Re: OT: Request for help with air core transformer/power transfer system

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi David,

On 6 Dec 2004, at 21:00, Tesla list wrote:

> Original poster: David Speck <dave@xxxxxxxxxxxxxxxx>
>
> List,
> I know this isn't directly TC related, but I hoped someone with more
> transformer theory experience that I have might lend a hand, with
> Terry's indulgence.  Please feel free to respond privately.
>
> ->Sound enough like electricity transmission without wires ;o))  But
> respond direct if not really related to Tesla coils) - T. <-
>
> I'd like to transmit about 6 volts AC at 60 Hz and 250 mA through a
> glass plate about 1/4" thick without drilling a hole in the glass, and
> without making any really esoteric circuitry like a high frequency
> drive circuit.  I don't have a lot of time for experimentation, so I
> though I'd run my idea past you and see what you thought.
>
> I have space to put a 4 1/2" OD flat circular pancake coil (about 1/4"
> thick) on each side of the glass.  I figured I'd run 6 volts from a
> wall wart into the primary, and have a matching secondary directly
> opposite it, on the other side of the glass, from which I'd feed a
> diode bridge and filter cap on the secondary side..  I know similar
> arrangements are used to recharge batteries of implanted medical
> devices, but I don't know if they work at 60 Hz or higher frequencies.
>  My main concern is making an arrangement that will not burst into
> flames during prolonged unattended operation. Questions are:  Would it
> be better to have primary and secondary circuits with a relative few
> turns of heavier wire, or lots ot turns of finer wire.  I expect that
> you'd get more inductance, but more resistive heating with finer wire,
> but in the absence of an iron core to corral the magnetic flux, I
> worry about drawing too much current from the wall wart into the
> primary with a few turns of thicker wire. Is there a way to calculate
> the optimum number of turns and wire size for this arrangement?
>
> Is there any simple arrangement of soft iron wire or sheet that would
> help to constrain/direct the magnetic flux for better energy transfer
> without involved magnetic core fabrication?
>
> Thanks in advance for your thoughts.
>
> Dave

The best way to do it is to use two halves of a potcore with a coil
on each half, the primary being driven at high frequency (sorry!) to
form a transformer with the best coupling you can get in the
circumstances you outline. Similar transformers are used in older
types of depth sounder/fish finder types of equpiment although these
are used as pulse transformers to transfer energy from a stationary
coil to a rotating one on which is mounted a neon bulb. I would say
that the larger the core, the better the results, at least up to a
point. The intervening material should make no difference to the
final result as long as it is electrically insulating and not
affected by magnetic fields. In effect, a heavily gapped transformer.

Malcolm