how long do capacitors hold their charge?

[Samuel]

I just got some old SCSI harddrives from work that I want to salvage the cases and a couple components from. Question: these drives directly took three prong outlets, so I know they have a power transformer built in to convert mains to 12V/5V (DC?).There's a 400V cap near the mains connection that seems the most likely candidate for danger.

I know for sure these things haven't been plugged in for 6 months, and I'm reasonably certain they haven't been plugged in in over 2 years. I know that large caps can hold their charge for "a while", but is this "while" measured in hours? days? weeks? months? millenia?

I've got a feeling the answer is "hours" or "days" but thought I'd be sure before I go poking around...

[Joep]

The rule of thumb is that it will take 5 "RC" times for a capacitor to discharge. If you have a 100uF cap and a 10K resistor to discharge it, it will take 0,0001 x 10.000 x 5 = 5 seconds.

Often in modern circuits, this discarge resistor or bleeder  is "hidden" somewhere in the circuit and the cap will be discharge automatically as the circuit is powered down.

But don't take a risk and use a volt meter to check the voltage on the cap.

Bye,

Joep

[petemoore]

Pretty much anything except millenia.
 After a month they're probably near fully discharged.
 After a week they'er probably about discharged
 After a few days "" "" "" ""
 Use a resistor is what some Ppl say and shunt them to ground.
 I just shunt [connect] them to ground and put up with the ZAP [arc/spark].
 I had a monitor sitting there for at least 3 weeks, and was surprized to see a sizable spark, but felt nothing, I was using insulated cutters.
 Some stuff has 'drain' [large] resistors that act like a teeny drain hole for the charge, so that the danger is lessened over a shorter time...I don't know that I could every tell which caps had that...lol.

[Samuel]

Thanks guys, just what I was looking for!