"TimPerry" wrote in message
...
Arfa Daily wrote:
"M.Joshi" wrote in message
...

Hello,

I was wondering if there is a method of calculating the value and
wattage for the resistor that goes across a mains capacitor to
discharge it once the power has been removed?


--
M.Joshi

Any cap that is placed across the mains in a piece of equipment,
should not in theory require any resistor to discharge it, since the
load presented by the equipment itself should do that. The value of
any such caps is usually very small anyway, so any residual charge
from the last half cycle of the mains that appeared across it, will
be more or less *instantaneously* discharged by the load.

Arfa

Arfa, some of the equipment i work on used high voltage supplies to power
tubs power amplifiers. large bleeder resistors are place across the HV
filter capacitor in the event of a tube failure (or some other type of
failure) that would leave a HV cap charged up.

these are often 100 k ohm 200 watt wirewound placed in series/parallel.
the
ides is to discharge 7,300 volts in about two or three seconds down to a
"safe" level (at least safe enough to open the doors without a giant arc
when the safety shunts engage.

other units switch the bleeders in when the doors open.

M.Joshi, the resistance is set by taking the voltage and capacitance into
consideration and deciding how fast you want to discharge the cap. then
wattage is determined by E max squared / R then multiply by whatever
safety
factor you feel comfortable with and round off to the nearest standard
value.

here's a place to read about time constants.
http://www.kpsec.freeuk.com/capacit.htm



Hi Tim

Agreed. Everything you say is absolutely valid, but I didn't think that was
what the OP actually asked. He specified " mains capacitor " which I took to
mean a capacitor across the mains, ( ie line power in the US ) which many
items of equipment have for transient suppression purposes, or as part of a
filter network to prevent crud from the equipment going back up the mains.

If he actually meant a resevoir or smoothing cap, or some other HT
decoupling cap on the back side of a rectifier, then that is altogether a
different matter. The only thing I would add to your explanation of
determining values for such a discharge resistor, is that the value of the
voltage that will be across the cap / resistor combination, is critically
important in this case. Everybody knows that resistors have ohmic values,
and power ratings, but many do not know that they also have a voltage
working rating, which with many resistor types, is not very high at only 2
or 3 hundred volts. If the intended application exceeds the maximum working
voltage of the selected resistor type, then the value should be halved, and
two of the same value placed in series, across the cap to be discharged.
This will double the effective working voltage.

I have also seen 4 resistors used ; two 2-resistor chains, in parallel
across the cap to be discharged. Presumably, this gets a single resistor, of
twice the voltage rating, and twice the power rating, with a built in safety
factor that if any one resistor goes open, there is still a discharge path,
allbeit a slower one, across the cap.

Arfa