Question about Capacitors in Parallel

[vanhansen]

I was looking at a forum for guitar wiring and someone there posting saying that if you wire caps in parallel the voltage capacity doubles like the capacitance does, i.e. 2 16 volt caps = 32 volts.  I say that's wrong, the voltage will remain the same because each cap sees the same voltage.

Would someone be kind enough to confirm my thinking?  I just makes sense but I'd like confirmation from an expert in the field.

Thanks.

[puretube]

only the capacitance adds in parallell.
max voltage is the lowest indicated voltage on any of the parallelled caps.
(e.g.: 10µF/10V || 10µF/16V > 20µF/10V)

[vanhansen]

Thanks, puretube.  That definitely confirms my thinking, and also a bonus.  I forgot about what the max would be if you had two different voltage rating caps in parallel.  That makes sense.  Thank you.

[Hal]

voltage adds in series...as long as the 2 caps are the same.

[vanhansen]

voltage adds in series...as long as the 2 caps are the same.

So if you hook up 2 caps rated at 16 volts, in series, then the max acceptable voltage is now 32 volts?  But of course, wiring them in series also descreases the capacitance.

Also, does wiring caps in parallel increase their amperage or power handling capability?  This is a question I was asked, and I say no, the voltage capacity remains the same, and that appears to be the case based on puretube's reply.

[Hal]

most caps aren't rated for amperage/power.  Are you confusing this with ESR?  ESR is equivalent static resistance - the internal resistance in the capacitor.  This effects the maximum current output of a capacitor when discharging.  Besides for this, cap's don't dissipate power.  Thats the point of them :-D.

yes, 2 16v caps in series will be a 32v cap of half the value.  4 caps - 2 series, 2 parallel can make a cap of the origional value with double the voltage.

[vanhansen]

No, I wasn't confusing this with ESR, just confirming what my original thinking was.  Someone in another forum for guitar wiring brought that up and I was just correcting what he had mentioned that wiring 2 caps on parallel will double their power handling, which is not true.

I appreciate the responses.  Good education.  

[Mark Hammer]

Gerald Weber, in his regular column in Vintage Guitar magazine, often describes use of two caps in series to attain higher voltage ratings and support SAFE use of higher voltage power supplies and higher plate voltages in the amp's power tubes.

[triskadecaepyon]

Capacitance is the measure of charge before the electrons jump, if I remember right.  The capacitance is the riciprocal in calculation of resistance, therefore if you add caps in parallel, you are adding C1+C2, and if series, 1/C1+1/C2=1/Ct.  you are decreasing the capacitence if you are adding them in series, increasing if in parallel.

[vanhansen]

Gerald Weber, in his regular column in Vintage Guitar magazine, often describes use of two caps in series to attain higher voltage ratings and support SAFE use of higher voltage power supplies and higher plate voltages in the amp's power tubes.

That's exactly what the discussion on the other forum is.  To help protect against a voltage spike hitting your guitar and you, star ground all your connections in the guitar and use a .33mf 400 Volt cap between the star ground and ground point.  The question came up about using 3 .1mf 200 Volt caps in parallel.  Sure, you'll get .3mf, but still only rated for 200 volts.

[moving_electrons]

One aspect of using 2 caps is series is that you are somewhat increasing the failure rate.  If either one fails then the overall value is incorrect.  Since failure is a statistical phenomenon the failure rate will be higher with two in series rather than one large one.   If expected failure rate is low then it is not that much of an issue, but worth being aware of.

[aron]

>Gerald Weber, in his regular column in Vintage Guitar magazine, often describes use of two caps in series to attain higher voltage ratings and support SAFE use of higher voltage power supplies and higher plate voltages in the amp's power tubes.

Yes for amp stuff, it's common to use two electrolytic caps in series with double the capacitance with balancing resistors to get the desired capacitance.

[puretube]

I`d say: balancing resistors to make the correct voltage balance
between the two
(especially at switch on).

(sure you need 2  100µF caps in series to get
a higher voltage rated 47µF)

[R.G.]

The info here is correct: caps in series have a composite capacitance lower than any one in the string, and can have a higher voltage rating.

The emphasis is based on this: voltage in series capacitors divides by the inverse ratio of the capacitances (that is, the direct ratio of their impedances as Xc=1/(2*pi*f*C) for transient voltages and by the ratio of their leakage resistances at DC.

Example: if I have two caps, each 1uf, both rated at 63Vdc; one has a 10M leakage resistance, the other a 100K leakage resistance. If I apply a 100V step to their series combination, then the voltage will initially civide by 50:50, in the ratio of their capacitances. As time goes on, the DC voltage on each cap will redistribute itself with a time constant of  a tenth of a second (1uF*100K), so that in a short time, the voltage across the 100K leakage cap will be  1V and the voltage across the 10M leakage cap will be 99V.

The cap with the low leakage will be subjected to overvoltage through that 100K resistor and may fail sooner or later, or if it's an electrolytic may form up to a higher voltage - and lower capacitance.

If you have a real leaker (say 10K) and a good cap, the leaker overvolts the good cap, which then fails because it's overvolted, and then the leaker fails because it too is overvolted.

The moral here is to always use external resistors less than 1/10 of the minimum value of the specified leakage resistance for the caps you're using to distribute DC evenly (relative to their rating) on them.

[Hal]

http://www.oz.net/~coilgun/theory/capacitors2.htm

[R.G.]

Hey, Barry's still doing coil guns!

I pointed out how to get some iron into the M-field flux paths to him a ways back, as well as a way to impart spin to the projectile - magnetic rifling!

[vanhansen]

Thanks for the great explanations.  Things are starting to make more sense.  Gotta do some more reading.