Winding a (big) inductor by hand...

[Al (astral highway)]

I thought I'd share this in case anyone is thinking of building their own receiver with limited tools/ space.

After initially balking at the idea, I eventually stepped up and wound this hefty inductor by hand. I don't have a workshop, just a tiny desk, so I don't have the option of making even a basic winding jig.

The winding length is 22 cm by 8 diameter, using 30 SWG wire. A back of the envelope calculation says that's, erm, quite a few turns of wire...

The method is immensely tedious but survivable with the right radio playing in the background. It's essential to maintain at all times the tension in the spool of winding wire. With one hand, the entire former is turned, and after each cm or so of winding, the fresh turns are taped off with medical adhesive tape.

For my application it's essential to avoid overlapping turns at all costs - there will be a hefty voltage gradient between one end and the other. In fact there are a few irregularities / slightly looser turns here but I think they are survivable once I've coated the inductor numerous times with lacquer.

Once I've done that, and made a robust termination at the earthy end, job done. The start and end points are adequately tensioned using just two tiny holes.

The former is a standard postage tube. I've tried this before and this grade of cardboard, once impregnated with a lacquer or varnish, is brilliant. (A lot less lossy than some plastics, for example.)


Cheers,

Al

[Aub]

Nice job. I'm intrigued about it's application. Is it a low frequency receiver?
On second thoughts, is it one of those Tesla coils?

Cheers
Aub

[Al (astral highway)]

Originally Posted by Aub

On second thoughts, is it one of those Tesla coils?

hi Aub. Yep, you guessed it. f res is low but not that low...around 100Khz, A larger one could be used as the basis for a very low frrequency receiver...saw an intriguing 1960s design from an era when enthusiasts were interested in detecting missile test launches, for example!

[Sean Williams]

I would be a little worried in the event of a coil flashover that the tube would then catch fire....

Tesla coil - on my round tuit pile

Be interested to see how you get on.

Handy hint, don't 'unwind' from the drum of wire merrily let the wire roll over the top of the supply drum. Then there is no inertia causing the wire to snap.

[Al (astral highway)]

Originally Posted by merlinmaxwell

Handy hint, don't 'unwind' from the drum of wire merrily let the wire roll over the top of the supply drum.

Exactly! I was doing this intuitively at the time! You're completely right, disaster if you lower the tension in that winding wire...

Originally Posted by Sean Williams

I would be a little worried in the event of a coil flashover that the tube would then catch fire....

Ah Sean, not a problem. When the coil fails, you typically get a small arc between adjacent turns, weirdly. I have only seen this with traditional, not IGBT half-bridge designs.

Some designers put a strike rail above the primary of interruptive designs. Otherwise, complete length flashovers don't happen except in very high power designs.