discharging capacitors

[Narcosynthesis]

well, when playing about with the insides of disposable cameras (i want to get the flash part, but not the camera i came across the problems of the big ass capacitors they use inside, i have been shocked a few times (it makes you jump, but its not that bad) and would like to know how to discharge the capacitors

i remember in school discharging them by conneting the two legs together and that drained them, this was using small caps though, and when i tried it with one of the bigger ones in the camera i got a nice big spark... which probably isnt a good idea, so anyone know how to discharge them safely?

David

[jimmy]

well yes the big screwdriver shorting the legs on one way to do it. or you could get a big resistor maybe a 1K 5W one and put it between the legs. this may seem obvious but dont do it with your hands! clip one leg to the cap and one to a screwdriver. the bigger the resistor the smaller the spark, although the longer it takes to fully discharge.

as you may be able to tell i have also been victim to the flash caps many a time  

good luck
Jim

[petemoore]

Give the R 'extra' time to drain the cap, they have  'memory' and can recharge somewhat too...

[R.G.]

Not every time, obviously, but electrolytics are not in general designed for high pulse current duty - like shorting the terminals. Flash power caps, like in cameras, are better than ordinary electros, but it's not a good practice to ever crowbar-short a cap.

Shorting the terminals of a high energy capacitor together causes sufficient mechanical stress from the suddenly huge currents that flow to make those arcs to damage the cap in some cases.  High power pulse capacitors are specially constructed mechanically to withstand the stresses of sudden discharge.

Use a resistor. Even a small one limits the current a lot.

[Narcosynthesis]

well i worked out they have some serious current in them when there were marks left on the legs of the cap where i touched it with the screwdriver... as is a little melted spot where the contact was...

David

[amonte]

I've got an Electar Tube 10 that I've done some mods to.  When I first started working on it, I had asked some folks how I should discharge it.  They told me to get some of the insulated aligator clip cables from RS, clip one end to the chasis and make contact with all of the the capacitor leads with the other end.  This seemed to work - I'm 90% sure that I checked the voltage with my multimeter before proceeding, and well, I didn't get shocked.

Is this the correct way of doing it?  Is it safe?  I've got a Peavey Delta Blues that I want to work on next, which is much higher powered than my Electar Tube 10.  Also, my multimeter is one of those small pocket portable models they sell at RS.  Is that ok to use for testing the voltage?  

I know I don't always check the voltage.  Sometimes I just repeat that process 4 or 5 times, to make sure I've gotten them all and start working on the amp.  I do make certain not to create a make any contact with the caps and the amp with both of my hands.

Sorry for all the questions - just want to make sure I'm doing this right.  Thanks.

[Fret Wire]

Close, but even better is to use around a 10watt 150ohm resistor spliced into the alligator lead (bare ends covered with heat shrink). Then, one end on the chassis, the other to the pos. lead on every cap. Let it sit a bit to totally drain them.

The screw driver method is old, and just doesn't cut it nowadays when the alternatives are just as easy and safer.

There's another old method for amps ( if the amp is still working). That is to play some chords, and unplug (leave the power switch on) the amp while still playing the chords. Depending on the amp, it will play for quite a few seconds unpluged while the caps discharge. Supposedly, the caps are now drained to a safe level. I've never measured the caps after doing this. I do this before I take an amp apart, and then discharge the caps the proper way in addition. I do use this method, however, before I put an amp away that's not going to be used for awhile.

[Peter Snowberg]

 :shock:  :shock:    :twisted:  :!: http://205.243.100.155/frames/shrinkergallery.html

[Fret Wire]

Wow! From now on, wear a lead cup when you discharge caps. Don't need any "coin" shrinkage!! :shock:

[niftydog]

ah, a man after my own heart. Tesla is my hero, and that site just blew me away. Did you check out the "arcs and sparks" section with the mpegs of sub station equipment going horribly wrong?!? Man, that made my day!

[amonte]

"Close, but even better is to use around a 10watt 150ohm resistor spliced into the alligator lead (bare ends covered with heat shrink). Then, one end on the chassis, the other to the pos. lead on every cap. Let it sit a bit to totally drain them. "

Wow - really?  Why the 10 watt rating?  Would I need a higher rating to discharge the peavey and I was just using some fairly thin aligator clips - would that be alright, or do I need something of higher gauge?

When you say "sit a bit" - how long are we talking?  I remember holding it there a few seconds.

Also, other than the caps, is there something else that needs to be discharged?  

I've noticed the "fade" effect that you mentioned  - I'll do this as well.

[Fret Wire]

The 10 watt rating is a safety thing. The leads should be about 18ga. You can take the alligator clips off your existing wire, and solder them on to the heavier wire (with their covers), with the resistor soldered in between (heatshrink insulated). This will work for any amp. A little more work, but a very safe method. Sounds to me like it won't be the last amp you work on. Now you'll have a good safety tool for amp work, not later after you've shocked yourself. You could use a 5 watt resistor, but think what you're skimping on: your safety. Doesn't matter what brand it is, all amps can kill you dead if you let em'.

10 to 15 seconds is good. Use a DMM to check, and you'll get a feeling for how long different rated & size caps take to discharge. You can shorten the discharging by the "playing the caps out" method.

[amonte]

I'm going to place an order at Mouser for the parts I need, so I'll add this to the list.

Is this the right item?

http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=428353

also, one question about my DMM - it's a radio shack mini dmm - it's this one:

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F008%5F002%5F000&product%5Fid=22%2D820

(I hope these come up as links)

The method I was using (if I am remembering this correctly) was to connect the negative end of the probe to an aligator cliped wire, with the other end going to the chasis (or maybe it was ground?).  I then probed with the positive end.  Is this the right method?  And if there's still power left in the caps, wouldn't that fry my DMM?

[eliktronik]

No, multimeters have a very high internal resistance, and hence very little current will flow through them. This is how they can measure large potential differences. Make sure though that it is set on a voltage setting above the maximum that the caps see. For example if the caps are charged to 250v, use at least a 250v setting on your dmm.  

[Fret Wire]

That resistor will work fine.