Discharging Cap Question

[skid20]

Hi all this is my first post here.... so I have heard that discharging caps should be done with a resistor of the proper value for the cap at hand.. I'm trying to find out if there is a problem with using a resistor with a much greater value than needed so it can be used to drain caps of many different values... I would like to make a insulated tool with a large resistor so I dont have to change values constantly.

thanks for any help, I'm just starting my first vintage tube amp project so I'm sure I'll have may noob questions to run by some of you more experienced individuals

[Conrad Hoffman]

There's no "correct" value. I just keep something of a few dozen ohms around for the purpose. If you're dealing with really huge caps, it should be a 2W or 5W wire wound. 10 ohms is fine, as is 100 ohms, for LV circuits. If the caps are huge, I'd stay with 100 ohms or more, and if the voltage is high, 1k might be more reasonable. The object is just to keep the current value low enough that the cap internals aren't damaged. Wire attachment to foil is less robust than one might think or desire.

CH

[beaner]

I never kept resistors around but did the light bulb trick. Mind you it's a 5 farad cap. Still works fine after many years of service.

[skid20]

Thanks for the responses, That should help quite a bit...

The largest cap I have in the amp I will be working on is the can cap 40-20-20-10uf @ 475v... are there any drawbacks to using a light bulb?

[Conrad Hoffman]

A light bulb is fine unless it blows out and you don't realize it. Then you still have energized caps waiting to bite you. Use a robust bulb, maybe rated for 220?

CH

[skid20]

I'll give it a shot... I'll probably have a meter on it as well so I can see that the charge is draining. Thanks for the help!

[Enzo]

You want to drain the cap slow enough not to vaporize the internal foil connection, as COnrad mentions, but you also don;t want it to take a week.

I assume a tube amp. One old trick is to clip a tes lead from ground to the pate of a preamp tube. That will discharge the caps on the B+ line through the (typically) 100k plate load resistor of the tube. Not real fast, but as convenient as the nearest clip lead.

Power tubes have screen resistors? Clip a grounded wire to a screen pin on a power tube socket. That will be a lot faster than that 100k.

I have a 10w 1000 ohm resistor sitting around that I grab for discharging. it doesn't need to be 10 watts, but I find that size real easy to hold without touching the leads, and convenient to clip test leads to.

[skid20]

Originally Posted by Enzo

One old trick is to clip a tes lead from ground to the pate of a preamp tube.

I think I heard of doing it like that before, thats not a bad idea either I'll give that a try as well sometime, thanks Enzo