"THERES RAJ, BLR" wrote in message
oups.com...
On Mar 6, 6:37 am, wrote:
Thanks for looking.

How does one discharge a capacitor when working on a PCB, so that one
does not get electrocuted? Are there any other components that
maintain a charge after turning off power source. Thank you. please
answer to .

the safe way to discharge capacitor are through resistor, use the
resistor in minimum 10 watts with resistor 10megaohms for AC capaitor
from 110v to 440v

for DC from 5 to 220v capacitor us 5 watts and 150kilo-ohms above 220v
DC use 10mega ohms resistor value,
use two long lead with insulation and one end ground to chase or PCB
ground, and other lead to discharge the capacitor.
take more care when discharging more than 110v capacitor which will
produced high sparking when discharging.


I wouldn't argue with using a resistor to discharge caps, and also, on
occasion, just whacking a screwdriver across them - although this can leave
a bit of a metal deposit 'splat' on the PCB. However, the suggested 10 meg
10 watt resistor, or even the 150k, seems rather high to me. I think that
you might struggle to even find 10meg at 10 watts.

I have a metal film resistor of 2k at probably 3 or 4 watts, that has lived
for years on the bench screw-magnet, and serves to discharge any caps that I
ever come across that remain charged due to fault conditions. That includes
up to 500 volts in some PA amps. This resistor has never even got warm
during a discharge. If you work out the math, yes, at first the resistor may
be massively overloaded from a high voltage cap, but it is for such a short
period of time that the resistor never has a chance to get hot, which is
what its power rating is all about. A higher value resistor than this (
though not as high as 150k even ) will do a gentler job of it, with less of
an initial spark, but will take correspondingly longer to do it.

Note also that many high voltage cap circuits have bleeder resistors, or
voltage sharing resistors across them when they are stacked, and these
should discharge the caps for you. You should always check that they are
doing their job though, as they do have a tendency to go open. You can also
discharge caps safely with an older analogue multimeter, which has a much
lower input resistance than its modern digital counterpart. This has the
added advantage that you can see the voltage dropping, but is somewhat
slower than using a resistor of just a few k.

In general, the following circuitry should discharge the caps for you, and
caps remaining charged will usually indicate a fault condition such as a
switch mode power supply not starting up, and leaving the main primary
filter cap charged to peak line volts, or a filter resistor between two caps
open circuit, leaving the first one in line, charged.

Arfa