Re: [TCML] Pulse Capacitors

[Nicholas Goble <ngoble@xxxxxxx>]

So for someone who is building their own capacitors, what do I have to do
differently to construct a pulse capacitor.  I mean a plate capacitor is a
plate capacitor.  Does the dielectric also determine the rate it can
discharge?  My understanding of capacitors was that the discharging current,
with respect to time was determined by the following equation:

I(t) = (-Q/RC) * (e^(-t/RC))
Where e is the natural logarithmic value and Q = C * V.  Am I right or just
way off?

I guess I¹m just trying to get a feel for how pulse capacitors work and how
to design capacitors.  Take this hypothetical scenario:

If I wanted to discharge 10amps in a thousandth of a second.  t=.001 and
I(t)=10.  I¹d just pick a voltage such as 9kV, which would tell me that I¹d
need an the relationship between my resistance and capacitance would be the
following:

C = -.001 / (R * ln((10R)/9000))

So a suitable capacitance would be .003F with a resistance of .025Ohms.

With this reasoning, if it¹s all correct, I¹d be able to discharge 10amps in
a thousandth of a second if I wired a .025Ohm resistor in series with a
.003F capacitor across a potential difference of 9kV, charged the capacitor
up, and discharged it.  Right?  So then why would I need a pulse capacitor
for that?

Nicholas Goble



On 2/27/09 7:50 PM, "bartb" <bartb@xxxxxxxxxxxxxxxx> wrote:

> Hi Nicholas,
> 
> A pulse cap is a high energy discharge capacitor. So we use them in the
> tank circuit. It is the capacitor that is in series with the primary
> coil inductor. The transformer charges the capacitor while the gap is
> not conducting. When the gap conducts (rotary, trigger, or static gaps),
> the energy stored in the pulse cap dumps it's energy across the primary
> coil which is of low impedance. So a large current is formed and thus
> the electromagnet field that envelopes and couples the secondary coil.
> Because of the low impedance at discharge, the decay rate will be very
> quick and the cap needs to be rated for this pulse energy (di/dt) which
> is simply the change in current over time (as well as the repetitive
> rate at which the gap will conduct). Because coils operating in the low
> to mid kHz frequencies, some RF losses can affect heating of the
> dielectric. Therefore, decent repetitive high energy pulse caps
> typically use polypropylene, polystyrene, etc.. (low loss at frequency).
> Regardless of professional pulse caps or even homebrew MMC caps, the use
> of high energy pulse caps is highly recommended in the capacitor defined
> in the primary circuit.
> 
> Take care,
> Bart
> 
> Nicholas Goble wrote:
>> Hi everyone.  I've heard some talk about using pulse capacitors in a tesla
>> coil and am a little confused as to how they are used, what they do, and how
>> they do it.  Sorry if that's a little vague, but can anyone give me an
>> overview of what pulse capacitors are?
>> 
>> Nicholas Goble
>> 
>> 
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