Re: Safety gap and first light
I would never say that a cap safety gap is needed, unless you are using
super-expensive brand-new metal-cased Maxwell pulse caps. The other
safety gaps should be fine here. Your cap is insufficiently rated, and
your main gap is too wide! There is no other way your cap safety gap
would fire unless this is the case. For 15kV you need *AT LEAST* two
layers of 60 mil poly, perhaps 2 caps in series or even 2 of two layers
of 40 mil. BEFORE YOU DO ANYTHING with your primary circuit make sure
your main gap fires without anything else connected, ie just the xfrmr
and the gap, at about 90% of max setting on variac. This is the only
way to determine the total gap spacing you will need. Also, when tuning,
close the gap down to, at the widest, 1/4 of this - then you can tune OK
at low power and not worry about overvolting your cap.
If your cap is a flat layer type, make sure
you have left plenty of margin on every side to avoid flashovers.
Where the poles leave the sides of the dielectric the next foil *must*
be spaced at least 2" in from the edge of the previous, opposing one.
Otherwise at even 12kV you are asking for trouble. Also make sure that
you have rounded the edges on all foils - time-consuming but worth it.
Either get rebuilding or do a MMC like others who got fed up of
chopping up umpteen pieces of poly and foil! (ouch, those scissor
It *will* work - congratulations on 1st light!
On Sun, 13 Jun 1999 15:03:27 Tesla List wrote:
> Original Poster: "Dr. Resonance" <Dr.Resonance-at-next-wave-dot-net>
> to: Bill
> It may be to high, but if you don't have enough turns on you will never
> know. If you have too many, no problem --- just tap at the point of best
> resonance, but if you have to few you could be switching caps and fiddling
> with everything and never discover the problem. Too many is always good to
> start with --- make it look nice AFTER you have found the best resonance by
> replacing with copper tubing, strap, etc with the correct number of turns
> plus approx 1-2 additional.
> Glad you discovered your wiring error. Did you do your cap under vacuum
> (evacuate and fill with oil)? If not, then an air bubble probably got it.
> -----Original Message-----
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
> Date: Saturday, June 12, 1999 7:54 PM
> Subject: Re: Safety gap and first light
> >Original Poster: Bill Fuller <wfuller-at-home-dot-com>
> >Dr. Resonance,
> >Well it turns out that part of the problem was operator error. I had wired
> >spark gap in series with the cap and primary like this: NST output-spark
> >gap-cap-primary coil-NST output. After connecting the NST across the spark
> >everything changed!!!! I immediately got 15" streamers from several points
> >the toroid. The safety gap on the cap was still firing quite a bit so I
> >some tuning and improved things a little. Since I couldn't get the variac
> >~60 volts without the safety gap firing I figured I could move the safety
> >to 1" and increase the voltage. At about 80 volts I BLEW THE CAP! BIG
> >learned.... The cap safety gap IS NEEDED. I'm using a single cap with 1/16"
> >LDPE layers. I had been cautioned by several posts in the past about rating
> >cap too aggressively but excitement got the best of me. Dis-assembly
> revealed a
> >1/4" hole clean through the LDPE.
> >You mentioned going to 22-24 turns. Tesla Designer calculates ~140 KHZ for
> >toroid/secondary, 120 KHz for the cap/primary with 12 turns (140 KHZ at
> >turns). 22-24 turns seems way too high?
> >Thanks for the Help,
> >Tesla List wrote:
> >> Original Poster: "Dr. Resonance" <Dr.Resonance-at-next-wave-dot-net>
> >> to: Bill
> >> Not enough primary for initial test with this large toroid. Use some
> >> 12 AWG wire and 22-24 total turns. Tap as necessary until you find the
> >> resonant point. Go back to the pretty copper tubing later --- after it
> >> works.
> >> Regards,
> >> Dr.Resonance
> >> -----Original Message-----
> >> From: Tesla List <tesla-at-pupman-dot-com>
> >> To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
> >> Date: Friday, June 11, 1999 3:29 PM
> >> Subject: Safty cap and first light
> >> >Original Poster: Bill Fuller <wfuller-at-home-dot-com>
> >> >
> >> >Well, after many months of seaching for parts and construction, I fired
> >> >up my coil last night for the first time. No arcs! I'm not sure where I
> >> >got the idea, but I have a safety gap across the primary cap (~0.6"). As
> >> >soon as my RQ air blast gap starts to fire(90v on the variac), so does
> >> >the safety gap. The RQ gap is set at 0.3". No amount of tuning fixes
> >> >the problem. I did a search in the list archives and it seems unclear if
> >> >this safety gap is needed. (I already have a safety gap and RC filter on
> >> >the NST's). Any idea's? Here's the coil spec's:
> >> >
> >> >15KV NST's at 90 mA (plan to use 150mA)
> >> >8" dia secondary 32" long 19 ga wire (29mH)
> >> >28" x 7" toroid
> >> >Flat plate primary cap 0.026 uF
> >> >10 deg. primary inductor 12 turns 1/4 tubing, 1/4" spacing (66uH)
> >> >
> >> >I know the cap is on the large side for 90 mA. I don't think this is an
> >> >issue though.
> >> >
> >> >Thanks for your help, can't wait to get back to it tomorrow!
> >> >
> >> >Bill Fuller
> >> >
> >> >