Safe and practical way to discharge capacitors.

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eric_kv2

eric_kv2

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I need to make a mod on my amp, but in order to do that I need to make sure the circuit is fully de-energized and the capacitors are fully discharged. What would be a prefered method to do so. One way I thought that would be safe is to use an electrically insulated screw driver with an aligator clip attached from the screw driver to the case or ground. Also, posibly soldering a resistor to the screw driver to slowly discharge the capacitor when touching it with the scre driver. Are there any safer or more practical ways for doing this?

Thanks,
Eric
 
All you need to do is get a wire with insulated clips on each end. Clip one end to the cap and the oher to the chassis. You can actually put a voltmeter across the same connection and watch it discharge. :)
 
A good, safe was is to talk to Lord Valve, and I believe he's making a discharge probe with an alligator clip on one in, and then you just touch the probe to the high side of the capacitor... There's a resistor in the handle to control the speed of the discharge as to not damage the caps over time and to avoid sparking as much as possible. Now, it's been a while since I've seen one of his catalogs, but I believe he's got them going now.

A very straight method is one you eluded to... the BFS method, which stands for "Big Screwdriver, the F is usually silent"... :) I'm not gonna argue a lot over this one... I wouldn't suggest it, but you have to make the decision of what you're going to do with yourself and your gear.

Good luck.
 
As a person who has used the BFSD method, I do not recommend it. Unless you like impressing your friends by powering lightbulbs with your fingertips.
 
If your amp has chassis mounted preamp tube sockets you can use the insulated alligator clips and clip one end to v1 pin 1 and the other end to the chassis, this will drain the caps slow enough. If you've got pcb mounted tube sockets I would use the insulated clips with a big ass resistor in the middle and drain straight from the caps to the chassis. Always check with a multimeter to make sure too.
 
1 croco lead between chassis and 220k resistor (2 Watt)
1 croco lead between other side of resistor and first filter cap.

Exactly in this order. Check with DMM.
 
i've been searching around for a 220 k 2 watt.. but haven't had much luck finding one. my local "radio scrap" stores are worthless. i did find a ceramic 100 k 10 watt that a guy from a t-v repair shop gave me. he said i should be able to use that.. it just might take a bit longer to discharge.
it isn't so much discharging caps.. it's powering them back up after they're drained that i worry about. i found a web site that had instructions for how to build a poor man's variac using some simple circuitry and a light bulb. it's supposed to keep the caps from getting slammed with voltage when you re-charge them. wish i could remember the url.. but i'm pretty sure i just did a google search to find it.
 
http://www.mouser.com/

I personally use the BFSD method, but I've never gotten a big spark. I think my filter caps are dying (they're original since 83), so I plan on soldering a resistor across the filter caps so they'll automatically discharge.
 
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Code001":b9762 said:
http://www.mouser.com/

I personally use the BFSD method, but I've never gotten a big spark. I think my filter caps are dying (they're original since 83), so I plan on soldering a resistor across the filter caps so they'll automatically discharge.
Click to expand...

You're lucky, then. I've given myself a good jolt that way on some filter caps. Sure knocked sense into me :D
 
My screwdriver is insulated and I put my other hand behind my back, making sure no body parts are touching the amp at all.
 
Well for laughs, When I'ze a kid, I discharged a huge QUART sized capacitor once.. just touched both posts with my fingers. It works AND you get a fabulously hard shock out of it! :D This one didnt have a killing charge but it hurt! ;)
 
I am not trying to single anyone person out and I am only mentioning the following in the hopes that it will be of help;

If your amp does not have bleed resistors (also called load resistors) then the most logical way would be to use two insulted alligator clips with a resistor in the middle (IE: 220K, 100K, etc). to control the discharge time.

A word of caution: If you are going to discharge a filter cap do not simply connect the positive side of the cap directly to ground and can easily cause internal damage to the cap. I know there is info on the net that states you can do this and with only half the information they correct in that it will discharge the cap but the other portion to this method is that this method will serve to cause internal damage to the cap.


Just trying to help - have a great day guys
Trace
 
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