Very stupid question about desoldering power supply capacitors

[Mp3]

Hello everyone,

I'm fairly experienced with desoldering and comfortable with it. However I need to repair some power supply capacitors and have never done that before. I need to do both machines which currently work and machines which have damaged power supply capacitors.


For the ones which still work and have been powered on recently, how do you safely discharge a capacitor before desoldering it without risking shock damage to the PCB?

For the ones which have bulging capacitors, do they need to be discharged or just desoldered?

I know you are supposed to discharge them before testing for ESR but i did not want to try desoldering them and have a shock from that.


Thanks in advance!

[james_s]

Generally I don't discharge them unless they are high voltage parts and the thing has been turned on very recently. In most things the capacitors will discharge themselves fairly quickly but if you are worried about it just short across each one with a screwdriver or make a discharge probe with a power resistor of a few tens of Ohms.

[Electro Detective]

Quick and dirty if you don't want possible screwdriver sparks (and mini stick welds) is to use an old school light globe in a desk lamp and make a poor mans lead set
It's not as good as an inline power resistor bleed lead, but beats having nothing

If you have lazy tendencies like me, a multimeter with Low-Z feature will let you know if there is voltage on the cap,
and may even discharge it via the meter's 3k resistor, as you watch the voltage go from ?? down to zero   

Otherwise manually put any decent multimeter on the highest DC Volts setting (600 > 1000v) and test the caps for voltage

If there's no voltage or very little, then do the screwdriver short thing to make sure there's absolutely no voltage lurking

Capacitors can magically recharge/recover btw, so suggest to perform the check n discharge routines just before you desolder, solder,
or test for ESR (and discharge again!)

You don't need any cap surprises with your face and grounded soldering iron in that zap area   
i.e. WEAR GLASSES !!!

And don't forget to do same biz with the new capacitors... TRUST NO ONE !     LOL   

[Shock]

Here are some options, pick one and enjoy:

Measure it with a DMM and see.
Use the LoZ voltage measurement function of a DMM multimeter or perhaps try a lower impedance analog multimeter. They normally load down the circuit under test.
Use an appropriately high voltage rated component that dissipates enough energy without excessive current/overheating.
Use an online discharge calculator to work out the resistance vs time, make a discharge cable with a beefy resistor.

Measure voltage before and afterwards if unsure, obviously it depends on how much voltage and capacitance with how much care should taken, but typically you should be measuring otherwise your soldering iron could become the fuse. Same goes for soldering a live circuit, bad mistake to make.

[nuno]

My advice is to use a slow discharge method (resistor, bulb, ...) but the most important thing is to always check the voltage on the caps or to discharge them any ways. Usually they will slowly discharge in a few minutes because there's a load on them, however, I have received mains connected high power equipment for repair where a fuse or something else disconnected all loads from those capacitors, and it takes weeks for the voltage to go from 320V to below a couple tenths of V. So it's better no never assume caps are discharged.