Hi Mike,There is really little need to worry about dielectric losses, bulk or surface resistivities, and even breakdown voltage specifications when you use coil forms made from PVC, Acrylic (PMMA), Polycarbonate (PC), High Density Polyethylene (HDPE), epoxy-glass, Bakelite (phenolic), or even treated cardboard forms. If they can withstand the voltage stress, they'll all perform in similar fashion in a TC.
PMMA and PC cost more than PVC, but both materials are transparent and definitely look "cooler". HDPE tubing is difficult to find in larger diameters and is often curved from being wound on large spools. Epoxy-glass or Bakelite tubing will also work, but these can be difficult to locate (at low cost) for larger diameters. Even dried and varnished cardboard forms will also work as long as any conductive carbon-based printing has been removed first. However, quality of cardboard forms varies considerably, and some coilers have found metallic debris mixed in with the cardboard (after the fact - NOT a good thing!).
PMMA's main advantage is that it's clearer than glass and it's very resistant to "tracking". Tracking is the formation of carbonized paths that can short-circuit part, or all, of your secondary winding when subjected to corona discharges. PMMA withstands corona better than most other common polymers, and is the material of choice for laboratory HV setups/fixturing. PC tracks more easily than acrylic. And, PMMA or PC can be destroyed by cleaning with certain solvents (such as alcohol). Many years ago, one of the TCML coilers cleaned a brand new PC form with alcohol to prep it for winding. A network of cracks (crazing) formed and grew before his eyes, and the tube disintegrated into small pieces within minutes.
HDPE pipe will also work, and this material is often used as distribution piping, usually colored a bright yellow, by natural gas companies. HDPE resists tracking quite well, but it is very slippery - it's sometimes called poor man's Teflon. When used as a coil form, it should be first coated with a layer of polyurethane varnish and preferably wound (while still tacky) to prevent the secondary winding from literally sliding off the form. This can especially be a problem if the secondary is subjected to wide temperature variations.
PVC pipe and tubing are widely available at lower cost and most hobbyist TC's are constructed using PVC forms. They can work very well when properly treated. Since PVC readily breaks down and tracks when subjected to corona, the trick is to prevent corona from occurring next to the PVC. The key is to remove the air spaces between the winding and PVC former by thoroughly impregnating your secondary winding with multiple coats of polyurethane varnish, or by applying a thicker two-part epoxy or polyester polymer that thoroughly surrounds and coats the winding. You'll need to continuously rotate the form as these products dry or cure to avoid unsightly sags. Coated PVC coils can be very reliable, and the coating also helps to protect the magnet wire from accidental dings or insulation damage from racing sparks or primary-secondary flashovers.
PVC pipe comes in many flavors. The kind you want is solid. DO NOT USE any PVC that has a foam core, since it may fail via internal tracking. Unfortunately, foam core PVC is often found at consumer hardware stores, including Home Depot. It can be difficult to tell the difference from solid PVC pipe, especially if the foam core is the same color as the parent pipe. To keep weight down, try to get thin-wall material, such as sewer and drain pipe (called SDR, with 1/16" - 1/8" walls) from a plumbing supply company. This material is also used for air ducts inside concrete floors for office buildings. Thicker Schedule 20 or 40 PVC pipe will work, but will make your secondary heavier than it needs to be.
There is lots of good information abut TC secondary materials and construction techniques in the TCML archives - it will be worth some time perusing the archives to save you potential time and grief.
Hope this helped, and good luck on your coil! Bert -- Bert Hickman Stoneridge Engineering http://www.capturedlightning.com *********************************************************************** World's source for "Captured Lightning" Lichtenberg Figure sculptures, magnetically "shrunken" coins, and scarce/out of print technical books *********************************************************************** Mike Gray wrote:
For the same money I can get roughly 250% more PVCu tube from here: http://www.pipestock.com/pvc/pvc-pipe/pvc-pipe-10-bar-metric/ So is there any real difference between acrylic and PVCu? What's the electrical property of primary concern here, relative permittivity? I'm still wondering about that volume resistivity figure I was given for the PVCu... anyone able to shed some light on that one? Or am I over-engineering this whole decision...? Is it just a case that any old (pure) plastic will do a good job? On 21 May 2013 08:50, Phil Tuck <phil@xxxxxxxxxxx> wrote:Use Acrylic tube from somewhere like this http://www.theplasticshop.co.uk/ (no connection) I've used it for three secondaries (8", 6.5", 4") and it works ok without being too 'lossey' Need about 1/8 inch minimum. (Varnish 1 layer initially, so wire grips better) Phil
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