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More thoughts on protection chokes
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From: Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: Sunday, March 01, 1998 3:17 PM
To: Tesla List
Subject: Re: More thoughts on protection chokes
Hi Gary,
> From: Gary Lau 27-Feb-1998 1242 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
> Sent: Friday, February 27, 1998 11:56 AM
> To: tesla-at-pupman-dot-com
> Cc: lau-at-hdecad.ENET.dec-dot-com
> Subject: More thoughts on protection chokes
>
> >> >> From: Gary Lau 26-Feb-1998 0837 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
> >> >>[snip]
> >> >> What I do see in the PSpice simulations however is my choke and bypass
> >> >> cap resonating and ringing down as soon as the gap conducts. The choke
> >> >> ringdown current here peaks at tens of AMPS and is due to the bypass caps
> >> >> discharging through the chokes, gaps, and damping R's.
>
> >> >Yes, indeed!
> >> >For all of you 'empiricists' out there, this result of
> >> >Gary's simulation is quite REAL, and is not at all obvious
> >> >from looking at the schematic. It was this same choke
> >> >ringdown current that _melted_ a 4" x 20" RFC in my old
> >> >coil! PSPICE revealed that the choke current was not the
> >> >expected 5ADC, but rather 20A RMS AC worth of ringdown!
> >> >Changing from an L-C to an L-Diode arrangement fixed this.
>
> >> While these peak ringdown currents can be suprisingly high, the duty
> >> cycle is quite low and consequently, average power dissipation isn't that
> >> high. The maximum power dissipated in the R's due to ringdown,
> >> excluding 60 Hz charging I*I*R, is .5*C*V*V*BPS, where C is the bypass
> >> cap and BPS is gap breaks per second. I doubt this could account for
> >> melting your choke, or my 500 Ohm R's dissipating >100W (finger test, not
> >> simulation).
> >>
> >> Actually, assuming your choke's resistance was reasonably low, I can't
> >> imagine why they would get hot at all, being a mainly reactive component,
> >> unless there was internal arcing.
> >>
> >> I believe there is some other undiagnosed and unmodeled mechanism at
> >> work here.
>
> >From: Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
> >I don't. You can easily test this: stick an R in series with your
> >bypass cap. I guarantee it will get warm if not hot. It doesn't
> >matter if the duty cycle is low. Peak currents are the key. Heating
> >is due to I^2.R and resistors do not get rid of heat quickly.
> >
> >Malcolm
>
> What? Could our definitions of "duty cycle" be different? Resistor
> heating is _definately_ dependant upon duty cycle of the ringdown,
> relative to the interval between bangs. In my simulations, the choke
> current peaked at ~20-30A and rang down to a neglegable value in about a
> dozen cycles of the choke/bypass cap's Fres. This ringdown period
> constitutes only 2-3% (duty cycle) of the 8.33msec period between bangs.
Exactly what I'm talking about as far as duty cycle goes. I agree
that heating has a strong dependency on the duty cycle but I was
attempting to point out that you will still get heating with a low
duty cycle because of the high peak currents involved. Resistors
lose heat a lot less effectively than they gain it from electrical
heating - at least all the ones I've ever used do.
> The other 97-98% of the time had only the 60 Hz charging current passing
> through the R & choke. Peak currents don't matter, the RMS current over
> the bang interval does.
Do you really need to resort to unexplained phenomena to explain
I^2.R heating? Have you tried putting resistors in series with your
bypass caps to check this out?
<snip>
> Ahhh. I forgot that you're working with DC and that your "bypass" caps
> are actually filter caps and store MUCH more energy that the few hundred
> pF bypass caps typically used with AC-based TC's. If you dissipated the
> energy stored in your .047 uF caps on each bang in your chokes and
> damping resistors, that would definately result in something toasting, a
> total waste of energy. That's more power than my NST-based TC uses in total!
>
> Gary Lau
> Waltham, MA USA
The chokes also store energy (or am I simply stating the obvious?).
Malcolm
>
>