On Thu, 27 Nov 2003 06:42:18 -0500, you wrote:

>On 27 Nov 2003 at 11:54, Koen van Leeuwen wrote:
>
>> Hi all,
>>
>> I have a circuit mounted on a fan, doing about 3000 rpm. This circuit =
has
>> to be powered with 5v, 30 mA. I can't use brushes because they slow =
down the
>> fan too much.
>> I tried to make an "open transformer": an open coil (not rotating) =
with a
>> smaller coil with iron core inside it (not touching the other coil, =
and
>> rotating).
>> Both coils have ~30 windings.
>> Using 50 Hz 5v 500 mA at the primary side,=C2=A0only 300 mV shows up =
at th
>> secondary side with short-circuit current of 1 mA.
>> Should I use a higher frequency, or just make the coils larger with =
more
>> windings?

A higher frequency, coupled with ferrite cores will  help a great deal. I=
 would suggest something
like 20KHz (remember to use fast diodes at the receiving end)
You may be able to scavenge suitable cores from inside the head drum of a=
 scrap VCR

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