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Re: Series vs parallel R's with chokes




From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Wednesday, September 03, 1997 1:50 AM
To: 	Tesla List
Subject: 	Re: Series vs parallel R's with chokes

Tesla List wrote:
> 
> From:   Gary Lau, DTN 223-1360, Maynard MA PKO3-2 pole 31D  02-Sep-1997 1611[SMTP:lau-at-hdecad.ENET.dec-dot-com]
> Sent:   Tuesday, September 02, 1997 3:21 PM
> To:     tesla-at-pupman-dot-com
> Cc:     lau-at-hdecad.ENET.dec-dot-com
> Subject:        Series vs parallel R's with chokes
> 
> There has been much recent discussion on different HV xfmr protection
> networks.  I recently posted my two cents worth, now it appears I may
> have done so prematurely:
> 
> > Agreed in full, but incomplete.  I was using a 15KV/30ma NST with 9 mH
> > chokes, 600pF caps, and safety gaps on each side.  As such, I was seeing
> > terrible (2") arcing between my tank circuit and ground (like the bottom
> > of my secondary and gap cooling fan).  Malcolm Watts on this list pointed
> > out that my chokes were being shock-excited by the spark gap, just as my
> > tank circuit was, and were ringing at their self resonant frequency.
> > The solution was to add some high power resistors in series with the
> > chokes (I used 3K/50W).  This fixed the arcing.  I now don't believe a
> > choke/cap protection circuit should be used without series resistance.
> >
> > Gary Lau
> > Waltham, MA
> 
> It began by adding some larger, 9mH chokes to my TC, which caused
> the arcing problems described above.  I added some 4.2K/5W rewsistors in
> series with my chokes, only to immediately smoke them.  Believing that
> the high power was in part due to the 60Hz charging current passing
> through the resistors, I found some 3K/15W resistors and put them in
> PARALLEL with my chokes.  This definately solved the arcing problem,
> things ran great, though even these R's got pretty hot.
> 
> I understood that placing the R's in parallel with the chokes will
> accomplish the desired damping, but at the same time will lower the
> effective reactance of the choke, reducing it's filtering ability.  But I
> didn't put them in series since I feared the added power dissipation from
> the 60Hz charging current would toast these R's too.  I reasoned that the
> same degree of damping would be accomplished by placing them in series,
> plus I'd be getting the full benefit of my choke's reactance, once I got
> some higher power R's, so I sent my post without actually trying the
> series R's (shame on me!).
> 
> I just installed some 3K/50W resistors in series with my chokes (as
> popular wisdom would have it), and now I'm back to my original tank
> circuit arcing problem!  I guess I'm going to have to go back to my
> parallel R's, even though I know it's a serious compromise with regards
> to the low pass filter effectiveness.
> 
> My configuration has my static RQ gap connected to my chokes/R's.  Can
> anyone speculate why my chokes are still ringing or what to do about it?
> 
> Embarrassed and hoping I've not sent anyone off in the wrong direction,
> Gary Lau
> Waltham, MA

Gary,

This definately doesn't sound right, especially with only a 15 KV 30 MA
source - something else is probably going on here. How many total turns
on your primary, and what turn are you tapped at? Is the "fixed" end of
your primary at the innermost or outermost turn? Autotransformer action
can elevate the "free" end of the primary (after the tap point) to quite
high RF voltages, and is one reason to make the "fixed" end the
innermost turn. Are your safety gaps firing at all? Finally, are your
neon center-tap (case), safety gap ground, resonator base, and
strike-rail all firmly tied to a good RF ground?

-- Bert --