[AMPS] Choke input PS question

Peter Chadwick Peter.Chadwick@zarlink.com
Mon, 22 Oct 2001 14:30:33 +0100

I've used negative lead filtering and tuned choke input very successfully. By
using the RIGHT value of capacitor, you can get quite good regulation - in
conjunction with a suitable bleeder, of course. The capacitors need to be fairly
hefty types capable of carrying a fair bit of current - I use some WW2 paper
caps, about 4 inches wide by 2 inches deep by 6 inches high, rated 0.1uF at 5kV
DC, with big ceramic standoff terminals on them.

The correct values were determined empirically, using a variac on the primary,
and a number of 100 watt wirewound resistors as a varying load. Switch OFF
before changing load, of course. You could use a string of series light bulbs,
or couple of high dissipation tubes as varying loads. 

One thing you do not want is to get the choke resonant. The idea of the
capacitor is to get the choke on the HF side of resonance, but where it looks
like a very much bigger inductance than it is. That's at the minimum current
drain: as the current goes up and the choke inductance goes down, it tunes
further HF, but the critical value of inductance that's required goes down, so
it doesn't matter. The other thing I do is to use a large (50 microfarad)
capacitor on the output to give good dynamic regulation. (actually a string of
computer grade electrolytics). 

The equalising resistors are well under run, and placed ABOVE the capacitors,
which are at the very bottom of the cabinet: the big bleeder resistance and the
glitch resistance (50 ohm w/w, 100 watts, 1.5 inch diameter, 12 inches long)
and diodes are above them, so the caps are as cool as practical. In practice,
they feel to be at about ambient temp even after a long period.

Note this is not a small power supply.................18 inches wide, 20 inches
deep in the bottom of the 5 foot high 19 inch rack that has the amplifier in it.
I've used equalising resistors on the diodes to, despite what people say. Next
one will use K2AW blocks....

Bear in mind Tom Rauch's tale of  tuned choke filters and learn from
that........I'd suggest it's an area where if you're not extremely sure of what
you're doing, DON'T!


Peter G3RZP

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