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More thoughts on protection chokes




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From:  Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent:  Thursday, February 26, 1998 1:43 PM
To:  Tesla List
Subject:  Re: More thoughts on protection chokes

Hi Gary,

> From:  Gary Lau  26-Feb-1998 0837 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
> Sent:  Thursday, February 26, 1998 7:52 AM
> To:  tesla-at-pupman-dot-com
> Cc:  lau-at-hdecad.ENET.dec-dot-com
> Subject:  More thoughts on protection chokes
> 
<snip>
> >Your addition of an external C3 improves upon this, but 
> >the task of de-Qing the filter still remains.  My guess is
> >that the required de-Qing resistances will dissipate 
> >quite a bit of heat.
> 
> Are you suggesting that the high heat is due to requiring a high R value
> to de-Q the chokes, heating due to 60 Hz charging current I*I*R, or the
> R's dissipating the ringing of the chokes?
> 
> >>[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.

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