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Re: [TCML] newer tesla coil parts (split Terry filter resistors)

Gary, DC -

Just an observation, for what it's worth:
My original Terry Filter had 3-ball spark gaps at both the "input" side (directly across the NSTs) and also at the "output" side (driving the tank circuit). Resistors were 1K 100W conventional wire-wound, bypass capacitors were 500pF ceramic doorknobs. No MOVs were used.
On a 6" X 30" spark gap coil powered by (2) paralleled 15/60 NSTs, occasionally there would be sporadic arcing at the input gaps, and sometimes sporadic arcing at the "output" gaps, depending on the power level (variac setting) and if the coil was generating air streamers or was arcing to ground. I don't think I ever saw arcing at both input and output gaps simultaneously. 

I can offer no explanation why this would occur.

Regards,
Herr Zapp
----- Original Message ----- From: "Lau, Gary" <Gary.Lau@xxxxxx> 
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Thursday, November 12, 2009 3:23 PM
Subject: RE: [TCML] newer tesla coil parts (split Terry filter resistors)


Hi DC,
Yes, I agree that the safety gaps appear to be redundant if one also uses the prescribed number of MOV's. However, even with a proper Terry filter using a properly set mid-point grounded safety gap, I still see these gaps fire on rare occasions following a primary strike or SRSG misfire.
The MOV's are great because they keep us honest. Short of disconnecting them, one can't open them up just a bit more. But they are relatively fragile and won't endure a continued overvoltage for long before they fail (fortunately in the shorted state). The ball gaps cost nothing, take up no significant real estate, and just look nice. I see no reason to suggest elimination them. If we were all perfectly honest and meticulous in setting the safety gaps, I think the MOV's could be eliminated, but human nature being what it is, would not suggest this. 

Regards, Gary Lau
MA, USA


-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf Of DC Cox
Sent: Thursday, November 12, 2009 5:38 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] newer tesla coil parts (split Terry filter resistors)

Gary:

Interesting question.

I was also wondering the precise requirement for the spark gap balls.  In
the days before varisters came along we definitely used them. But, now, the 
varisters should shunt any overpotentials to ground directly, essentially,
make the spark gap balls unnecessary as there should not be any high
potentials present to fire the gaps.

I would assume this might be of assistance in the rare event of horrible
mis-tuning, but with JAVATC this has almost become a thing of the past.  I
usually add 1/4 turn beyond JAVATC predicted pri tap point, and it's dead-on 
at full power levels for most small and medium size coils.

Comments on this idea (eliminating the spark gaps) welcome.

Dr. Resonance




On Thu, Nov 12, 2009 at 9:22 AM, Lau, Gary <Gary.Lau@xxxxxx> wrote:
> I'm not sure I see the value in having the caps & varistors at the > midpoint > of the resistors. The R-C filter will only see one half of the > resistance, > so its low-pass frequency will be twice as high. The only benefit I see > to > this is the bypass caps will be cushioned against the discharge current > when 
> the safety gaps fire.  But given that this should not be a frequent
> occurrence, I'm not sure this is worth compromising the R-C filter's > cutoff 
> frequency for.
>
> While splitting the 1K between two 500Ohm units is fine, I think it is > bad 
> advice to suggest placing the caps at the midpoint.
>
> Regards, Gary Lau
> MA, USA
>
> > -----Original Message-----
> > From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> > Behalf Of Jim Mora
> > Sent: Tuesday, November 10, 2009 3:30 AM
> > To: 'Tesla Coil Mailing List'
> > Subject: RE: [TCML] newer tesla coil parts
> >
> > Hi DC, All,
> >
> > I agree - nice, thoughtful construction! It never even occurred to me
> split
> > the halves.
> > We will all look forward to more pictures of our art form at work!
> >
> > Jim Mora
> >
> > -----Original Message-----
> > From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> > Behalf
> > Of DC Cox
> > Sent: Monday, November 09, 2009 2:46 PM
> > To: Tesla Coil Mailing List
> > Subject: Re: [TCML] newer tesla coil parts
> >
> > Nice construction work.
> >
> > Both Digi-Key and Mouser usually stock them.  I split mine into two 50
> watt,
> > 500 Ohm resistors putting one ahead and one behind the common
> varister/cap
> > connections.
> >
> > D.C. Cox
> >
> >
> >
> >
> > On Mon, Nov 9, 2009 at 10:53 AM, Shaun Epp <scepp@xxxxxxx> wrote:
> >
> > > Here are some pictures of my new NST filter.  I ordered parts from
> digikey
> > > this time and used the correct values and MOVs( before I was using
> 3Kohm
> > > resistors and door knob caps). It's built so that I can change from > > > 7 
> > MOVs
> > > per side to 8. After reading posts on this list, I think I'm going > > > to 
> use
> > > all 8 for my 12Kv, 60mA NST because it puts out more that 12Kv at > > > 120V 
> in
> > > and I'm using a variac. I'm using 50 watt resistors for the 1Kohms, > > > I 
> > can't
> > > get ahold of the 100w versions, does anyone on the list have some > > > for 
> > sale?
> > >
> > > http://i487.photobucket.com/albums/rr239/growl99/NSTfilterfront.jpg
> > >
> > > http://i487.photobucket.com/albums/rr239/growl99/NSTfilterback.jpg
> > >
> > >
> > > Question:
> > > My capacitor bank is currently on 2 boards and I'm going to make a > > > 3rd > > > board of CD 942 caps (14 caps 0.15uf in series spaced). What are > > > other > > > coilers using for board to board spacers?, I want to stack them one > > > on 
> top
> > > of the other, and legs to keep them off of the floor,  right now I'm
> using
> > > 1/4 inch nylon bolts with heat shrink tubing over them to strengthen
> them.
> > >
> > > thanks,
> > >
> > > Shaun Epp
> > >
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