chokes (and BIG Resistors)

From:  Gary Lau  25-May-1998 1526 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent:  Monday, May 25, 1998 2:59 PM
To:  tesla-at-pupman-dot-com
Subject:  Re: chokes (and BIG Resistors)

>>Gary Lau wrote:
>> Then a BIG resistor,
>> like 500-1.5K, 50-100W from each NST terminal to the main gap.  I don't
>> know why they dissipate so much power but they do, at least if your main
>> cap is well matched to the NST and is mains-resonant to draw maximum
>> power.

>From: Jim Lux <jimlux-at-earthlink-dot-net>
>In a series RC charging system, the amount of energy dissipated in the R is
>the same as the energy stored in the C. So, if you have a 450 VA NST
>(typical), then you are going to be putting half that in the C and half
>into the R, hence 200+ Watts dissipation in the R.  The fact that the NST
>is really an inductive source actually reduces the dissipation in the R
>somewhat, but this should give you the general idea. 

Hold on.  Not true.  "Half" of the energy goes into the R?  Shouldn't the
value of the R factor into this somehow?  I think a first-order
approximation of power would be I*I*R.  Assuming a 750 Ohm R and 60mA
NST, that would be 2.7 Watts.  This could be doubled if using resonant
charging.  Say 5.4 Watts.

A second-order effect is that, assuming one gap firing per half-cycle,
the energy stored in the bypass caps is also dissipated in the resistor.
Assuming a 15KV NST and a 500 pF bypass cap per side, charging up to
15KV*1.414/2 =10605 V, energy is .5 * C * V*V = .5 * 500E-12 * 10605 *
10605 = .028 Watt-seconds, times 120 bangs per second = 3.37 Watts.

So, I can account for 5.4 plus 3.4 = 8.8 Watts per resistor.  This does
not explain why my 750 Ohm, 50 Watt resistors are burning up though.

>Seems to me, it would be better to build a LC low pass network as a filter.
>Which is what a choke and cap combination is. The problem is that your
>interference source is really broadband (a series of impulses) and it can
>excite all sorts of parasitic resonances in the filter components.

True.  And it's not the parasitics but the L and C themselves that will
be excited to resonance, and that's why I avoid them.

Gary Lau
Waltham, MA USA