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What's the best way to secure the ends of single layer windings on a plastic form?

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Nicko

Wed Sept 30 2009, 07:26PM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

Harry wrote ...

Nicko wrote ...

I'm a bit of a beginner at all this HV stuff, but I use that nice adhesive copper foil to place a small patch (1cm x 1cm) at the end of the winding to act as a strain relief and solder pad - terminate the winding on that and solder the flying leads to the pads also...

That would probably be good for fine windings Nicko, but when you get onto thicker wires that you have to straighten as you turn, I'd suggest a rather stronger anchor at the end.

I've been winding some inductors for a Pulse Forming Line using 3mm enamelled copper wire, (11 SWG) and holding down the end and getting a straight pull is no easy matter for the man with ten thumbs, even though there are not many turns.

For thicker windings I've used two small holes about 4mm apart with a diameter slightly bigger than the wire - pull the end of the wire into the first hole, then out of the next one, then solder to the copper pad (see above)... note that its a good idea to smooth the edges of the holes to lower the turn radius of the wire & to reduce strain...

Herr Zapp

Wed Sept 30 2009, 08:48PM

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

jp -

I like to provide a sturdy mechanical attachment for the magnet wire at both ends of the coil to prevent any chance of breaking the wire (like when when after running your coil outdoors at night, you go to lift the secondary out of the system and forget to disconnect the secondary ground wire). A good mechanical attachment should be able to withstand at least a 10lb pull without failing.

I usually use acrylic or PVC secondary coilforms, so it's easy to bond a copper plate (with a brazed-on nut) directly to the coilform using epoxy adhesive.

Since you're using an UHMW polyethylene form, no adhesive will form a reliable bond. I'd suggest that you use the copper plate method, but attach it to the coilform with at least four nylon screws. If your coilform wall thickness is adequate, you can just tap the holes in the form to match the thread size of your screws and not use nuts on the inside.

Here's a photo of the bonded-copper-plate method on the bottom of a 6.25" dia secondary.

Regards,
Herr Zapp

doctor electrons

Wed Sept 30 2009, 09:30PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Beautifull Herr Zapp!!
Just a side note to the coating! I use polycrylic because it does not yellow over time, dries crystal clear, and it cleans up with water.

jpsmith123

Wed Sept 30 2009, 10:18PM

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

My coil forms are about 0.25" thick...I wonder if that's thick enough to tap successfully without going all the way through?

Oh, BTW Herr Zapp, what kind of coating did you put on that coil?

doctor electrons

Wed Sept 30 2009, 10:56PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

30% is usually enough for thread engagement.

teravolt

Thu Oct 01 2009, 03:59AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I use masking tape to hold the winding on the tube temporally. Then spray it with a clear coat or clear polyerothane if intend on leaving it for any length of time. I like ...'s idia of super glue for tacking the start windings before turning.

Arcstarter

Thu Oct 01 2009, 04:43AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I use scotch tape at either end to hold the wire, then when i am done winding i coat it with Minwax quick drying polyurethane. The poly covers the tape, and the tape is literally as hard to cut through as fabric. The tape is not needed after the coating, though.

Herr Zapp

Thu Oct 01 2009, 05:25AM

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

electrons -

You wrote: "30% is usually enough for thread engagement".

30% of what?

jp -

It would be extremely difficult to drill and tap good quality blind holes in a hollow cylinder with only a .25" wall thickness, even if you have a setup with a precision depth stop (milling machine, etc) and a special plug (bottoming) tap. Your UHMW material has very low tensile strength, so you will need all the threads you can get, and you will need to be carefull with screw torque to avoid thread stripout or screw breakage.

If you DO decide to use a bolt-on copper plate, I'd suggest just drilling completely through the coilform wall and tapping the hole with #6-32 or #8-32 threads, which should give you at least six full threads of engagement. Secure the plate using nylon screws, and apply a sealant (RTV silicone, etc) to the threads when you install the screws. Done carefully, this should provide good dielectric isolation between the plate and the inside of the coilform.

The secondary in the photo is coated with a single application of Envirotex Lite epoxy (eti-usa.com) , applied while the rotating secondary was still mounted on the winding jig. I can provide a detailed procedure for applying the two-part epoxy if you are interested. Done correctly, it yields a near perfect mirror finish, like the secondary is encased in a glass sleeve.

Regards,
Herr Zapp

Wolfram

Thu Oct 01 2009, 07:02AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Herr Zapp wrote ...

The secondary in the photo is coated with a single application of Envirotex Lite epoxy (eti-usa.com) , applied while the rotating secondary was still mounted on the winding jig. I can provide a detailed procedure for applying the two-part epoxy if you are interested.

I would be interested in hearing your procedure.

Anders M.

Dr. Brownout

Fri Oct 02 2009, 01:52AM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

Here is how I secure the ends of the winding, I use Kapton tape because it is very thin and has a very high temp and dielectric rating, when I start the winding I make a small loop around the first turn, then I lay the flag end of the tape flat on the coil form and wind over the top of it and that anchor will hold the the winding in place. Now for the last turn I make a loop again and lay the flag end of the tape on the coi form, create this loop about 8 to 10 turns before you get to the end of the winding, can be more but you want to wind over the top of the flag end again just like you did at the start except this time let a small amount of the flag stick out between the turns and then wind tightly over the rest until you get to the loop, then when you get to the last turn run that turn trhough the loop and pull on the flag that will tighten and secure the last turn, trim the edge of the flag close to the winding and then varnish the coil as usual and you will have very secure start and finish turns. Not only is the Kapton very good tape but it has a color that when varnished is almost invisible and you really cannot see it.

You can get rolls of it off of Ebay for a really good price it is kind of pricey stuff otherwise.

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