🚀 go-pugleaf

bernst@husc8.harvard.edu (Noam Bernstein) writes:

> I was taking apart my power ampilfier to check for a leaky capacitor in the
> power supply.  I figured I'd check the peak resistance of the capacitor (as
> t -> Inf).  Things were behaving strangely, so I started checking the
> voltage across the two terminals, and it appeared to be increasing very
> slowly.  In fact,  a couple of hours later, it had increased significantly.
> I took it out of the circuit to make sure no other capacitor was
> discharging into it.  I then discharged it almost completely (voltage
> across it was down to about 50mV), and waited a few hours.  It had built up
> to about 0.7 V.  Within two days or so it seemed to stabilized around 2.5V.
> It's an 18000 microF, 75V capacitor (electrolytic, of course).  I gave up
> on trying to check if it was leaky, since it did not fit into my universe
> in an explainable way.  I have no clue why it's behaving (or how it can be
> behaving) the way it is.  Does anyone else have a clue ?
> 
> 						Noam Bernstein
> 
> 						noamb@das.harvard.edu
> 
> And no, this isn't a joke.


Noam,

        What you have experienced is common- you have to think 
about what you have there, an ELECTROLYTIC capacitor.  
Electrolytics contain "electrolyte", a fluid dielectric betweenn 
the plates.  This fluid is not pure, and it is the impurities that 
cause the "battery" action you have noticed.  Short the pins for a 
couple of days and THEN retry.  Chemically the impurities in the 
oil will neutralize and the capacitor will no longer "charge 
itself up".  The action is similar to a very poor battery (try 
sticking dissimilar metal nails into an orange and measure the 
voltage across the nails- you should get about half a volt or 
more).

        Those of us who work in physics (electronics) are quite 
aware of this action, especially on 350 volt electrolytics, as we 
have all gotten a bite at one time or another..............sq

(I even noticed this action a few days ago when my hand crossed a 
known discharged Xenon strobe cap.  The cap had 60 volts or so on 
it when my hand crossed it *after* I shunted it with a screw 
driver!  I shunted it almost 3 times before it was below tingle 
voltage...)

        (another thing to note is that discharging capacitors have 
internal resistance as well as inductance.  When you discharge a 
capacitor, imagine that the short is not zero ohms, but more like 
through a resistor- the resistor is the less than perfect 
conductance of the metal foil within the cap.  You have to shunt 
the cap for a long time because of this internal resistance...)