EPE Mini Theremin construction (and problems)

> If anyone is REALLY stuck and cannot get IF
> transformers, I have a few spares (enough for 4
> Theremins..) send me an email, and I will post
> a set to you FOC.

Jaycar sell them - www.jaycar.com

Part no. LF1050

>>"I figure that if I'm getting the same
>> inductance with fewer turns, then I must also
>> be getting far less self-capacitance."

> First - I am not an expert on inductors!

I too have always had the impression that their construction is something of a black art, and have managed to avoid them whenever possible!

> I do not think the thicker wire increases the
> inductance.. I suspect that the increase in
> inductance is due to different core
> charactaristics...

That makes sense to me - though I'm not sure that I follow that fewer turns =/= lower capacitance. The wire I used was only marginally thicker than the recommended wire (possibly just metric/imperial equivalents).

Imagine the winding capacitance of 1 turn of wire... I figure that's got to be lower than an inductor with 2 turns, which in turn must surely be lower than an inductor with 3 turns. And so on...

Kit

Here's another thought - when you jumble wind an inductor, the coil takes up a significant amount more volume, meaning that there's more air between the turns of wire. This would also translate into reduced capacitance.

Kit

"Here's another thought - when you jumble wind an inductor, the coil takes up a significant amount more volume, meaning that there's more air between the turns of wire. This would also translate into reduced capacitance."

Interesting idea! - you may be right about this (it is the only explanation which I can find no solid argument against..) - If this is true, then thin wire with thick insulation with good dielectric, should be the best option for constructing tidy, repeatable (Ie Manufacturable) inductors

I think this errata/note from Silicon Chip may be interesting:
"(2) Equalising coil L1 needs to be wound so that its self-capacitance is as low as possible. In practice, this means that the windings should be jumble-wound by hand without regard to neatness. Do not wind each layer with each turn placed adjacent to the next as would be done by a coil-winding machine. (12/06)"
See: http://www.siliconchip.com.au/cms/A_105891/article.html

Just to throw in my two cents on coils.
An air wound coil's inductance(Xl) is a function of the # of turns, length and the dia. The higher the number of turns and the smaller the diameter of the turns, increase the Xl. Whereas the length( spacing between turns) of the coil decreases the Xl. Winding a coil very neatly decreases the overall diameter and length and increases the Xl. A good model is adding bar magnets together to increase the field, each turn acting as a magnet.
The Xc (cap) of our coil comes from the distance of the turns between each other and the amount of
surface area(the number of parallel turns). A capacitor is formed by two plates separated by a dielectric (air in this case); the Xc is a function of the surface area of the plates and the distance between them. Two parallel conductors will always have some amount of Xc and Xl, Coax cable is rated with the Xr( The function of Xl&Xc at design Freq) expressed in Ohms. So the more turns that are parallel and the closer the turns are to each other, the higher the Xc. Jumble winding adds more distance between turns and breaks up the amount of 'parallel-ness'; lowering the Xc but also the Xl(to a much smaller effect).
Chokes are a special class of inductors that are designed to have a very low Xc. At high inductance values they can be air wound in a weave pattern where each turn is at an angle to the axis of the coil and spaced from the last turn. Take a look at the Eq coils of the Etherwave.
In closing ( at last!) all the above effects are only applied to AC signals, at DC a coil is a short.
Just my two dollars worth,
yogi

"just my two dollars worth"..
That description is worth more than two dollars.. Thanks Yogi!
One thing I still have a little doubt though, is the cumulative (ie total) capacitance as a result of different winding methods.. As I see it, assuming the windings are packed into the same area, the total capacitance should be the same.. regardless of winding technique (albeit, this is a unlikely practical situation, as jumble-wound is likely to have a lot more area, and therefore more air spaces).

What I think would be true is that, with a regularly wound coil, the L-C-L-C distribution will be constant and definable - and aproximate a standard transmission line equivalent circuit - whereas a jubmble-wound coil will have the 'Cs' in this circuit (and also Ls) randomly "interconnected".. This would mean (?) that a regularly wound coil is likely to have a more clearly defined 'natural' resonant frequency, but a random wound coil would be less 'inclined' to any particular frequency (?)..

However - regardless of theory, I have not found much (if any) difference in the results when using various differently wound coils - but perhaps this is because I have not played with circuits where the coil is critical - or perhaps I have just been lucky.. Before I put anything into production I will check to be sure it isnt just 'luck' which caused my prototype to work!