[sdiy] How do old electrolytics go bad?

[Bob Weigel]

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
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