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Old 6th April 2008, 11:46 PM   #1
sbelyo is offline sbelyo  United States
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Default Did I hurt this capacitor?

I'll just start by saying I will now always use a bleeder resistor in a PS.

I built a psu and fired it up only to realize I had no way to discharge it when I turned it off.

I took a resistor held by a pair of pliers trying to discharge a cap. I slipped and saw a nice spark.

Did I hurt this cap by shorting it to ground for a split second?


Before anyone yells at me, I safely discharged the PS and but a bleeder resistor in place
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Old 7th April 2008, 12:06 AM   #2
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Did you hurt your capacitor? I doubt it. While shorting them out is not the best thing for them, they can withstand some abuse especially when new.
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Old 7th April 2008, 12:09 AM   #3
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If you discharged to chassis ground you should be OK. If you shorted to the metal case of the cap in question (if it has a metal case) then you might have some problems if the arc "punched" through the insulation of the "can". Check for arc marks on the capacitor in question (not good - but not always fatal) bulging or bulges of the "can" (usually a bad sign), electrolyte leaking from seams or holes (it could be oil or could look a bit like a colored "paste"). If the capacitor looks "good" then power up the unit keeping in mind a way to turn things off quickly should things start to "smoke" or spark - thus the term "smoke test".
Be aware that large capacitors in power supplies can actually "explode" when failing due to internal shorts and can have enough explosive force to make things very interesting - read dangerous!! Losing a finger or eye is not out of the question in extreme cases - although just a loud "bang" and some smoke are the most common result. Hope that this helps -
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Old 7th April 2008, 12:12 AM   #4
sbelyo is offline sbelyo  United States
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I didn't think so... I knew shorting them was not really good, but I recall people doing that to discharge them in a hurry.
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Old 7th April 2008, 12:17 AM   #5
sbelyo is offline sbelyo  United States
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Quote:
Originally posted by c2cthomas
If you discharged to chassis ground you should be OK. If you shorted to the metal case of the cap in question (if it has a metal case) then you might have some problems if the arc "punched" through the insulation of the "can". Check for arc marks on the capacitor in question (not good - but not always fatal) bulging or bulges of the "can" (usually a bad sign), electrolyte leaking from seams or holes (it could be oil or could look a bit like a colored "paste"). If the capacitor looks "good" then power up the unit keeping in mind a way to turn things off quickly should things start to "smoke" or spark - thus the term "smoke test".
Be aware that large capacitors in power supplies can actually "explode" when failing due to internal shorts and can have enough explosive force to make things very interesting - read dangerous!! Losing a finger or eye is not out of the question in extreme cases - although just a loud "bang" and some smoke are the most common result. Hope that this helps -
I shorted it to chassis ground. It's shrink wrapped and the case is not connected to anything.

There's just a small mark on the negative lead of the cap where it touched

I fired it up afterward and it was fine
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Old 7th April 2008, 12:37 AM   #6
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hmmm.....Sounds as if you have just performed your 1st "smoke test" and things went OK. Congratulations!!

BTW - Shorting smaller caps to ground - or terminal to terminal - is a fairly common practice and won't hurt the cap unless you get careless and damage it. Circuits that have high voltages (usually 100's or 1000's of volts) are usually discharged with the use of a "bleeder" resistor to prevent large and rapid discharging that can result in large "arcs" - or damage to the circuit due to some issues with large currents generating reverse voltages etc.
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Old 7th April 2008, 01:02 AM   #7
sbelyo is offline sbelyo  United States
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I aint doing that again if I can help it
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Old 7th April 2008, 07:01 AM   #8
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Sorry to hijack your thread...

I keep reading about beeing carefull and discharge capacitors before touching the inside of the amplifier... Every time I do work on mine, I switch it on to measure, after switching back off, I measure voltage around the big caps, but find nothing... Does it mean the amplifier takes care of discharging...

I blew a cap the other day, 220uF 100V, reversed some new schotky diodes that apparentely didnt work, well they did, at least reversed, and PUFF went the cap, luckily I had moved back a bit before switching power on, I still find pieces of cotton like material that flew all over the place...
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Old 7th April 2008, 02:24 PM   #9
sbelyo is offline sbelyo  United States
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Yeah... that's what happens. Reverse polarity is no good for caps.

I would suspect that if you measure no voltage after switching off, but the voltage is fine while on then you have a bleeder resistor in place already.

Do you have a schematic of the psu?
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Old 7th April 2008, 02:39 PM   #10
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Antonio - well .... if there is no voltage on the capacitor then something is discharging it It might have a discharge circuit as part of the design - but remember - if those components fail then you might have a capacitor with a charge stored in it and the little bugger is just waiting to reach out and ZAP your unsuspecting rear-end! Capacitors can hold a voltage charge for a very long time - easily several minutes and in some situations for days - even months!! So - never assume that a capacitor is discharged - make sure it is discharged!! It's a good work habit to form - and one that will one day save you from becoming the discharge path. Take a little look at this video to see a cap "blow" http://www.youtube.com/watch?v=B8gFgIQl2HI
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