[AMPS] HV PSU Question

[David Kirkby]

Phil wrote:

> I'd like to feed the anode of the GS35b with 4 to 4.5Kv DC. Many others have
> had great success with this level of voltage on the anode so I've chosen
> this as my goal.
> 
> I've managed to get my hands on an HV Transformer which features a
> 3950-0-3950 secondary, and a 10-0-180-200-220-240 primary. The only ID on
> the unit is "M.E.P. M1921 SR/T 2468".
> 
> The question is: How do I tame the beast so as to provide the required level
> of DC from a 240v AC supply?

You need to use a full wave rectification. Take the centre to earth and
put each end of the transformer on the anodes of two high-voltage
diodes. Join the cathodes together, to make the +Ve output. That will
give you a peak of 1.414x3950=5585. 

The exact on load voltage is difficult to calculate, as you need to know
the equivalent circuit of the transformer. However, it will be around
1.2 times the RMS voltage of the transformer, or 4740 V, which is a bit
higher than you want. However, if you feed the primary at the 10 and 240
V taps, you will be feeding the UK mains voltage (230 in theory) to a
250 V primary. That should reduce the voltage by 230/250, or make it
about 4360 V on load. In practice, it might be a bit more, as the mains
in the UK is normally closer to 240 than the 230 is is supposed to be.
So you will be very close to the 4500 maximum you set, perhaps a little
over it. 

A variac could reduce it further, but it will need to be very big,
expensive and wasteful of a much more useful piece of equipment.

A lighter/cheaper option is to use feed a low voltage (say 15 V)
transformer with the 230 V mains, and use the output of the low voltage
transformer in series opposition with the mains, to feed the HT
transformer. If you get the phasing of the mains and the 15 V
transformer right, you can produce a voltage of 230-15=215 V. (You can
also produce 230+15=245 if you get it wrong). That could then be fed
into the HT transformer. If you do this, the low voltage transformer
needs to have a secondary current rating equal to the current the HT
transformer takes. A 10 A transformer should be fine, which is quite a
modest transformer - unlike a 10 A variac. (If you want to see a circuit
for doing this, I could sketch one and email it to you. It is not a very
obvious way around the problem, but I believe could be the best option
in this case). 


-- 
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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