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More thoughts on protection chokes
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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.
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.
>> 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.
>From: Greg Leyh [SMTP:lod-at-pacbell-dot-net]
>Here's how the choke melted in my ill-fated original filter:
>
>R2 = ~4 ohm (at DC)
>L2 = 4 mH
>C3 = 0.047uF
>VDC = 26kV
>
>The peak ringdown current when the gap fired was about 90A,
>and the RMS function in PSPICE averaged it to about 20 RMS.
>I*I*Rchoke = 1600W
>With 15J sitting in the filter cap and a BPS of 300, approx
>4500 VARs were available for filter cooking. I believe that
>at _least_ 1/4 of the available VARs were doing just that.
>
>With diodes taking the place of the filter cap, the filter choke
>now has scarcely a noticable temperature rise.
>
>-GL
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