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Secondary keeps burning up...

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NicktheGeek

Tue Feb 19 2008, 09:43PM

Registered Member #1335 Joined: Tue Feb 19 2008, 09:14PM
Location: Woking, Surrey, UK
Posts: 6

Hi guys,

I'm a NOOB to this forum and have been coiling for about 9 months. What a great, if somewhat anti-social hobby !
I'm looking for some advice on a new problem I have encountered:

I recently upgraded the 10Kv 23mA OBIT i was using to power my medium sized coil to a NST. The power has jumped from 230VA to 400VA. Since then the secondary has started to arc internally. very severly and 'burn up'. The coil had proved 100% reliable prior to the upgrade. The gap was set to fire at about 3-4Kv. Is this purely a case of overpowering the coil ? The specs are as follows:

30awg (.25mm) double insulated magnet wire wound on a 2.2" (55mm) PVC former, sealed with polyurethane varnish, apporx 900 turns.
10.44nF MMC tanks cap.
Dual gap static gap (fan cooled).
Primary tapped at 8 turns. (11 turns of 16awg (1.25mm) solid copper wire 1/4" (5mm) spacing.
NST is 8Kv 50mA.
'Proper' RF ground installed, base of secondary and strike rail is connected to this ground.
It was producing bright, 16" streamers before death with the new NST.

I have two other coils, one 2" (portable) and one 4", they both work faultlessly (the big one does 40" streamers without any stress from a 10Kv 100mA supply).
I'd like to keep the existing base and primary and just construct a more suitable secondary.
Is my assumption regarding overpowering correct ? Or am i missing something else ?
Should i just use a smaller transformer ?

Thanks in advance,

Nick

Any advice would be greatly appreciated.

PS if you 'youtube' the search string 'tesla nick' you'll find 3 vids of my coils.

Here's a link to my big coil:

[Edit: Double post]

Coronafix

Tue Feb 19 2008, 10:26PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

If the coil isn't completely dead then you could try sanding back where it has arced internally, (all the black stuff) and coating the inside with varnish or whatever you use.
My understanding is that this occurs because the coupling of the coils is too tight,
but this usually results in racing arcs on the outside of the secondary.
That being said, I happily powered a 55mm secondary with about 1300 turns with a 15kV60mA NST.
I just had to raise the secondary until there was no more racing sparks.
Also check the grounding, it may also do it if not properly grounded, and make sure you don't have any metal protruding into the inside of the secondary.

GimpyJoe

Wed Feb 20 2008, 01:53PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

Make sure you put a cap on the top. Otherwise sparks will track down the inside to ground.

HV Enthusiast

Thu Feb 21 2008, 01:04AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Sorry for the bad news, but even worst news is yet to come. . .

If the winding itself is whats arcing and burning up, take that secondary and trash it. You can attempt to salvage by splicing, but its already weakened at that point and the solder joint will become a discontinuity which can lead to more failure.

What likely happened is that tuning was off, or you had a previous kink in the wire, so the high voltage either formed a "peak" at the point (bad tuning) or the kink just burned up.

NicktheGeek

Thu Feb 21 2008, 01:34AM

Registered Member #1335 Joined: Tue Feb 19 2008, 09:14PM
Location: Woking, Surrey, UK
Posts: 6

Thanks for all your replies guys, what I had failed to mention is that this is the third secondary in a row that has done exactly the same thing (internal carbon track almost the length of the secondary winding). The three secondarys were each tuned slightly differently (various lengths) and all produced nice long, bright streamers. My latest idea is to make a fouth, tuned slightly higher than the rest and reduce the 'bang' from the primary circuit (smaller cap, less turns), thus reducing the overall power throughput.

The coupling was quite loose (apporx 1.5" above the primary), with no racing sparks. The carbon tracks are very severe, one causing the PVC former to crack...

I'm now wondering if the system might be over-coupled and causing hot spots and general stress without causing racing sparks..? The coupling was set to approximately the same as when I was using the old transformer/cap configuration which never caused these problems.

GimpyJoe

Thu Feb 21 2008, 02:02AM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

Is the top capped?

NicktheGeek

Thu Feb 21 2008, 02:22AM

Registered Member #1335 Joined: Tue Feb 19 2008, 09:14PM
Location: Woking, Surrey, UK
Posts: 6

Yep, capped and sealed, i create a contact on the cap by sandwiching the bare end of the secondary wire, a coin and aluminium tape, which is then aluminium taped to the cap. The toroid then sits on top of said contact.

Andyman

Thu Feb 21 2008, 06:38PM

Registered Member #1083 Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256

If there's dust or dirt on the inside of the pipe, that makes an excellent conductor for high voltage, high freq. You should try to sand the inside of the coil form up to a super fine grit (400) and then use some spray polyurethane varnish and coat the inside.
I think I read somewhere about a guy who filled his secondary with oil for insulation. It would sure make the thing heavy, but it would serve your purpose. Of course, there would be extra dielectric losses.(I think)
I've also heard that it's good to have your toroid a few inches above the top of the coil and not resting directly on top.

NicktheGeek

Sat Feb 23 2008, 05:00PM

Registered Member #1335 Joined: Tue Feb 19 2008, 09:14PM
Location: Woking, Surrey, UK
Posts: 6

Thanks to everyone's advice, i have now constructed a new secondary paying more attention to cleanliness and sealing. The topload has been raised by about 2", I also installed an internal baffle 'baffle' seal about 1/2 way up the former. Reduced the couping a little and finally I re-routed the grounding system. So far so good, nice hot 15" streamers without failure (so far).

So thanks again..

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