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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.

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.

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.

Today, sombody mentioned (in the discussion on "paper capacitors")
that Tesla used his caps in series:

Tesla List writes:
 > >From i541771-at-imsday-dot-comSat Sep  7 09:37:34 1996
 > From: Chris Singletary <i541771-at-imsday-dot-com>
 > > From:          Tesla List <tesla-at-poodle.pupman-dot-com>
 > > >From hullr-at-whitlock-dot-comThu Sep  5 22:16:22 1996
 > > 
 > >
[snip]
 >  I believe that you mean Maniton (not Manitou) water bottles and while 
 > he did have then break from time to time he attributed it to a a cascading 
 > effect. The bottles (is placed in series) were able to withstand an 
 > EMF of 30,000 safely and stated that he thought the glass was 
 > excellent.
[snip]

What did I do wrong in my experiment?  

I am currently doing (very low budget) experiments with large
flash lamps, and would love to make large capacity, 5000 volt
capacitors by hooking lots of photoflash capacitors (which I
can get for almost free, from those disposable cameras) in 
series.  It seems that other people have built high voltage
caps by connecting lower-voltage caps in series.  What must be
done in order to avoid the over-voltage conditions that occur
when they charge/discharge unevenly?

Thanks for any help/suggestions you can give me!

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
                     Kevin D Christiansen
      User Interface Group - The University of Virginia
              "The best VR that pizza can buy" 
   kevin-at-virginia.edu    http://www.cs.virginia.edu/~kdc4n/

"The universe is composed of space, galaxies, and intergalactic
 dust.  Galaxies themselves are composed of space, stars, and
interstellar dust.  From the omnipresence of dust, we conclude
that nature abhors a vacuum and won't pick up a broom, either."
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