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Secondary with thin wire




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From:  Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent:  Wednesday, February 18, 1998 4:06 PM
To:  Tesla List
Subject:  Re: Secondary with thin wire

Hi Antonio,

> From:  Antonio Carlos M. de Queiroz [SMTP:acmq-at-compuland-dot-com.br]
> Sent:  Tuesday, February 17, 1998 1:47 AM
> To:  Tesla List
> Subject:  Secondary with thin wire
> 
> Hi:
> 
> I was looking what I have around to build a small capacitor-discharge
> Tesla coil. I found a PVC tube with 8.8 cm (3.5") of diameter (more 
> than enough, about 2m) and many spools of 32 AWG magnet wire (relay
> coils). By my initial calculations, I think that I can build an 
> acceptable secondary with this rather thin wire, with 1500 turns, what
> results in a winding measuring 30 cm (I don't want sparks longer than
> about this for awhile).
> The inductance would be 20.8 mH, the self-capacitance 10.4 pF,
> with resonance without top terminal at 305 kHz. Considering only the 
> DC resistance of the wire (223 Ohms), the Q reaches 435 (I didn't 
> compute the skin effect and other losses yet). I will have to use 
> five of the spools to complete the required wire length.
> Some advice on how to make this work? Is the thin wire a so serious
> problem? An it is really necessary to dry and coat the PVC tube with
> polyurethane varnish before the winding? (saves a day).

I would expect the Q to come in at no more than 150 with that wire 
and probably less. Skin and proximity effects mean a very great deal 
when it comes to quality. I have noted the odd experience with 
extreme experiments in the past, one of which should be in the 
archives from last year. The coil was literally a throwaway. My 
current guide for wire size:

a minimum of 3 skin depths at the lowest frequency of operation 
(i.e. largest topload you will ever use) for a space wind where 
spacing = wire diameter

a minimum of 5 skin depths for a closewind under the same conditions.
4 skin depths is still OK but allows no leeway for experimentation 
with larger terminals and the Q starts dropping badly if you do use 
them.

I base these figures on measured Q degradation using the best 
techniques I have developed for measuring unloaded Q.

Malcolm