[sdiy] How do old electrolytics go bad?

[Bob Weigel]

OH yeah...it's just good design protocol to incorporate 220K ish 
resistors across the caps.  (Especially if they are stacked like in some 
fenders and ampegs so that the voltages don't leak differentially and 
put all the load across one cap which then will fail of course and the 
other after it leaks all the voltage to the other one :-).  Kind of like 
when you're doing squats and someone leaves the pin sticking out and all 
the weights get knocked off one side...then off the other of course :-) )

Some years back one of my regulars was trying to 'help' me... and when I 
wasn't looking he turned the standby switch off and I didn't see it.  
NORMALLY when working on tube amps I'll turn off the mains or hopefully 
just unplug the cord.  In this case I had because I was replacing the 
cord for him after doing some other work.

In soldering on I never thought about it having charge since I didn't 
see him turn off standby...which of course isolates the caps that have 
the bleeders on them.  The tubes weren't heated so there was full charge 
still on it at 440V or like that.  And I managed to hit a hand on 
something hot while hitting my other arm on the chassis.  GANK righ 
through the heart.  Could have killed me.  I hate it when people 
*help*.  But I shouldn't get sloppy like that and presume anything.  
Should always short something.  Safety first. And safety is SO much 
easier when you don't have to hung for a special tool :-)  -Bob

Steve Carter wrote:

> 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
>>>>
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