[AMRadio] Series capacitor equalizing resistors

[Donald Chester]

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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