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Electrical properties of plastics

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brtaman

Mon Aug 24 2009, 07:14PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

Hello,

Being in the process of starting a new (medium) secondary for an SSTC, left me wondering as the to suitability of the coil form material I had chosen (LDPE), so I started searching the internets. I came across this website

, with a very useful table, listing important electrical properties for the plastics we commonly use in the HV hobby (Dielectric Strength, Dielectric Constant, Dissipation Factor).

Anyways I am finding the table very usefull (it has found its way into my bookmarks). I posted the link because I think it could be quite usefull for the members of this forum? I would definitely not mind seeing it in the 4hv wiki. If anyone has been using an even more thorough website, please post.

Thanks,
brtaman

Sulaiman

Mon Aug 24 2009, 10:29PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Electrical properties of coil formers for TCs are important,
but unless you make a very wrong choice (e.g. wet cardboard)
physical properties are more important,
e.g. all the 'best' dielectrics (ptfe, xxxxPE etc.) are so 'slippery' that
they are hard to wind on and very hard to glue to or machine.
A final layer of laquer or some other method of securing the windings is essential
to prevent the coils dropping down and bunching up when hot, and most laquers have much worse dielectric properties than ptfe, PE etc.
Most 'coilers' use pvc, acrylic, abs, fiberglass or dried cardboard due to availability and cheapness.
Overall the losses of the secondary former are negligible compared to winding resistance for example.
Still a useful table of data though.

brtaman

Mon Aug 24 2009, 11:24PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

I agree 100% with your post, Sulaiman.

However, my brain seems to recognize the secondary as the "heart" of the coil, due to this reason I can't stand to spend hours winding the secondary on anything but the best material possible, even though secondary losses are negligible in TC operation compared to switching losses wiring resistance etc. It is just one of the Monk like quirks. ;)

Hot glue seems to stick to the LDPE quite well though?

While we are on the note off laquer losses, I could use your advice. I have two different lacquers at my disposal (both have been used before for secondaries). One is Boat Lacquer, which goes on very thick and makes for a very nice finish. The other is something new to me (picked it up at Bauhaus), it is an acrylic water based lacquer, which applies quite thin and its finish leaves a lot left to be desired, its plus it that after many layers the winding is encased in in a plastic cocoon which is very resistant to physical damage. Think its worth it to use the acrylic based, which I assume has lower losses than the boat lacquer?

Best Regards,
brtaman

jpsmith123

Wed Aug 26 2009, 03:54PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I'm about ready to put 400 turns each onto two pieces of 2" OD x 1.5" ID x 6.5" long, UHMW polyethylene tube. (This UHMW PE tube is nice stuff, BTW).

Although I've machined the tubes, I haven't started winding them yet as I'm still trying to figure out how to mechanically secure the ends of the windings. I suppose I will put a coating of some kind over the completed single layer windings, but I'm still debating how to properly anchor the ends and at the same time connect the magnet wire to the heavier wire which will connect to the FWVM.

How do people generally do it with Tesla coil secondaries I wonder?

Sulaiman

Wed Aug 26 2009, 04:25PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I like a nice thick encapsulating laquer/varnish, I use several coats of polyurethane,
mainly because when the secondary is stored away the wire doesn't get 'dinged'
it also looks nice, but can sometimes hold quite a shocking static charge.

If there is corona or arcs around the loss due to any laquer will be negligible so go for
the best looks and durability within your budget.

Marko

Wed Aug 26 2009, 04:39PM

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

If the SSTC is going to operate in CW mode I would highly recommend using a heat resistant material. You don't want a thermoplastic that will melt at 80 degrees C, and I'd say cardboard or fiberglass are materials of choice. Just dry the cardboard tube out in sun and douse it with varnish; sand before and after if necessary and wind the coil.

Proud Mary

Thu Aug 27 2009, 11:29PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Marko taises an important point. How many times have we seen 4HV projects fail because the constructor thought wishfully that it would somehow be alright, or nearly as good, with under-rated components.

Conundrum

Fri Sept 11 2009, 10:55PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

hmm.. i did wonder about making a TC secondary as "stacks" of smaller secondaries, like the construction of a ccfl transformer. any ideas?

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