[AMPS] auto transformers

[Dave Kirkby]

Peter Chadwick wrote:

> I wouldn't go so far as to 'never use' an autotransformer. If you do,
> it needs to be suitably rated that means BIG and HEAVY. The existing
> ....

> Has anyone any experience of 3000 volt 1A switching type PSU's?While I
can see that increasing the voltage into an amp (or any equipment for
that matter), needs *very* careful thought (ie its not something I can
see myself doing), it can be done without a BIG and HEAVY transformer.
Take a low voltage (say 10V at 20  A =200 VA= quite small) transformer
and power that directly from the mains. Take the secondry output of this
(10 V), and put it in series with the mains, making mains+10V. Feed,
mains+10V into the amp. As long as your amp draws less than 20A  (the
secondry rating of your transformer), the small transformer will be
okay. If you get the phasing of this the wrong way around, you can get
mains-10V, so measure to make sure you are not reducing the voltage!

This is basicaly all an autotransformer does, but low voltage
transformers are more common than autotransformers.  Unlike running from
a step-up transformer which provides all the amps power, the transformer
only supplies a small part of it, which means the transformer can be
much smaller.

I've not bothered going back to the original post and seeing the size of
the amp, and how much extra voltage/current is needed, so the 10V 20 A
were plucked from the sky, but you should get the idea. 200 VA cant add
more than about 100 W to the output power of the amp, so perhaps this is
a bit small (depending on how much extra power you expect to achieve).

As for 3kV 1A switch mode supplies, there was the QEX article on one
giving 2.2 kV at 1.5 A. This was reported here recently, but if anyone
missed it, I can dig out the referece.

--
Dave Kirkby G8WRB
Department of Medical Physics,
University College London,
11-20 Capper Street,
London,
WC1E 6JA.

Tel: 0171 209 6406 (International +44 171 209 6406)
Fax: 0171 209 6269 (International +44 171 209 6269)



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