RE: Capacitor - series?

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Original poster: "Philip Chalk" <phil-at-apsecurity-dot-com.au> 



-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Saturday, 7 February 2004 12:41 PM
To: tesla-at-pupman-dot-com
Subject: RE: Capacitor - series?

Original poster: "Luke" <Bluu-at-cox-dot-net>

The only way you would get 5KV across both caps at 10KV is if the two
cap values were equal and therefore their reactances equal at the
operating frequency.  If you have unequal capacitance values you cannot
simply divide the total voltage by the number of caps to see what
voltage they will see across them.

Luke Galyan
Bluu-at-cox-dot-net


That's right, but in the example Dan chose that's how it is.

It depends a bit on what the capacitor is for. What you're talking about
is more appropriate at DC, with appropriately sized resistors) but still
not generally considered 'a good idea'.


-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Friday, February 06, 2004 3:34 PM
To: tesla-at-pupman-dot-com
Subject: RE: Capacitor - series?

Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com>


You have to be careful that the voltage will equally divide across the
capacitors.

For example, if you have a 10kV, 1uF cap and a 1kV, 1uF cap, you
couldn't put them in series to get a 11kV, 0.5uF cap.
You would get about 5kV across each capacitor assuming ideal conditions.

Dan


Can we series caps that are diffrent in working voltage and capitance?
Example if we have a 1 nf 10 kV cap and a 10 nF 1 kV cap? If i series
them il get 11 kV capacitor? What will happen with capitance, will this
work?

Considering this particular example:-

For DC  (e.g. an energy-storage cap)

The series combination would yield a value of about .909nF  On DC, I
seem to recall that theoretically voltage distributes in inverse
proportion to capacitance (Q=CV conserved). In real caps it could be way
different.

That is, on 11kV DC, it would tend toward 10kV across the 1nF capacitor
& 1kV across the 10nF capacitor. To ensure this though, it would be very
wise to put equalizing resistors across the caps, again in inverse
proportion to capacitance - say 10M across the 1nF & 1M across the 10nF.

It serves to jack up the dc voltage rating of your 1nF 10kV cap by 10%
while reducing its capacitance by about 9%.  But it seems like a bit of
a waste of the 10nF 1kV cap.

On ac, 'thinking out loud' :- At 50Hz, the 1nF cap has Xc= 3.18M Ohm
approx. & the 10nF cap Xc=318k Ohm. When passing current at 50Hz (e.g.
as filter cap after a 1/2 wave rectifier) these values begin to swamp
the effects of the equalizing resistors, but the ac voltage still
divides in inverse proportion to the reactances, so the higher voltage
still develops across the smaller capoacitance. So it sounds, on the
surface, O.K. to me.

At 100kHz, (TC tank maybe) 1nF Xc=1.59k Ohm, 10nF Xc=159 Ohm.  Now the
equalizing resistors have virtually no influence, but the voltage will
divide as before.

Sounds OK then to me, but still not usually done. Better I think to use
equal voltage ratings (& ideally 'identical' caps) when seriesing
capacitors.

The particular example you chose with the 10:1 ratios happens to work
out 'OK', other more random combinations would not.  You need to
consider whats going on in each case.

Regards,

Phil Chalk.