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Convert 12 to 270 VDC for Vacuum Tubes


Dimirtij Klingbeil

Jan 1, 1970
Hi there,

This question may sound a little bit silly, but I recently ran into trouble
converting 12VDC to approx. 270VDC and hope someone can help me out.

Basically, the only thing needed is to run the 12 volts through an
alternator circuit and a step-up transformer and then rectify the result,
but that is what I could not get working. In order to avoid noise (and a
big trafo) I figured, that the frequency should be somewhere above 20kHz
and the trafo shoud be rated for high frequencies. For this reason I
decided to use an ond AT PSU power transformer. As its primary is usually
powered by ca. 150V, using the 5V secondary instead of the 12V one and
setting a slightly higher frequency to avoid overheating seemed to be a
reasonable approach (provided the primary's isolation can handle 300V). The
secondary is symmetric: left one -- center/ground -- right one, so I
thought, using 2 power transistors in reverse-phase would make sense.

Requirements: At most 200mA at slightly less than 200VRMS on the primary*.

Now my questions:

1) Can this sort of transformer be operated at a nearly constant frequency?

2) If yes, at which one, else within which range?

3) Does the high remanence imply that a special signal waveform must be used
or would it be reasonable to run it off a simple square-waved signal?
(I neither want to fry the transistors nor the transformer :)

4) The output will be rectified straight into a big (ca. 200 microF) elco,
so the current at startup is expected to be way higher than during usual
operation. What special precautions apart from limiting the transistors'
maximal current have to be taken?

5) The whole thing is likely to emit a lot of radio static. Do you think it
is enough to put filters on all connections or does it need to be placed in
a grounded metal case, especially if it runs off square-wave?

6) If a transformer heats up, but the core is heating far more than the
coil, is this only a result of a too-high frequency (assuming the ohmic
resistance of the coil is VERY low for the given current)?

Thanks for any help in advance.

*The terms 'primary' and 'secondary' refer to the 'normal' connection mode
of the transformer: 150V primary and 5+12V secondaries.