Re: 6" Coil Problems

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Jonathon,

At 09:22 PM 9/23/2002 -0400, you wrote:
>Hello everyone,  (forgive me for this being long, there is a lot to include 
>:-\   .)
>      
>       I've had my coil put away for a while, and recently brought it out for 
>some tests. 
>       If you don't know, my coil is as follows:
>
>- 12/60 NST
>- Terry Filter
>- Multi - Pipe Static Spark Gap   ( 12 gaps, each at .025" )
>- .017 uF Geek Cap
>- 10 turn, .25" Cu pipe primary coil
>- 6" * 30" (Wound) secondary coil, #20 AWG wire.
>- Controller consists of just a switch/light combo, and one outlet.
>
>Max sparks to date were 22" Ok, for my first coil, but a 12/60 should deliver 
>more. The problem during the first runs was that the spark gap would only 
>fire at 3 or less gaps! That is only .075 total gap! And never once did the 
>properly set safety gaps fire.

A 12/60 coil can do 45 inch arcs according to John's formula if everything
is perfect.

>
>Yesterday, I had just my NST, filter and spark gap hooked up, to see how many 
>gaps that would let me use.  Suddenly there was smoke coming from somewhere, 
>which later turned out to be a MOV in the filter. :-( While investigating the 
>smoke, I noticed a piece of what I think is soldier, stuck under the leads of 
>the MOV array, making contact with at least two of the MOVs. I removed it and 
>tryed again. I guess MOVs fail closed, because I let the smoke out of a 
>second MOV.

If the MOVs really overheated bad, you will have to replace all the MOVs on
that leg.  MOVs limit voltage well and recover, but it sounds like they may
have been pushed to the point where they start to short out permanently.
The leads are soldered inside them.  If the solder melts, it flows inside
the move and basically shorts it out.

>
>I believe the soldier shorted out one half of the MOVs, which lowered the 
>voltage at which they clamp. This level was under half of my NSTs voltage, so 
>they were sitting there, dead short to ground. I guess the 60mA for a while 
>burned it up.  When the one failed, the same scenario occured, and the second 
>MOV failed. Does that make sense to every one?

Yes,  The MOVs on that leg have probably all shorted or now have vastly
reduced voltage.

>
>After that I connected the NST straight to the Spark gap, with no filter, cap 
>or anything. It let me use all 11 or 12 (I forget the total) gaps. Pleasing, 
>that my NST was still OK, I took out the actual filter, but left the power 
>resistors, and the safety gaps in circuit. That gave me max of 7 gaps, any 
>more and the safety gaps would fire. Sounds good to me.

Sounds fine.

>
>Now that makes me wonder, how long has that short been in the filter? Could 
>that have been giving me the less-than-satisfactory performance before? 

The MOVs would not suck up power very long.  If your runs were short, it's
possible they would not obviously burn up.  However, the MOVs will not run
very long at all if there were a serious problem.

>I 
>would like to try the coil now, but I am a bit worried about the filter not 
>being in there. I still have the safety gaps (which seem to work now) and the 
>power resistors in place, and I know that some people use NSTs with out any 
>protection at all. But hey, I'm 14, and poor; I have built a Tesla Coil and 
>just bought an electric guitar, so I dont feel like burning up this NST. 

The safety gaps will help a lot.  You are "90%" safe ;-)  Depends on how
lucky you feel...  Normally the MOVs don't do anything.  They are just a
last chance NST saving safety backup when everything else goes wrong.

>
>Would it be ok to run the coil the way it is?
>
>I thank you for reading this, and hope to get this working right...

Cheers,

	Terry

>
>---------------------------------------
>Jonathon Reinhart
>hot-streamer-dot-com/jonathon
>