Draining caps

[Joecool85]

Ok, well, I was working on a stereo today and had to drain some caps, the biggest were 2200uF, I just shorted them with a screwdriver.  I know thats the ghetto way, so I want to know, how do you guys do it?  This is for solidstate btw, not tube.

[Herr Masel]

I think the proper way is similar, but with a wire that has the other lead clipped to ground with an alligator snap. Wouldn't it be pretty dangerous with just a screwdriver, as in sparks flying etc.?

[Joecool85]

Yeah, it sparks quite a bit, and leaves scorch marks on the PCB.  You are suppose to use a resistor.  The thing is, the only things I've seen recommend 25k resistors, but that is for tube amps that deal with like 400-500volts, not solidstate amps dealing with like...50volts or so.  So I've heard using as small as a 2.7ohm resistor.  I was thinking something like this: http://www.radioshack.com/product/index.jsp?productId=2062292&cp=&kw=resistor&parentPage=search

[bancika]

IIRC any resistor will work, just larger one will drain it faster and you could see sparks

[no one ever]

Yeah, it sparks quite a bit, and leaves scorch marks on the PCB.  You are suppose to use a resistor.  The thing is, the only things I've seen recommend 25k resistors, but that is for tube amps that deal with like 400-500volts, not solidstate amps dealing with like...50volts or so.  So I've heard using as small as a 2.7ohm resistor.  I was thinking something like this: http://www.radioshack.com/product/index.jsp?productId=2062292&cp=&kw=resistor&parentPage=search

kinda hard to find a 25k 5-10w resistor

[bancika]

not really, I found 100K 5W with no problem

[Mark Hammer]

A small appliance bulb (like the ones inside your refrigerator) will be content to have 120v pass through it, and will drain the caps nicely.  If there is a lot of current stored, your bulb will light up briefly, but pass the current to ground safely.  Happily, it doesn't matter what side of the filament connects to ground.

[Joecool85]

Thats a pretty good idea with a lightbulb...

[no one ever]

not really, I found 100K 5W with no problem

really? where

[gulliver]

Thats a pretty good idea with a lightbulb...

I wonder if a lightbulb went off in his head when he though of it? 

After wards he was quite drained 

[Joecool85]

Wow...that reminded me of my dad.

[bancika]

not really, I found 100K 5W with no problem

really? where

in local electronic supply store

[R.G.]

Some electrolytics do not survive the short-with-a-screwdriver draining. The stored charge changing so fast actually distorts the internal plates mechanically and can short them out. If they short during a drain, they weld, and good-bye cap.

A resistor across them, any resistor will drain them. How fast? Hmmm... let's see.  The time constant of any R-C is just R*C, in seconds. In one time constant, the cap will drop to 0.632 times (1-1/e) its original voltage. In two time constants, it drops to 0.632*0.632 = 0.399 of the original. At three time constants, you get 0.252... and t five time constants, 0.10008, which most people take to be discharged.

So if you have a 100K (for instance) and a 2200uF cap, the time constant is t= 1E+5*2.2e-3 = 2.2e+2, or 220 seconds, and a 100K will drain a 2200uF cap to 1/10 of its original voltage in 1100 seconds, or 18.3 minutes. Too slow. A 10K will drain it in 110 seconds, and a 1K will drain it in 11 seconds. The 1K sounds like the right thing here.

What current flows in a 1K? Depends on the voltage. I'm guessing if the amp has main caps of 2200uF, it's a 30W or less amp, and the voltage is perhaps 25V. So the 1K pulls 25ma initially.

Good practice in power supplies is to put a resistor across all power caps that pull the capacitor down in under a minute or so. So if you have a 2200uF cap and want to drain it in one minute, that is roughly equivalent to saying "I want a bleeder resistor that makes a time constant that's 1/5 of 60 seconds, or 12 seconds."

The resistor is then 12sec/2.2E-3F = (12/2.2)*1E+3 = 5454ohms. A 5.1K will do that nicely.

If we leave a 5.1K resistor across the cap all the time, what power does it dissipate? P = V^2/R or (25V)^2/5100 = 0.122W, or about 1/8 watt. A 1/4W resistor will work fine there.
So you could put a 5.1K resistor across a 2200uF cap all the time. It would eat 1/8W, and would discharge the cap to 2.5V in one minute.

[Joecool85]

Yeah, I guess the power supply on the chipamp I'm building does that so you don't have to worry about it.  Just shut it off, unplug and wait a couple minutes and its drained...I like that.  Thanks for all the help guys.  I *probably* won't drain any more caps by shorting them.  Although you do get one heck of a spark!

[Dave_B]

A small appliance bulb (like the ones inside your refrigerator) will be content to have 120v pass through it, and will drain the caps nicely. 

I had a similar tip when repairing a fuse-blowing Onkyo reciever.  Wiring a light bulb across the fuse socket allowed me to fix it without going through a box of fuses.  If the bulb lit up when I hit the power switch, I hadn't found the problem yet.

Now if I could figure out how to get around a speaker protection circuit, I could fix my old Yamaha...

[Xlrator]

I have light bulbs with wires soldered onto them. Drain caps on all kinds of stuff that way. It will also give you a visual when it is drained.