ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Cap voltages in series/parallel

:

10/12/1999 4:37 AM

Cap voltages in series/parallel

I have many occasions to combine capacitors to make a value I don't have. Usually temporarily to see if it's the problem and then I can purchase the one I need. I've been limiting my combinations to adding capacitances in parallel that are the same voltages. I have read this info somewhere but would appreciate someone spelling out the formulas for combining caps in series and parallel both for the capacitances and voltages.
Thanks
Tom Lennon

10/12/1999 6:08 AM
A.S	Caps in series: 1/C=1/C1+1/C2+1/C3.....and so on. The formula is the same as the one for resistors in parallel. Caps in parallel: C=C1+C2.... I hope this makes sense to you and I hope this was what you wanted. A.S

10/12/1999 2:12 PM
Benjamin Fargen	Tom, Running the PS caps in series (Like the first two PS caps in bigger fender amps) gives you half the rated capacitance of each cap. You then add the voltage rating of each cap together to get your new max VDC rating. I.E. 100uf/350V + 100uf/350V in series would give you the same rating that a 50uf/700V cap would. Does that help? Benjamin.

10/12/1999 4:25 PM

Caps in parallel gives you the sum of the capacitances (Ct = C1 + C2.....) and the LOWEST voltage of all the caps in parallel.

Caps in series (just like AS and Benjamin said) add exactly like resistors in parallel (Ct = 1/[(1/C1) + (1/C2) + ...]) and give you the total voltage rating ov the caps in the stack.

Caps in series in a power application also need a resistor in parallel with EACH cap in case one cap fails (usually fail by going open-circuit). Don't recall what value resistor, though.

That last note ONLY applies when the voltage of the circuit exceeds the LOWEST rating in the stack.

Anyine recall the R value?

10/12/1999 4:35 PM
peter	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.

10/12/1999 4:41 PM
Don Symes	Thank you, sir.

10/12/1999 5:32 PM

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

Page 1 of 1