[AMPS] HV PSU Question

[David Kirkby]

paul wrote:

> Having just re-built my own 3.5KVA power supply which was originally
> built around a surplus transformer that burnt out (it was intended for a
> choke input filter) I had to perform major surgery to the enclosure and
> electronics to get it all to fit. I don't recommend this repeat process
> to anyone, furthermore a rewinder who I knew quite well would not rewind
> the transformer since he reckoned it was too dangerous using old
> laminations and formers at these sort of voltages.

I had a similar problem once. We needed a large HT transformer for a
pair of 4CX1000As, and I managed to find one of 2.3 kV, 4.5 A in a skip.
A company near by where I worked had gone bust and all their equipment
was thrown out. With some friends, I managed to get the transformer out
of the skip. 

The transformer was tested off load and gave a reasonable voltage. After
welding the enclosure to fit the transformer, and fitting all the bits
inside, the PSU was switched on. The off load DC voltage was as
expected, but after taking only 10 mA, the DC output had fallen to close
to zero !! The transformer seemed to have a huge self impedance. In
fact, the transformer did not mind a short circuit on its output  - it
did not smoke as one might expect. 

I never did know why this was, but when we tried to get a replacement
made, it became apparent that the old transformer was smaller than it
was possible to get a new one to fit. Hence the whole supply had to
junked and a new box welded up. 

Why a transformer marked at 4.5 A should be able to supply only a few mA
before the voltage fell to zero, I never did find out. However, it
taught me to perform 'short circuit test' on a transformer, where the
output current is measured into a short circuit, with a very low input
voltage. This will not destroy a transformer, but gives one a reasonable
confidence the transformer can supply a lot of current. Had I have tried
this on that transformer, it would have became obvious it was unable to
supply any appreciable current.


-- 
Dr. David Kirkby Ph.D,
email: drkirkby@ntlwold.com (formally davek@medphys.ucl.ac.uk)
web page: http://www.david-kirkby.co.uk       
Amateur radio callsign: G8WRB

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