Re: [TCML] capacitor charge time?

[Steve Ward <steve.ward@xxxxxxxxx>]

In the case of a constant current source (or even otherwise as long as you
can do some calculus)

Q = C*V = I*t

Assuming the charging is limited purely by R is not valid in most tesla coil
charging systems.  Generally its limited by some inductive impedance at
60hz.  The impact of this impedance on charging the capacitor is
non-obvious, particularly as the capacitor is discharged at various phases
along the 60hz mains cycle.  Using spice would be a good option to gain more
insight.  Most tesla coilers dont bother to dig this deep and just pick a
ballast (inductor) that gets their average charging current within an
acceptable value.

To get at your original idea... the resonant behavior of the tesla coil is
damped and long gone before you really even start to charge up the capacitor
again.  You are dealing with time scales and power levels that are on very
different magnitudes.  The capacitor charging is in the 10's of mA and mS
range, while the resonant discharge is in the 100s of Amps and 10's of uS
range.  Interestingly enough, the DRSSTC is a solid-state approach to
driving a TC where the tank capacitor is always in a state of being "charged
up" by a special power source that can keep up with the 100's of Amps and
can switch polarities along with the resonant frequency of the system,
somewhat along the lines of your idea in question.

Steve

On Thu, Oct 15, 2009 at 6:26 PM, <jhowson4@xxxxxxxxxxx> wrote:

> Hey guys.
> I was explaining the tesla coil to one of my physics friends (we are both
> sophmores) and he asked a question that i could not answer.
>
> I was asked if it would be a good idea to match the time required to charge
> a capacitor to (1/4) of the resonant frequencies period. with the general
> idea that the capacitor would be completely charged when the resonant wave
> form was at a max or a min, thus maximizing efficiency.
>
> sounds like a good idea to me.
>
> But when we went to try and do an example calculation we realized that our
> standard RC capacitor charging equation would not work because our current
> output from the transformer was fixed and would only decrease after the cap
> charging current became less than or equal to our source current.
>
> and this spawns my question. how does one calculate the charge time of a
> capacitor with a fixed source voltage and current.
>
> does the capacitor ignore the fact that the transformer is current limited
> and draw that initial huge current anyway. or is there some other special
> equation that accounts for a limited source current and fixed source
> voltage.
> or are we both just missing something.
>
>
> t=-CR ln(IR/V) derived from I=V/R e^(-t/RC)
>
>
> thanks for the help
> Jay Howson
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