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Designing a transformer - RF Cafe Forums

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Post subject: Designing a transformer Posted: Thu Mar 13, 2008 8:38 pm


Joined: Thu Mar 13, 2008 8:22 pm
Posts: 1

I wish to build an oscillator of the multivibrator type, for induction
heating experiments. The transformer will be 1:1 with center-tap
on the primary. Frequency between 1khz to 50khz. Needs to be
small as possible to fit on the PC board. Now here's the rub;
needs to handle 115V-230V @ 1-10Amps, but should run off as
little as 12V.

Torroids are fine, have the ARRL Handbook 2008 and many online
L calculators but am still way over my head. No idea what core material,
size, wire size, number of turns, etc. Don't want to inductively heat
the transformer, just the workpiece! Suggestions welcome, and
thank you.



Post subject: multivibratorPosted: Fri Mar 14, 2008 12:41 pm


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
You sure have some stringent requirements.
Start with the simple ones:

Assuming your output is 115-230V@1-10 A:
230V at 10 Amps is 2300 watts, so you'll need a big core with thick wire.

At 12V input, this is about 192 Amps - so you'll need transistors rated for at least this much current. Likewise, at 230 V input, your transistor should be rated for at least 500V.

200 A and 500 V transistors are rare and expensive, and need special heat sinking and drive. You're into specialist territory, not hobbyist or experimenter territory, if I've interpreted your requirements correctly.

Good Luck!


Post subject: Posted: Mon Mar 17, 2008 6:36 am


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
As Paddy once said: If I was goin thar I wouldn't start from here.
I agree with Fred: 12V is the wrong starting point for a system consuming more than 1kW, and using transistors is also probably the wrong approach.
I would be thinking in terms of 56 volts at least, and using Triacs, not transistors. The triacs would be driven from a reliable DIL TTL or CMOS multivibrator 4047 or similar.
Why 12V?
Also you state the transformer is 1:1 ratio, but with a primary of 12V and a secondary of 115V or 230V, the ratio would be more like 10:1 or 20:1.
Are we missing something here?

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