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previous: peter The parallel resistors aren... -- 10/12/1999 4:35 PM view thread

Re: Cap voltages in series/parallel

10/12/1999 5:32 PM
Gil Ayan
Re: Cap voltages in series/parallel
quote:
"The parallel resistors aren't there in case a cap  
shorts (in which case no current would flow through  
the resistor) they are there to make sure the  
caps get equal voltages despite differing leakage  
currents. Typical values are in the ballpark  
of 220k."
 
 
Peter, Don was right as well, so both of you were. His words were "in case the caps OPEN," and I guess caps do open... So, the resistors are there to ensure both caps get the same voltage, AND to ensure one cap still works even if the other one opens up.  
 
As for the value of the resitor, yes, about 220K for a 100uF cap is right. It boils down to how long you want to wait for the caps to discharge after you turn the amp off... a 470K resistor would cause the same cap to discharge over twice as slowly... The lowest I have ever seen there is 150K, as used by MESA/Boogie.  
 
Since there is always a chance a cap may short out, in that case, the one resistor left over would have to carry the whole power. Considering you probably have say 500V is a lot of amps, the power dissipated by a 150K resistor is 500 x500/150,000 = 1.67W, and using a 220K = 1.13W, and by a 270K = .93W, and a 470 K = .53W  
 
Guess what? The 270K looks good, and you could get away with a 1W resistor safely -- remember the likelihood of the cap shorting is very small.  
 
Cheers,  
 
Gil