Rewinding an old solenoid coil

[McMurdo]

I have here a solenoid coil which I'd like to attempt to rewind, it has gone open circuit somewhere.

The strange thing is that as I've started to backwind it, I've found it's wound trifilar. Is there any reason for this? It's an AC coil, 120V 50-60Hz. It's only a small bobbin, maybe 20mm long by 20mm diameter. There is only the one winding on there, brought out to insulated flex tails.

Thankyou!

[brianc]

The only thing I can think of is to make it multi-voltage - windings in parallel = low voltage, windings in series = high voltage. Are the three winding connected in series for 120V? If so, it may be for another reason altogether.

[McMurdo]

The strands are soldered before the lead-outs are joined; so 3 inaccessible at the centre and 3 on the outside.

[ppppenguin]

Perhaps the thicker wire for a single winding would be a bit too stiff or give a poor fill factor?

[McMurdo]

I think you might be right Jeff. I have completed backwinding it and there are 12,000 turns on a bobbin not much bigger than an octal relay coil. The bobbin core is square to accomodate the plunger; lots of sharp corners.

[ppppenguin]

That's getting into the region where the wire is fine enough so that the insulation is taking up a lot of the space. That's a long way from the case I was suggesting previously.

Where's Ed Dinning when we need him. He's the expert round here on all wound things.

[kalee20]

Almost certainly this is wound trifilar for practical reasons - the 3-strand bundle is more flexible than a single solid wire, and lies against the square-section bobbin better.

Or could have been simply that they had the 3-strand wire in stock at the time.

Let us know the sizes and wire gauge, this should settle it.

[Ed_Dinning]

Hi Gents, just back fom hols to find that Kalee has given the answer.
It could have been what they had in stock, or as at one place I worked, we used huge quantities of 0.5mm wire; we also needed 0.71, but used 0.5 bifilar instead as it brought total costs down and it wound better.
This bifilar was coded with red and green enamel to identify the cores, but we simply used them paralleled.

Ed

[McMurdo]

I guess on a mass-produced coil like this, production time and reliability have to be engineered-in together.

I have ordered a reel of single 46swg and will see how it goes. I can wind it as steadily as i like, a new coil is the princely sum of £175 from the states so I'm prepared to learn as I go.

[McMurdo]

Just a follow-up, the 0.05 wire arrived today and I gave it my best shot. Unfortunately, the square-section bobbin causes (predictably) the tension to vary in jerks. Even with no tension at all on the supply spool, I can't get a speed up on the winding spindle before the wire breaks. I got to 1300 turns before it snapped. At that slow rate, I'd be there 2 days watching it!

I think I'll leave the lathe for speaker crossovers.

Oh well, you win some, you lose some.

Volunteers, anyone?

[FRANK.C]

Hi Kevin
You could try building a sprung loaded jockey pulley, so that it will take up the slack and also give some wire when the bobbin needs it faster, this will then enable the supply reel to turn at a constant speed and not jerk.

I built one into my winder and have wound down to 0.06 mm on a rectangular bobbin with ease. I don't believe there would be any problem with 0.05 wire either.

Frank

[kalee20]

That's exactly what you need for winding fine wire on a square-section bobbin - a tensioning device with no inertia.

Winding directly from the reel, with the reel on a spindle, the inertia of the reel can't respond to jerks and the wire suffers mightily.

With wire like this, if the flanges of the reel are smooth, you should be able to pull the wire off vertically if the reel is on the floor resting on one face. Although the wire, in theory, gets a turn of twist for each turn unwound from the reel, it is common in the winding industry.

Then, you can lay it on the bobbin as it is turned, tensioning between your fingers.

Have fun!

[McMurdo]

Mm that's a possibility. The main problem is that 12,000 turns gives alot of scope for error! I dont like the odds!
I might just try the end-on reel approach tomorrow, (and I'll dig out my cotton gloves) and give it one last try before I send the bobbin to an expert. At least I have plenty of wire to play with.

[kalee20]

I wound 15,000 turns (centre-tapped) of 0.05mm a few months ago, on a rectangular section former. Cotton gloves not used - but I did thoroughly wash my hands immediately before. Actual winding time taken - 20 minutes x 2, (plus the time to form terminations).

If you get it wrong, the time to strip and unwind is far longer than the time to wind!

[McMurdo]

Well Kalee, I did as advised and allowed the wire to spill off the bobbin from a distance away. After a few false starts when my sweater got caught up in the wire, I managed to wind the 12,000 turns on successfully, cotton glove gingerly applying slight pressure.

Unfortunately, the solenoid won't pull in hard enough! A comparison with a good one shows the coil resistance is alot higher...5.6K as opposed to 3.4K on a good one. The resistance is obviously too high for the voltage.

I've now ordered a couple of gauges thicker and I shall try again now I'm confident the spool-off technique is the one I should persue. My digital calipers must be out!

[McMurdo]

Just a follow-up to this, I wound it with the next gauge down and the 12000 turns just went on with just enough room for the empire tape. The cotton glove was again used to hold and guide the wire.

It now pulls in well and runs cool enough, takes 35mA at 110V AC so I shall press it into service!

[valvekits]

£175 sure is one hell of an incentive Kevin.
Nice one!

Eddie

[McMurdo]

carriage and VAT on top of that!