RE: [CAD_CAM_EDM_DRO] resistor for power supply

Jan, Peter:

Wouldn't one way to calculate the resistor value be to assume that, at
turn-on, the capacitor is a dead short and peak voltage is across the
transformer leads. You could calculate the resistor required to prevent the
diode from seeing its maximum one-cycle peak current. Once you've calculated
this value, subtract the transformer resistance (and wiring resistance, for
that matter) from it and you've got your resistor. You would probably find
that the actual resistance that you'd have to add is quite small.

-- Jerry

|-----Original Message-----
|From: JanRwl@... [mailto:JanRwl@...]
|Sent: Monday, December 10, 2001 6:27 PM
|To: CAD_CAM_EDM_DRO@yahoogroups.com
|Subject: 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
|
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