> Hi all, perhaps someone can help with a wee power problem. I've
> got a circuit board mounted on a motor shaft. The circuit is there to
> measure the movement of an escapement mechanism (kind of
> like a governor) as the shaft turns from 100 to 3500rpm. The
> circuit itself is not a problem, I've managed to attached a small
> slider pot to the escapement and it's measuring OK with an ADC.
> What I'd like to do is induce power to the circuit using some kind
> of magnet/coil system. I need around 15mA at 6VDC. A possibility
> is to use a 7-segment LED (synched with the rotation) to indicate
> the escapement movement, hence the power requirement. Using
> batteries is not really an option because of the duration of the
> measurements and particularly if the LED idea goes ahead. I've
> got plenty of envultured materials - strong HDD magnets and
> more scrap transformers (enamelled wire of all gauges and
> laminate cores) you can shake a stick at. I thought I may be able
> to attach a small DC motor to the main motor's shaft as a
> generator but that's proved too tricky because of space limitations.
> So, any ideas on the most efficient way to induce power into coils
> on the PCB from stator magnets ?

More high Jinx ???

If you have room for a magnet field all the way around the stator this is a
trivially simple task. It becomes a basic alternator. The task is not much
worse with a coil passing a fixed magnet once per revolution although
mechanical balance may be more of a problem. Typical olde days motorcycle
alternators and magnetos tended to use a coil in one place which was passed
by a rotating magnet once per rev. Come to think of it, most every lawn
mower I have ever seen does just that.

Main requirements are half a closed magnetic path around the coil (eg a
metal u of laminations) with the moving magnet almost closing the gap across
the U as it passes - air gaps of mm's are going to be OK.
              N
   IIIIIIIIII   N
L II L     S
   IIIIIIIIII   S
               \/

You COULD use a coil to coil transfer with a highish frequency field in the
stator coil but this is probably less simple than a magnet and coil and adds
extra complexity. If using coil to coil then resonating the receiver can
give good results with largish air gaps (many millimetres).

The power level of 100 mW should be easy enough to achieve.


            Russell McMahon

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