Discharging Capacitor?

[karma2122]

I'm new with electronics and don't have much money but TONS of old, broken, and obsolete electronics sitting around. I want to take them apart and take out any useful parts. The problem is with discharging capacitors. I've just been using a screwdriver but I know it usually just destroys the capacitor and need advice on actually discharging it without destroying it. I've constantly been told by other people online to use resistors but don't tell me actually how to use a resistor to discharge a capacitor. I've searched online and still no answers...everyone just seems to expect me to know how to...if I did then why would I be asking??? Sadly I'm a visual learner but there's no one that I know of to show me how. So how would I go about doing it safely?

Also what would be the right tools and things to use? I've been told to get well insulated gloves just for a precaution but where would I get them?

 

[Diver300]

It is very unlikely that any significant charge would remain on a capacitor after a day. Also it is only high voltage capacitors that could do you any damage.

The energy that a capacitor can store depends on the physical size, so only larger ones need any precautions. Larger ones normally have the voltage written on them so you only need to discharge those over 25 V.

Discharging with a short circuit, like a screwdriver, isn't likely to damage them. If you don't get a spark, the capacitor was discharged anyhow.

A resistor will reduce the current so that you don't get a spark, and it is a better way of doing it. Just about any value of resistor will do. A small incandescent mains lamp connected to bared wires would be good, as you can hold the insulated part of the wires.

 

[karma2122]

I'm guessing using the screwdriver will work for power supplies? I have two 240 watt power supplies that've been sitting around for several months that I want to take apart.

 

[jakeselectronics]

My old electronics teacher used to just drop the PCB in the bath, then let it dry out for a few days.

But if your that concerned about it, i'd go with the incandesant bulb...

 

[Sceadwian]

If the power supply has been sitting around it should have self discharged. Bleeder resistors are standard practice, even on cheap supplies, though it does't hurt to be careful. Dead shorting a capacitor probably isn't going to damage it, I've done it many a time, but for some types it might not be as good as a slightly more controlled discharge. Just attach a wire to ground and put a 10k resistor or so in series with it, perhaps attached to something like a screw driver, attach another one directly to ground. Apply one to one lead and one to the other of the cap for a few seconds, then dead short the two leads. The 10k resistor will do a low current controlled discharge (really large caps just hold it there longer) and then the dead short will bleed off the rest. though remember large value high voltage electrolytics tend to slightly recharge themselves from dielectric absorption, at the same time however the leakage on caps like that will cause them to self discharge over time. I wouldn't be so paranoid over it unless you're working with TV tubes, which upon pain of death should be discharged carefully even after sitting for extended periods of time.

Aside from that the only tip I can offer you is get yourself a decent temperature controlled soldering iron (MPJA.COM has several inexpensive ones) as soon as you can afford it. And for bulk desoldering your absolute best investment is a heat gun, which you can pick up at most DIY store for 20-40 bucks. Variable heat control would be awesome but a low and high setting are fine. You can make your own custom tips using thin sheet metal, scissors and a ltitle sand paper to deburr the edges, you can desolder just about anything with that and a pair of plyers, and learning to control the distance you hold the heat gun from the board. Low heat to preheat then high heat for a few seconds while applying pressure with the plyers. SMD components will tend to just fall or get blown off by the airflow.

 

[karma2122]

I just opened one up and there's a bunch of white junk over several components...what the hell is it? Looks like glue to me.

 

[Sceadwian]

You'd have to post an image, but it's likely a high temp hot glue, it's used as a potting compound to keep components from moving around, can be a pain to remove but not horrible, especially with older supplies as it grows brittle over time.

 

[saken]

A resistor will reduce the current so that you don't get a spark, and it is a better way of doing it. Just about any value of resistor will do. A small incandescent mains lamp connected to bared wires would be good, as you can hold the insulated part of the wires.

i was going to say that