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Re: Wiring caps in series
Tesla List wrote:
>
> >From kdc4n-at-cs.virginia.eduSun Sep 8 20:04:32 1996
> Date: Sat, 7 Sep 1996 16:46:20 -0400 (EDT)
> From: "Kevin D. Christiansen" <kdc4n-at-cs.virginia.edu>
> To: tesla-at-pupman-dot-com
> Subject: Wiring caps in series
>
> A little while ago, somebody on this list asked if it would be
> practical to make a high voltage capacitor by wiring a bunch of photo
> flash (330 V 160 uF) caps in series. I never saw an answer to this
> question.
>
<SNIP>
These are electrolytic capacitors. They are not useful in Tesla
applications for a number of reasons:
1. MUCH Too high a capacitance
2. The Killer: They are made for DC applications (or low duty cycle DC
pulse service into a non-inductive load - a flashtube). They cannot
tolerate the major voltage reversals seen in an oscillating LC circuit
3. If pressed into this type of service, they will rapidly short out
(and some will probably "blow" their pressure vents releasing hot
gas/steam).
> As an experiment, I tried wiring 6 of them in series and then
> running them through several charge/discharge cycles of 1500 volts -
> by putting 1500 volts between points A and G (see "schematic" below),
> and then bridging points A and G with a heavy wire (BANG!!!).
>
> || || || || || ||
> o-||--||--||--||--||--||--o
> || || || || || ||
>
> A B C D E F G
>
> The first time I charged them up, each of the caps had about the same
> voltage across them (Vab=Vbc=Vcd=Vde=Vef=Vfg=250 volts). After
> dischaging them by bridging A and G with a wire, I assumed that all
> of the caps would have discharged to some low value. What I found
> instead is that the caps had more or less random voltages on them
> (between -50 and +50 volts). Charging the set up again (by putting
> 1500 volts across points A and G) resulted in widely varying voltages
> across the various capacitors.
>
<SNIP>
The problem with tieing large numbers electrolytics in series is that
the capacitance of each varies significantly. Also, the Equivalent
Series Resistance (ESR) can vary significantly between identical units.
When you discharge the chain, the maximum surge current will be limited
by the SUM of the ESR's of all caps in the chain. A much larger voltage
will be (temporarily) seen on those caps having the highest ESR's. This
surege voltage can easily be higher than the peak voltage rating of the
cap.
Once the discharge current goes to 0, what actually happens is the SUM
of all the capacitor voltages becomes 0. However those caps with higher
ESR's will retain a positive voltage, while those with a lower ESR will
be "reverse" charged to a negative voltage. Reverse-charging polarized
electrolytics lowers their capacitance (and shortens their life). When
you then "recharge" the chain, those with lower capacitance will charge
more quickly thatn those will higher capacitance. This can easily result
in one or more capacitors being subjected to too high a voltage. A
number of charge-discharge cycles like this and one or more of your caps
will probably be history.
By the way, the trick (used for non-polarized capacitors in series) of
simply adding equal-valued balancing resistors or RC networks across
each cap does not prevent these problems for electrolytics...
> Given this situation, I could easily see some of the caps failing
> due to over-voltage conditions, which are caused by the varying
> voltages that result when the capacitors are charged and discharged
> in series.
>
<SNIP>
Agree!
<BIG SNIP>
Safe Coilin' to ya!
-- Bert --