The only proviso I'd add to Phils' comments are that the current
requirements of the transformer are altered going from a bi phase to a
full wave rectifier, additionally when using surplus transformers often
you never know if it was built for a choke or capacitor filter.
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.
New techniques and process's produce a smaller transformer which is more
reliable with better insulation, less subject to moisture and low
insulation breakdown whether it's a toroid or EI laminations.
Paul
In message <3A510FCB.B1C8FAE@ntlworld.com>, David Kirkby
<drkirkby@ntlworld.com> writes
>
>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).
>
>
--
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