variable voltage

Roff said:

A square wave at 12V peak-to-peak will only give you 6V RMS.
What kind of transformer do you have?
Why do you need 220V at low current?

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Sorry it is not 12 volt peak - to peak....it is 12 volts rms
step up transformer
i need high volt and low current in a next circuit needs high volt and low current
the problem is how to convert the variable dc (from 3 to 12 volt dc) to variable ac (from 3 to 12 ac)

aymanok said:

Sorry it is not 12 volt peak - to peak....it is 12 volts rms
step up transformer
i need high volt and low current in a next circuit needs high volt and low current
the problem is how to convert the variable dc (from 3 to 12 volt dc) to variable ac (from 3 to 12 ac)

Click to expand...

What I was trying to say is, if you simply take your 12V DC, switch between it and ground at a 50% duty cycle so that you get a 12V p-p square wave, you will only get 6V RMS.
If you want 220VRMS out of a transformer that only has 6V RMS input, you will obviously need a different transformer.
If you want to use your present transformer, you will need to double the input square wave to 24V p-p, using an inverter or a 24V DC supply that you can switch at a 50% duty cycle, giving you 12V RMS.

Oh....No solution?

aymanok said:

Oh....No solution?

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I thought I offered some in post #22.
I would use an inverter with adjustable output voltage, that creates 3-12V RMS AC from 12V DC. Of course, you would probably have to design one, because it doesn't sound to me like something you could buy off-the-shelf.