Just and idea but is it possible to make an electronic variac?

Obviously, the PWM frequency will need to be much higher than 50Hz.

Some snubbing before the filter will also probably be required.

Attached Images

PWM Variac.GIF (3.7 KB, 109 views)

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Try this;

QHB Extra - #9 - Nov 2002

-BaC

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A light dimmer is not the same as a high frequency PWM circuit.

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That's true, phase control delays the firing of the triac which gives a totally different waveform than PWM.

The PWM circuit will chop the sine wave up in to many tiny pieces and the filter will smooth them all out to form a pure sine wave on the output.

There will be some voltage loss in the filter and a variac often steps up the voltage as well so a small autotransformer can be added to the input to step up the votage to say 270V (assuming we want 0 to 260V operation from 230V).

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Very true, eremm I didn't read that post, my fault. Should be sure what I am posting before I do such. My bad, I should have known as it's an old project.

-BaC

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Hero999,

What you showed is a buck switcher. The output will be less than the input.

If you are looking for information on a similar think, look at PFC switching power supplies. It is a boost up type of supply. The error amplifier is very slow because you do not want to regulate part of the sign wave. (10hz response time)

 

I've seen buck switching DC supplies, they're very common, but I didn't know that they exist for AC too.

A PFC stage on is normally added after the bridge rectifier on a mains switching power supply. It boosts the input to a steady DC value so current is always drawn all the way through the cycle. I don't see how they compare with this circuit.

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The reason I brought up PFC is that; Most boost up switchers have a fast error amplifier. So fast that they will try to filter out 60hz noise. A PFC’s has a very slow error amplifier. It takes about 6 cycles of the 60hz for the amplifier to respond.

In your case you want to buck down 240 volts to lets say 120 volts (RMS). If you have a fast error amplifier, anything above 120 volts will be bucked down to 120. You want to run the FET at 50% duty cycle. The error amplifier will look at the RMS of the output voltage and adjust the duty cycle to maintain 120 volts AC.

If the input voltage moves the output voltage will move. In about .1 second the error amplifier will move the duty cycle around to cause the output voltage to remain constant.

 

Very interesting Ron / Hero cheers

-BaC

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I see what you're saying, as the duty cycle is varied the input voltage is divided by the same factor. However a buck is totally different, it only works on DC and a freewheell diode keeps the current flowing from the inductor when ther switch is turned off.

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I learn something everyday here, at least once a day. Except when it was down (Pulling My hair out") I was going to ask them if they needed any help..lol This CCIE I have should be worth something...hehehe JK best cert I ever got, and hardest as well Just need to be to specialized to Cisco to continue your certificate yearly, I really rather remain more widely Scoped.

-BaC

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it won't work without 2 more FETs as the flying diodes since you need to conduct the inductor current when the switch FET is off in the proper direction.

You would be better off rectifying, bucking, and then using a power bridge to make it AC again.

 

Yeah, I know, the above circuit is for a typical DC buck converter.

I don't see how it's similar to my PWM variac circuit; I don't see how it can be called a buck converter.

A buck converter uses a freewheel diode or another MOSFET to keep the current flowing thorugh an inductor.

My circuit, just PWMs the sinewave and puts it through a filter.

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Filter it how? A capacitor would instantaneously charge to the line voltage, and an inductor would, in it's attempt to keep the current flowing when the FET turned off, blow the FET.

 

Well I can no longer see the original schematic as there is a problem with the file.
Only hero can answer this, but I now am curious. Where is that schemo!..lol

-BaC

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