Re: [CAD_CAM_EDM_DRO] Re: Capacitor hunting

I found a bunch of old 12,800, 75V working/100V surge capacitors at an
industrial surplus sale for peanuts. I had to reform the dielectric by
applying the DC voltage through a high-value resistor, and letting them
"soak" for a few hours to see which ones would recover. All but one were
restored to operation. I'm using a pair on my 72 volt, 15 amp supply. Don't
go way overboard on capacitance as it will appear as a dead short to the
rectifiers at power-on and pop your bridge if you're not careful.

As far as ratings are concerned, you can choose filter capacitors with a
working voltage about 5 or 10% above your maximum voltage. The surge-voltage
rating, which is typically 30% higher, will save your cookies for short term
voltage rises (electronics not withstanding). Don't, for example, choose a
capacitor that's double your max voltage. It's not good for the capacitor as
it tends to deteriorate the dielectric more rapidly, just like too much
voltage.

Another tip about ratings. Choose a bridge rectifier with a PIV (peak
inverse voltage) rating greater than 1.414 times your transformer secondary
for bridge configurations (2 wire), and 2.828 times your transformer
secondary for full-wave center-tap configurations (3-wire). A minimum bridge
PIV rating of 2 times your secondary RMS works. Current capacity should be
at least 2 to 3 times you transformer secondary rating (just to keep it
simple). A 100 PIV, 35 A Motorola bridge cost me about $5.50 at a local
electronics store. The expected maximum voltage for a bridge power supply
will be (1.414 times the transformer secondary RMS voltage) minus about 1.5
volts bridge drop.

Regards,

Paul R. Hvidston, N6MGN
ACKSYS Engineering
Upland, CA