On Wed, 19 May 2010 22:44:37 +0100, "Ian Iveson"
wrote:



I've read this a few times but....perhaps you misunderstood.
I was entertaining the theory that the 120 ohm resistor
drops the difference between 220V (or whatever the motor
hopes for) and UK mains, i.e. 10 or 15V, whatever it was
when the deck was made.

IIRC the setup was that the motor was a 120 volt motor used on a 240
volt mains. Not a 220 volt motor on a 240 volt mains. If the desired
drop is only 12 to 15V then you are correct for the value. But that is
not what I understood.

If that difference has increased by,
say 25% from 12V to 15V, then the change that has been
mentioned, from 120 to 150 ohms, makes sense, roughly.
Everything that Arthur has said seems to fit this theory.
It's simple, too.

Anyway, as I understand it, Arthur's worried because if it's
supposed to be a much higher value, he might do some damage.

I'd agree, given the conditions in my comment above, that a 120 ohm
resistor might in fact do damage.

That's why I homed in on the voltage dropping issue.
Whatever else it might do, a resistor in series will
certainly drop voltage. If the value is much too low, then
it could result in a burnout. This is true whatever its
effect on direction, speed, torque and lots of other stuff I
don't know about but probably everyone else does but didn't
fancy saying so until now.

I wonder if he's tried it yet?

Ian


I thought he had, but I'd have to go see if there is a reply.
Something about using three or four larger resistors in parallel...