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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Hi all. The availability of cardboard tubes from the inner pieces of some packaging (Christmas and all that) got me thinking.
Could one of these be coated inside and out with a suitable HV varnish such as Krylon triple thick glaze while spinning and then once dried be used as a TC former?
Not sure but I think it would be fairly lossy, however when properly dried not so much.
Also, why can't I modulate the input to an OBIT (designed to run at 50Hz) to energise the resonant circuit directly? The plan here is to use relatively cheap 240V rated parts such as a pair of MOSFETs in series with the neutral side of the OBIT so that I can run the coil at resonance i.e. 240V 50Hz and 31.2 kHz say.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I have used cardboard TC formers previously and they worked well. I didn't coat them or anything. They were fine in a room environment, but would probably soak up moisture when allowed to.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Also, why can't I modulate the input to an OBIT (designed to run at 50Hz) to energise the resonant circuit directly? The plan here is to use relatively cheap 240V rated parts such as a pair of MOSFETs in series with the neutral side of the OBIT so that I can run the coil at resonance i.e. 240V 50Hz and 31.2 kHz say.
Well, think about it; OBIT's huge parasitic capacitance and inductance will make it's own resonant frequency far lower than of any practical TC, even if the losses in iron at 30kHz weren't extreme. Base driving with ferrite transformers has been attempted but failed miserably for the same reason.
On topic, many people including Steve Ward have used cardboard tubes as their secondary forms with great succes, though they may indeed be prone to absorbing moisture unless they are impregnated with varnish or epoxy first.
PVC tubes are cheap and easy to wind on though, and probably by far best for all pulsed coils which won't heat up significantly during operation... and for others, I prefer fiberglass tubes anyway ;)
Registered Member #3037
Joined: Mon Jul 26 2010, 06:42PM
Location: Los Angeles
Posts: 20
The losses with cardboard are really not noticeable as long as you varnish the tube as best as you can. Also, it might help to dry it out with a blow dryer or by leaving it out in the sun, as they can absorb moisture. For my large 12" coil I used sonotube, which is similar to cardboard. I dried it out thoroughly and varnished the cardboard 3 separate times.
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Cardboard Tesla coil?
