Re: Equalizing resistors for electrolytic capacitors

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>

Hi Eric,


> Original poster: "Hydrogen18" <hydrogen18@xxxxxxxxxxxxx>
>
> List,
>
>      I have a large number of small electrolytic capacitors I am trying to
> connect in series. Since they have the nominal -10% to +85% value control
> on them the potential exists for a unequal division of voltages. I
> understand than resistors across each capacitor can help solve this
> problem,

I don't think anyone has correctly explained the problem or I did not spot
it.

The unequal value of the caps will only cause an unequal division of voltage
when they are first charged up or when they are discharged.  Eventually the
division of voltage will only be determined by the leakage current.

Adding a resistor across each cap will have little effect on the initial
variation in voltage unless they are charged slowly in comparison to the
time constant of the R and C . Similarly for discharge.
Resistors will eventually reduce the differences but as others have said
they are normally used to equalize the leakage currents.
The only way round the -10+85% problem is to provide more margin by having
more caps in the  string such that worst case the smallest cap does not have
its working voltage exceeded.

Actually the problem may not be as bad as the above figure indicate. Check
the spec on the caps. Those figures probably  refer to the various due
voltage temperature and time.  The initial variation between them may be
much smaller say +/- 10%.  In addition some caps have a higher voltage surge
rating so provided that is not exceeded during charge or discharge you will
be ok. You will then be reliant on the change of each cap value tracking
with time etc more likely if they are all the same type and age.

If the cap spec does not give a value for the initial variation assume 10%
or for a more robust design 20% i.e. its a trade off between fewer caps and
reliability.

The value of the resistors used for equalization is a similar trade off.
One rule for the selection of them is to select a value such that the
current drawn by them is ten times the worst case leakage current.  If
safety is an issue check the discharge time is reasonable and reduce the
value if needed.

Bob









but have found no formulas or examples on what values should be
> used. Is it an arbitrary thing? Any help you could provide would be
greatly
> appreciated.
>
> So far I have come up with:
>
>   I understand the time constant of Tau(R*C) correctly it would be:
>
> 47000 * (330*(10^-6) = 15.51 seconds.
>
> so in about a 78(5 Time constants = 98% charge gone) seconds provided my
> caps are at full voltage they will only be at 2 volts each with a 47k
> resistor across each one.This is still a high voltage when you factor them
> being in series. Although at that low of voltage I should be able to short
> them and they wont have energy to internally damage themselves I think.
> This is for a DC tesla coil filter cap
> ---Eric
>
>
>