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Vintage threads from the first ten years

Power Supply Resistors

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12/28/1999 1:42 AM

Daver

Power Supply Resistors Amps that have series filter caps have paralleled resistors to equalize the voltage across them. These resistors also act as bleeders when the power is off. My question is, if you are using only one filter cap in the first stage (lower B+), can you or should you use a paralleled resistor with it to act as a bleeder? If so, what value and power rating would you recommend? Thanks!   Daver

 

12/28/1999 5:54 AM

Bruce

It never hurts to have some positive way to bleed a filter cap off and it also helps (albeit minor) with the regulation.   Think of Ohm's Law here,   I don't know the voltage of the circuit you are using so here is another way to look at this.   If you want to limit the current across a 1 watt bleeder resistor to say, .5ma to 1ma then you can use this:   R = P/I^2     R = 1w/.001a x.001a   R = 1w/.000001a   R= 1M     R = 1/.0005^2... R = 5M     I don't know how long it would take a 1M to 5M resistor to bleed off, say 400vdc out of a 22uF cap, but I'd guess too way long. Ha ha... the smaller the resistance value, the shorter time to drain the supply off.   So, I'd just use a 1W to 2W 360K to 470K metal oxide resistor and be done with it.     More Ohm's Law...     400v/470K = .000851a   .000851a x 400v = .34 watts     400v/360K = .0011a   .0011a x 400v = .44 watts     A 1 watter will work OK but 2 watter will probably last a lifetime.     Bruce

 

12/29/1999 12:03 AM
Daver	Outstanding, Bruce! That's just what I was looking for. Thank you.   Daver

 

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