The original "schematic" was more of a block diagram showing a FET in a diode bridge going to a filter block as a GIF file. ( I can still see it here )

There was no detail other than that. That is why I posted about the effects of different filter types on the FET and why people were comparing it to a buck regulator.

 

Are you talking about this circuit?


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I presume the filter would be made of inductors and capacitors and yes, I think thre might need to be a snubber network to protect the MOSFET.

I just don't see how it compares with the DC buck switching regulator shown below.

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A snubber will not protect the FET. The energy stored in the inductor will overload any snubber you put there and blow the FET.

if you supply an 1A at 75V when the rail is 150V you will be drawing 1A at a 50% duty cycle. Normally the energy stored in the inductor is supplied to the load through the diode when the FET is off.

If you want to rectify-buck-unrectify you can do that, you can not just chop and filter.

The similarity is that you are trying to suggest an AC buck regulator.

 

Ok sorry guys sorry my firefox has a whacked out extension. This lappy needs a re-partition and fresh install.
-BaC
PS Was logged in, just kept refreshing that link in a new tab for some reason.

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Error[888] "while trying to load":[reality.sh] kernel: [Panic!]...{Universe has been Modified!}...

 

any idea to make Variac for 20VAC supply?

 

Well I don't have any specific circuit in mind but it would seem to me that it (an active variac) would be pretty straight forward. There are class D audio amplifiers designs available today and that is a form of PWM and of course audio amps can cover 60/50hz.

So it would seem to me just a matter of a putting together a precision low level audio oscillator followed by a variable gain stage followed by an audio class D power amp stage. Make sense?

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Measurement changes behavior

 

Very inefficient, overly complicated and power limited.

This thread had scrolled out of sight...

 

Alright it's probably less efficient than a variac but class D amplifiers are used in power converters which change the frequency.

Suppose the power supply is 400V 50Hz three phase and you want 115V 400Hz three phase supply to test some aircraft electronics.

A transformer and rectifier or SMPs converts the 400V 50Hz to a DC voltage just above the peak voltage required at the output and a class D inverter converts it to 115V 400Hz.

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I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

True, I have done that as well, but that is not what we were talking about. We were talking about simply changing the voltage of the existing powerline frequency with out a transformer.

Dan