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Coil form materials for HF tesla coils

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LAN_403

WaveRider

Wed Oct 11 2006, 09:53AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Hi all,
Something strange happened today... The inside (and just the inside) of my coil form for the 5 MHz coil went up in flames.

Granted, this was a bog-standard cheap gray PVC drain pipe, but I would have imagined its electrical properties to be fairly good. In fact (as I mention in the Projects thread), the windings themselves were not harmed.

Has this happened to anyone before? What materials are you using for coil forms at high frequency (> several MHz) and powers greater than 100-200W?

Reaching

Wed Oct 11 2006, 10:28AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

you even dont need 5mhz to set up a coilform in flames. i had 2 failures which were exactly the same with a 500khz and a 280khz (a drsstc and a sstc) but the higher the frequency the bigger are the chances to burn up the secondary. for very high frequency coils you should use glass or ceramic formers or totally formerless coils. some pvc sorts and other poly materials can heat up at a certain frequency

Plasmaniac

Wed Oct 11 2006, 10:36AM

Registered Member #206 Joined: Sat Feb 18 2006, 03:17PM
Location:
Posts: 72

I'm using a simple glass jar for my 20MHz VTTC's resonant coil's former. I'd prefer a ceramic tube but I dunno where to get them...

WaveRider

Wed Oct 11 2006, 11:26AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Plasmaniac: That's a great idea.. I should use glass myself... A wine-bottle perhaps.......

Marko

Wed Oct 11 2006, 11:48AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

That's wierd: I always considered PVC great insulator for HV, although biggest problem with CW is that secondaries used to get hot enough to soften the PVC and deform.
Turns would start buldging into each other until they finally short and blow everything up.

Your pipe must have had some kind of defect if it arced as such O_O
It's color may have been conductive, or you jammed something conductive between windings (once a tiny particle of carbonisation starts it becomes conductive and spreads the mess around)

I never had such failures except by human error :p

I fixed this by pouring poliester resin inside entire tube (wich makes bigger coils heavy and bulky, but they usually disipate heat better and don't need it) wich resists much higher temperatures than PVC.

After seeing some people using paper forms I tried it too, and it works great.

I also vote for teflon, glass, thick PE/PP pipes and similar stuff.

ragnar

Wed Oct 11 2006, 11:53AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

WaveRider, with my quadraphonic system (~3.55MHz, not 4MHz as it claims on the front panel, hehehe) I could not use a PVC former... If the system ran for any longer than ~20 minutes, the formers got so hot they drooped and wilted -- once a few turns on the secondary shorted, it was as good as gone.

This is unscientific, but a technique I use for comparing the copper losses versus the dielectric losses is to run the coil for a few minutes, then feel the OUTSIDE (copper) of the former... then feel the INSIDE of it.

Even with my thick (1/8" nominal wall thickness) teflon tubes, the dielectric losses were practically nil at 3.55MHz, so I'll be using the same teflon PFA tube for my 12MHz coil. If you look around on the net for datasheets you'll find the loss tangent for various permutations of PVC is something like two orders of magnitude worse than that of various permutations of teflon... or something ^^

WaveRider

Wed Oct 11 2006, 11:56AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Firkragg: That's what's so weird. I thought PVC was a good material too..

My secondary was made with thick gauge wire...altho' it got slightly warm after operating for a while, it never got really hot. Also, my failure was on the inside of the tube. The windings were fine.

There may have been a metal particle or some other imperfection inside of the pvc that may have started the carbonisation process... I'll give the PVC tube another go.... I'll buy one of those 1.5 liter bottles of red wine.....to use the bottle for the form if my next PVC tube blows up like this one....

HV Enthusiast

Wed Oct 11 2006, 12:01PM

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

wrote ...

Has this happened to anyone before? What materials are you using for coil forms at high frequency (> several MHz) and powers greater than 100-200W?

I have had this problem happen to lots of coils. Has little to do with high frequency and mostly to do with the thin wire used.

There are two failure mechanisms for this:

1. Coil not running at tune. This creates maximum voltage node other than at the top of the winding and kills the magnet wire where the node is existing. I have done this many times.

2. Sometimes, an unforseen kink in the magnet wire, especially if you are using thin wire, will cause a breakdown in the PVC. I've had this occur also.

WaveRider

Wed Oct 11 2006, 12:17PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

EVR: I'm not so sure that these theories fit. Nice, thick 1.5 sq mm wire was used...even with 200W drive, only ever got mildly warm (at the base of the coil..wher max current exists). Wire has plastic insulation (it's that pvc insulated wire that you use for wiring your house, etc.)and there were no kinks...and as I mentioned before, the wire insulation after the tube failure was still perfectly intact...no signs of melting or burning. Only this funky discharge branching pattern of carbonisation inside the tube.

Also, the coil seems to be at resonance. Good fat arc was at the top before suddenly, flames and smoke shot out of the tube. Frequency was 4.73MHz. Low power measurements with an electric field probe showed the maximum to be at the top of the coil in this frequency range... Arc loading never shifts the resonant frequency more than a few kHz, so I don't imagine the fundamental mode structure to be much different..

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