Did I hurt this capacitor?

[sbelyo]

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

 

[HollowState]

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.

 

[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 -

 

[sbelyo]

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.

 

[sbelyo]

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

 

[c2cthomas]

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.

 

[sbelyo]

I aint doing that again if I can help it

 

[Lowrider]

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...

 

[sbelyo]

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?

 

[c2cthomas]

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

 

[bigwill]

Regarding shorting out capacitors, will the hot arc vaporize any solder on its terminals creating dangerous lead vapour? Is there enough to considered a problem? (Not that I short out capacitors all day! I use bleeder resistors!)

 

[Lowrider]

 

[sbelyo]

Lowrider...

It looks like there is a discharge circuit in there

 

[sbelyo]

Regarding shorting out capacitors, will the hot arc vaporize any solder on its terminals creating dangerous lead vapour? Is there enough to considered a problem? (Not that I short out capacitors all day! I use bleeder resistors!)

I can't really say... I suppose if the arc lasted long enough it would vaporize something. And lead is no good in any amount for extended periods of time

 

[Lowrider]

Probably RY5 opens when I switch it off, letting R26 discharge the caps...

 

[c2cthomas]

Regarding shorting out capacitors, will the hot arc vaporize any solder on its terminals creating dangerous lead vapour? Is there enough to considered a problem? (Not that I short out capacitors all day! I use bleeder resistors!)

Bigwill - When I find myself around circuits that have enough "juice" to draw large discharge arc's I'm more concerned about not turning myself into "vapour". I have discharged some high voltage circuits (+250 KV) and drawn arcs - but usually those devices are so large that they have steel terminals that leads are bolted or screwed into - so there isn't any lead involved. I have seen "pits" get blown into said terminals and it did get my attention!!

 

[multi-volti]

I remember a guy about 30 years ago discharging a fairly screw-terminal computer-grade large electrolytic in a low voltage supply. Why he bothered I don't know.

The funny part was he forgot to turn the power off. He welded a screwdriver to the capacitor.

Then he showed me the screwdriver. He said he paid more for the screwdriver than the capacitor so he hacksawed the capacitor terminals off...why throw away both? It was unusable in it's present configuration! The mutant screwdriver with 'growths' on it reminded him in the future to be more careful

He got his BSEE shortly afterward. I still get a laugh out of thinking about it.

Murray

 

[multi-volti]

1200V Schottky diodes?

Where did you find those 1200 V 5A Schottky diodes?

 

[ThSpeakerDude88]

discharging a 47uf cap with 400v makes a nice spot welder

I shorted out my bench supply one time, and it took the cap a while to recover. My GFCI would trip each time I would turn it on for about 5 cycles then it quit doing it. me thinks the cap may be bad/sickly?

Btw blown up caps smell NASTY! I've done my share.

 

[Lowrider]

Re: 1200V Schottky diodes?

Where did you find those 1200 V 5A Schottky diodes?

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