1

Is it safe to short (discharge) an AC capacitor before you remove it from the circuit. Or do you have to wait until after you remove it from the unit?

6

Always short the capacitor as early into the disassembly process as you can. You may accidentally discharge it when handling it or removing it from the unit, and these components have enough energy to kill you.

I make sure to wear jeans and leather boots with a rubber sole when discharging capacitors, and always when it's relatively dry out. I keep my left hand behind my back and short the capacitor leads with a screwdriver that has an insulated handle. After about ten seconds of shorting, I double-check that the voltage across the leads is zero before considering it safe.

  • 3
    I just making sure it was safe for the equipment, you can always regrow a heart. – Evan Carroll Sep 5 '18 at 13:39
  • 3
    I grab a big (ohmic value and wattage) resistor when I want to drain a cap. But you do you. – Harper Sep 5 '18 at 15:20
  • @Harper I would prefer that, too, for the capacitor's sake. But I don't like the idea of using two hands to short a cap (in case one's fingers would contact the terminals before the resistor). Does your resistor have clips so you can do one terminal at a time? – calcium3000 Sep 5 '18 at 15:33
  • Sorry, by "grab" I mean in the figurative sense: read "obtain". It goes without saying that you should follow good electrical safety practices. – Harper Sep 5 '18 at 15:45
6

I wouldn't short the cap to discharge it. When you do that, a huge current flows for a very short time. This current is almost certainly far beyond the rated current capabilities of the cap. You may not destroy the cap, but you are overstressing it and shortening its life.

I would recommend disconnecting one terminal of the cap first, then shorting it through a resistor applied to both terminals. The resistor limits the current flow, while disconnecting one side prevents you from accidentally shorting/improperly loading your power supply if its still hot or turns on without warning.

  • 2
    Sorting the cap might not destroy the cap, but it might destroy whatever you used to short it. I learned that many decades ago, working on an audio amp with a 40,000 uF 50V cap in the power supply (They don't make them like that any more!) Discharging it with a small screwdriver produced a loud bang - and vaporized the entire screwdriver blade, leaving a piece of metal rod with a somewhat chewed-up end in the screwdriver handle. Don't try that at home, kiddies! – alephzero Sep 5 '18 at 20:18
  • 1
    Also, leave the resistor in place until you want to power up the cap again. Large value caps can recharge themselves from charge that is "hidden" in the electrolyte, and slowly percolates back onto the capacitor plates. If you remove the resistor, after a few hours you might discover (the painful way!) that the voltage has recovered to half what it was before you "fully discharged" the cap. – alephzero Sep 5 '18 at 20:23
  • Nice! Hope you didn't lose any fingers. The instantaneous starting current when you bleed the cap is I_short = V_cap / R_limiting_resistor. On a dead short, your R is very low, so your current is sky-high. Voltage thrills, but current kills. – schadjo Sep 5 '18 at 20:23
  • No damage done, fortunately. But you never forget what you learned from that sort of practical experience. – alephzero Sep 5 '18 at 20:26
0

In most cases properly designed electrical equipment will have built-in provision for draining the capacitors. So you shouldn't need to drain the capacitors, only verify they are drained, which you can do with your voltmeter.

If you do need to drain a capacitor, then best practice would be to drain it through a high value resistor.

In my opinion the least bad set-up would be two test probes connected via suitable resistors. A chain of ten 2K 0.6W resistors seems like a reasonable choice (chains of resistors have the advantage over single resistors that if one fails it's not a disaster)

Be aware that electrolytic capacitors can partially re-charge themselves if they are left open-circuit after discharging. Most capacitors used directly on AC won't be electrolytics though, electrolytics are normally found in DC applications.

Unless the capacitor has some kind of connector on it I don't think trying to remove it from the circuit while still charged is a good idea. Too much risk of an accidental short.

  • How do you recommend connecting the 10 resistors? If they are chained serially and one fails open, then the capacitor won't discharge. If they are in parallel and one fails shorted, then you'll get a big spark when the capacitor discharges. Seems like you'd want both, like 2 parallel banks of 5? – Johnny Sep 5 '18 at 23:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.