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Properties of a Secondary Coil Form

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quicksilver

Tue May 25 2010, 02:22PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I have read that PVC was less appropriate for a secondary coil form due to the properties of the polymer but that these could be addressed by coating same with polyethylene lacquer. While looking at the local store for additional size forms I came across a black polymer pipe (that was not PVC). However I had also been told that often black polymer materials contain carbon & were less appropriate for secondaries because of that. Is this factual? This issue has not been addressed to a greater degree in many books, etc.
In picking a secondary form some time back I had success with a paper tube from a carpet roll that I coated with clear lacquer prior to, & after winding. However I don't remember what the nature of what that lacquer was (polyethylene, polypropylene, whatever...). Could someone shed some light on guidelines on Secondary coil forms in terms of what is appropriate & why? The larger tubes are not as easy to find & the materials appear limited. - Thanks

Goodchild

Tue May 25 2010, 02:32PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Ive personal have built several coils that use the black pipe (BTW it's called ABS) and have had no problems with it ever, and some of my coils
push 3.3X

Proud Mary

Tue May 25 2010, 02:49PM

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

Goodchild wrote ...

Ive personal have built several coils that use the black pipe (BTW it's called ABS) and have had no problems with it ever, and some of my coils
push 3.3X

Black plastic water pipe (now officially blue in the UK) is fabricated from Medium Density Poly Ethylene (MDPE) sometimes known by the old ICI Trademark 'Alkathene, but ABS is Acrylonitrile butadiene styrene.

Carbon black (typically 1%) is sometimes added to polymers to give them UV resistance.

chris rockman!

Sun Jul 03 2011, 01:52PM

Registered Member #3981 Joined: Sun Jul 03 2011, 04:10AM
Location:
Posts: 1

how thin can a secondary form be?

Ash Small

Sun Jul 03 2011, 05:05PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I understand that the best material for coil forms is polypropylene, due to the fact that it has virtually no inductance (if that is the correct term, or it might be due to virtually no eddy currents, maybe someone else can elaborate?)

I don't know where you'd obtain large diameter polypropylene tube, though.

The only air core transformer I've built was 1" diameter, using a 'scrubbed' effervescent vitamin C tube. (scrubbed because apparently the paint or dye has a negative effect)

U4R1A

Sun Jul 03 2011, 05:36PM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

I've built a small TC using a beer bottle and I'm winding a bigger one around a wine bottle. I haven't been able to find any thing on the effects of using glass as a secondary form. Any one have any data on this?

Dr. ISOTOP

Sun Jul 03 2011, 06:41PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

The secondary former material doesn't really matter; sure, PP has less losses than cardboard, but in both cases it's small enough to not affect performance or break things.

U4R1A

Sun Jul 03 2011, 08:09PM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

That is what I wanted to hear. I was going to spruce up a little 811a VTTC with a wine bottle secondary to give it a bit more of a vintage look.

Andyman

Sun Jul 03 2011, 10:40PM

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

chris rockman! wrote ...

how thin can a secondary form be?

As thin as you can possibly get it and still wind it!

Herr Zapp

Sun Jul 03 2011, 10:46PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

qicksilver -

In the US, the most commonly used secondary coil form material is PVC pipe. This is available in many diameters and wall thicknesses. The preferred type is "SDR-35", which is commonly used for low-pressure (drain) applications and has the thinnest walls and lightest weight. This is most frequently seen in white and light green colors.

Also found in the US is black ABS drain pipe, which is also suitable as a secondary coil form. As Proud Mary mentioned, black polyethylene water pipe is available in the UK, but adhesives and and protective coatings (varnish, epoxy, etc.) will not readily adhere to it.

Scrap pieces of thin-wall PVC pipe can frequently be found at construction sites, and can usually be obtained for free if you contact the site supervisor and explain that you need it for a science project.

I don't think that anyone has ever demonstrated that "sealing" a PVC coilform with varnish, etc. makes any improvement in coil performance or reliability. After winding the coil, lacquer, polyurethane varnish or 2-part epoxy are the coatings most frequently used to protect the wire.

The black ink used to mark the outside of the PVC pipe can be marginally conductive, and is usually sanded off before the windings are applied.

Herr Zapp

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