designing your own ac-ac variable supply is not the way to go, especially if this is a one-off.

In my opinion you will save yourself a lot of time and money if you purchase off-the-shelf.

eg. the Behlman BL1350pf would suit your requirements

Agreed mostly. Though that unit is very nice it is more like laboratory equipment than hobby gear. There isn't much available for the hobby market unless i an very mistaken, there isn't all that much for the laboratory market either.