Re: [CAD_CAM_EDM_DRO] resistor for power supply

Peter:

This is "theoretically" necessary when the CAP is very, very large, and the
efficiency of the rectifier is 99% (no such thing!), and the transformer has
ZERO internal resistance. Then, you can get "nearly infinite" current-surge
for ONE cycle, and some who realize they often forget to wear their diapers
fear this will BLOW things. Prolly DID, back when Cu-O rectifers were used,
etc.

Calculation? That R will reduce the power-supply efficiency, make heat,
introduce inherent unreliability, etc., etc., and the only way to "calculate"
how large to make it is to TRY a dozen or more low-R values (fraction of an
ohm?) until "something blows" Then, use the next-higher size (resistance)
that "worked" OK. You can't really "calculate" this R, as it exists only as
a "shock-absorber" for nasties not really very accurately calculable. For
example, if a C has "68,000 uF" stamped on it, it may-well have a practical
actual capacitance of 100,000 uF. Even more! Or, as low as, say, 0.8*68,000
uF.

Do NOT confuse this "R" with a "bleed resistor"! The latter should be in the
high-K range, depending on the physical room for whatever R-wattage required
for the supply-voltage involved, and how fast you would want to "bleed" the C
down to, say, 65 VDC (a "safe" level).

Lotsa luck! Jan Rowland