I believe in using a fairly heavy bleeder resistor on hv supplies. Why not
take full advantage of the bleeder and use it to equalise the caps? If you
have a large wirewound resistor with provisions for a slide tap, all you
need is several of the sliding connectors. Just space them at the proper
distance apart to equally divide the voltage, one for each capacitor.
Otherwise, divide the bleeder resistance by the number of caps, and use that
value resistor across each cap. Of course, you also divide the wattage of
each resistor by the number of caps.
Smaller w/w resistors are probably easier to find in any case. It seems
that large 225-watt wirewound resistors, like most other heavy metal parts
have about disappeared from the hamfest flea markets, and are hard to find /
prohibitively expensive new.
I use choke input filters in my rigs, so I always use enough bleeder drain
to maintain critical inductance when there is no external load on the power
supply. Otherwise it tries to become a capacitor input filter under
no-load, and the voltage soars.
With capacitor input, you need a large amount of filtering and substantial
bleeder current to maintain any kind of decent regulation.
I hadn't thought of the idea of a low resistance in series with the string
of series electrolytics in case of a short-out, but it makes sense, that
with an instantaneous discharging current of several amperes, even a slight
unbalance in the caps could cause back-bias on one or more of the caps,
which would like damage the cap the high discharge current passed through
the capacitor backwards.
To protect the caps from overheating, locate the bleeder/equalising
resistors well away from the capacitor bank.
I still use oil caps, possible pcb hazard notwithstanding, whenever
available.
Don K4KYV
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