If you multiply the capacitance by the resistance, you get what is
known as the RC time constant. The capacitance is normally in ufd or
farads times 10^-6. So if you multiply by resistance in Megohms ( ohm
times 10^6 ) you get the time constant is seconds. In one time
constant, the voltage drops by 63%. So the rule of thumb is to wait
about three times constants so the voltage is only 5% of what it was.

For 100 mfd, try 10,000 ohms. (100 * 10^ -6 ) times ( 10,000 /
1,000,000 ) or
( 100 * 10^6 ) times (1/100 * 10^6 ) or one second. So waiting three
seconds for that size discharge resistor would be prudent.

With a 10 K resistor and 240 volts, the current thru the resistor would
be 24 milliamps. And the power in the resistor would be 5.7 watts.

So maybe a two watt 100 K resistor would be a better choice. It would
mean you ought to wait at least 30 seconds before assuming the
capacitors are safe.

Dan


Ignoramus677 wrote:


It was interesting to see how disconnecting the cap produced sparks.

What was even more interesting is that, after unplugging, when I
shorted my 3 parallel big behemoth vodka bottle sized capacitore, to
remove the charge, there was a huge spark and a sound like a .22 shot.

I think that I will add a discharge resistor, too. Any idea how many
ohms for every 100 mF is best?