Re: Cap. Discharge Res.

Arfa Daily wrote:
"M.Joshi" <M.Joshi.2a2vlj@xxxxxxxxxxxxx> wrote in message
news:M.Joshi.2a2vlj@xxxxxxxxxxxxxxxx

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


.

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