Discharge a capacitor ??

[joessportster]

What's the best way to safely discharge a capicitor. I am trying to check a transformer and the caps are holding 100 vdc. I know at work after one is discharged we short the leads to keep from a static recharge. I already had one lead touch the chassis and give me a nice pop. They have been setting about 10 hours and still holding same charge. Joe

[Arkytype]

Joe,

I was taught never to discharge a cap with a direct short. In addition to possible pyrotechnics, you will probably damage the capacitor by shorting the terminals directly.

Using the time constant formula (T=R X C), a 100 ohm resistor will discharge a 10,000 uFd capacitor in about 1 second. In actual practice, the capacitor won't completely discharge in that short time due to dielectric absorption. Just keep the resistor across the capacitor for a few seconds and then measure the voltage across the terminals.

Use a 5-10 watt resistor for safety as the discharge current can be high.

Lee

[fini]

Great question and answer!!

[joessportster]

Thanks guys. I also read on the net about shorting direct (did that for a milisecond on accident, pretty loud) I also read on the net that a lightbulb could be used. I took an old lamp and used the lightbulb method (as I have no resistors) worked like a charm. Thanks. Joe

[JJkizak]

Clip a ground lead on it while you are working to prevent it from recharging. Sometimes they will charge back up and nail you. That's why heavy electronic equipment comes with big *** ground rods on chains.

JJK