RE: Capacitor - series?

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


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



-----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?

Looking at Dan's 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 5.5kV across each 1uF
capacitor. To 'correct' this, you could put equalizing resistors across
the caps, in direct proportion to their voltage rating - say 10M across
the 10kV cap & 1M across the 1kV cap. This would tend to produce 10kV
across the 10kV cap & 1kV across the 1kV cap.

In the DC case equalizing resistors of suitable values should swamp the
very high internal DC resistance of the caps, so I think with 'good'
caps this voltage division should apply pretty well regardless of the
individual capacitance values (?) AT DC.


On ac:- At 50Hz, each 1uF cap has Xc= 3.18k Ohm approx. When passing
current at 50Hz (e.g. as filter cap after a 1/2 wave rectifier) these
values almost totally to swamp the effects of the equalizing resistors,
so the ac voltage still divides in inverse proportion to the reactances,
and with equal value caps will tend to divide almost equally across
them. Thus you will get close to 5.5kV ac across each cap.  One will be
happy, one will not.


The particular example you chose with the 10:1 ratios happens to work
out 'OK'.

The values in Dan's example, at ac would not work out. You need to
consider whats going on in each case.

Regards,

Phil Chalk.