Dear All, I am using a motorized variac to generate 50 Hz 40-180 VAC signal from a 220V source (The load draws 10 A max). The motorized variac takes a lot of space and its response time is slow. I would like to replace it with a solid state circuitry. I guess It would be a AC - DC
- AC (variable amplitude) converter. Can anybody help me out? How can I built such a Converter? Any schematics? Thanks
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-)
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