[sdiy] How do old electrolytics go bad?

[Steve Carter]

Count me in on that Bob!  I've done the same thing over the many  
decades without any problems although some engineers regard it as  
being  more than a little uncouth! I remember shorting a capacitor  
that produced an arc welding sound leaving the screwdriver unharmed  
but the aluminium rivet securing one of the terminals was arcing to  
its terminal in a big way. The high resistance at that junction, due  
to corrosion that couldn't be seen, was just waiting to become a  
problem - after that I was convinced it was good practice in some cases.

I must admit, nowadays I usually incorporate a bleed resistor in my  
valve designs so that it goes safe pretty quickly after switch-off.

Steve


On 9 Feb 2009, at 05:52, Bob Weigel wrote:

> You never do it in huge capacitors of course.  For one thing they'll  
> blow the end of your screwdriver off and send fragments flying that  
> might hit you in the eye :-)  But on ones typically found in your  
> garden variety guitar amps ( eg. 70uF 350V, 22uF 500V etc.) you can  
> safely discharge their total energy without damage.  Years of  
> experience has taught me this...(not to mention the calculations on  
> the material heating if the connections are within normal parameters  
> are not that hard to do.  And I've done those kinds of calculations  
> in the past.  One knows the amount of potential energy is just the  
> charge times voltage.  Q=CV so for example the 500V 20uF say has  
> 10mC of charge on it.  UNLESS something is wrong with the  
> connection /wire you should have about .01ohm or less there for  
> sure.  If there IS something wrong..hey..now's the time to find out  
> baby!  It's in my shop!! :-)  But if not the energy I believe should  
> be 10mC x .01ohm ^2 = 10^-6 joules WERE it a perfectly sparkless  
> short.  That's why us all pro capacitor bangers use dirty  
> screwdrivers!!  :-)  That way the lion's share of the energy is  
> absorbed in the higher resistance surface of the screwdriver rather  
> than in the capacitor!  But even in the worst case Aluminum has a  
> heat capacity of 800 J/Kg-K and we're talking about a practical  
> distribution of this heat in a unit with a strong connection over  
> two lead's connection area with a practical mass of relevance in the  
> area of .3G at least.  So I believe that'd be 10^-6j/800x.0003kg ) =
>
> 4x10^-6 ºK
>
> Much wisdom can be achieved from doing the actual physics of an  
> approximation of the situation.  Questions like 'but what if I'm off  
> by a factor of 100?' are returned with 'who cares?' :-)  Unless I've  
> really done something horribly wrong in my analysis (hey it's always  
> possible..check my work) then this correlates strongly to my  
> practical experience in this area.  NEVER discharge power grids...  
> car batteries... or very large capacitances again using a  
> screwdriver because pieces might hit you.  But for small caps...   
> Years ago in a discussion ABOUT how to discharge them in common  
> small tube amps I said 'let er rip'.   But close your eyes while  
> doing it I should have said just in case something flies but I think  
> others had already covered that nicely :-).  But on small caps I've  
> done it hundreds and hundreds of times.  Nobody gets hurt.  And it  
> doesn't wreck your capacitors.  I do NOT do it on caps above a few  
> thousand uF at 50V or above 100uF or so at 500V because I don't like  
> to damage screwdrivers and the sound can be fairly loud :-)  But  
> again even at those values the calculations should make it obvious  
> that nothing is really going to get damaged *unless* it's a  
> capacitor with a rusty interface somewhere...in which case isn't it  
> better to find out now then down the road after vibrations and a  
> little more use cause it to manifest with a loud hum during thy  
> performance?
>
> -Bob
>
> Oren Leavitt wrote:
>
>> The instantaneous current flow caused by a short-curcuit discharge  
>> can damage (or degrade) large value electrolytics - usually by  
>> breaking the point where the lead connects to the inner foil like a  
>> blown fuse.
>> Some capacitors, such as photoflash capacitors, are designed with  
>> heavier electrode structures for instantaneous discharge.
>> If you have to discharge a capacitor, a "soft" discharge using a  
>> resistor is the preferred way.
>>
>> - Oren
>>
>> Samppa Tolvanen wrote:
>>
>>> Hi,
>>>
>>> What about a discharge using a screwdriver?
>>>
>>> Samppa
>>>