Series vs parallel R's with chokes
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
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,