winding air coil inductors - removing springiness?

[Sadface]

G'day Guys,

I am winding some inductors for the output thiel network on an amplifier:
5uH.

I've gone with 18awg wire. According to this calculator https://www.circuits.dk/calculator_single_layer_aircore.htm
I need 18 turns on a 25mm mandrel.

After winding the copper wire around my mandrel (a 25mm electrolytic capacitor) the wire wants to 'spring' back and loosen up.

Any suggestions on how to get the thing to hold its form nicely so that I can dip it into some epoxy?

 

[newvirus2008]

• Do not use trial & error, always calculate the required dimensions / turns beforehand.
• Do not reuse wire from elsewhere.
• Wind tightly while maintaining tension in the wire, with the start end tied, or use a coil-winding machine (if available). The machine is capable of maintaining the correct pitch and tension.

 

[Preamp]

Maintaining tension on the wire while winding is a good advice. Try to get some PVC pipe with a suitable diameter as a coil former. You can make a little notch at one end to aid in fixing the wire there, and it is very helpful if the coil former is much longer than the final coil will be. You can actually leave it inside the finished coil afterwards and just trim it to the correct length.
Personally, I have had good success with wrapping the coil into several layers of (tightly wound) capton tape and carefully removing the coil former afterwards. The tape stays on the coil.

 

[jackinnj]

Don't you want to lower the "Q" by winding the coil on a 2 Watt resistor?

For other purposes you can still purchase Air Dux coils which are secured on polycarbonate rods.

 

[Ultima Thule]

There's a spring back factor at play here depending on the material and other mechanical factors which you have to accommodate for in order to reach your target diameter, simply put, in your case you have to wind your coil on a coil former with smaller diameter, try starting with someting like 20-22 mm.

Also, if you want the coils longitudinal direction to contract and hold together, you may also have to slightly twist around the coil former while winding, but usually holding the wire snug and almost letting it climbing on the previous turn at angle of something like 70-80 degrees instead of normally straight on 90 degrees perpendicular angle usually will make a fairly tight coil.

 

[Conrad Hoffman]

Same problem with spring winding. There are formulas for the size of mandrel you need to get a specific size spring. IMO, they're near to worthless. You have to do some trial and error, but as said above, try starting with 20-22 mm.

 

[Bonsai]

Put a thin (say 1cm wide) piece of grease proof paper on your former and tape it in place either side of where you will wind your coil. Then wind your coil. Once wound, secure the windings in place with a few drops of superglue along the winding over strip of grease proof paper. Let it dry and then just slide the coil off the former. You can then further secure your coil by filling the centre with silicone sealant.

I use a piece of dowel usually.

(Separately, 18 gauge seems a bit light and 5uH quite high!)

 

[tomchr]

I seem to recall that there's magnet wire out there that has slightly sticky enamel. That's intended for keeping the coil together after winding. The inductors I use in my amps are custom made. The manufacturer gives them a layer of yellow transformer tape to prevent them from unwinding. I think they also use the wire with the stickier enamel.

Tom

 

[JMFahey]

Yes, there are many versions of "self sticking enamel" wire, some are rated for voice coil use.

Some are activated by wiping wire with alcohol, they become sticky and dry up as a solid block, others use a low melting temperature enamel, similar to hot glue pistol bars; you pass some current through wire to heat it up, it partially melts and solidifies as a block.

But to avoid buying a pound roll, for home experiments just cover mandrel with a single layer of grease proof paper or mylar as suggested above, and while still under tension run 2 or 3 beads of Krazy glue, the gel type if possible, along coil.

In the old days {40s 50s) they used nitro cement or plain old nail enamel but then drying takes, say, 1 hour.
.

 

[Mark Tillotson]

Definitely compensate for the spring back - thinner mandrel, wind a bit further than 18 turns, then let it spring back, bend the ends at the 18 turn mark with the coil relaxed.
The more you bend copper wire the more resistance it presents, although its a fairly small effect.

 

[tomchr]

Another thing to remember is that the inductance of an inductor is rarely all that critical, at least not if we're talking air core inductors for Thiele/Zobel networks. You can be off by a factor of two on the inductance without much consequence, so if the diameter of the inductor expands a bit as the coil is moved off the coil former it's not likely to be noticeable. Furthermore, remember that the various equations and calculators for air core inductances make several assumptions, for example about uniform distribution of the magnetic field within the inductor, so you're likely operating with one or two digits of precision at best anyway.

So don't fret it too much. Just don't have the inductor unwind itself and poke you in the eye.

Tom

 

[Simon1972]

I just over wind them and let it spring back then make the last leg bend. Half a turn difference in a 20 turn coil ain't worth worrying about in almost any case.

 

[Mark Johnson]

Attach left end lead-out wire to coil winding rod with 3 wraps of masking tape. Tightly wind the correct number of turns. Keeping lots of tension on the wire, tape right end lead-out wire to coil winding rod. Cut away from wire spool.

The coil is now tight and non-springy. Tightly wrap it with two layers of Kapton tape. Since Kapton tape is not liquid like cyanoacrylate superglue, you don't need a barrier paper on the winding rod. Remove finished and tight coil, trim lead-out wires, and measure the final inductance on a DER-EE DE5000 digital LCR meter.

 

[avtech]

There are many good suggestions here.

I usually put some kapton tape sticky side out on a greased, slightly tapered mandrel to stop the wire from springing open.
Then once wound and cut to size, I put some superglue on the coil to hold then slide it off the mandrel.
A quick squeeze in a vice takes out any remaining springiness and it is ready for covering or epoxy dip.

 

[Sadface]

Thanks for all the advice. I should have enough to work with now.