Lance Allen wrote:

> I would add that there is a gotcha with this technique..... the PWM
> duty cycle should be calculated to the power NOT the voltage. As
> in 80V down to 14V at 500mA is derived by V div by A... so at 14
> volts the solenoid uses 7 watts but at 80 volts the solenoid uses
> (instantaneously) 228 watts, so the ratio for the PWM is 3%!!!!
> (NOT 17.5% as in voltage only).

  No, the gotcha bit *you* on this one, Lance.  Your calculations apply
for resistive loads, this one is inductive.  He needs (as Thomas notes)
to keep the chopper frequency high enough so that the coil doesn't
saturate.

  The duty cycle will correspond to keeping the specified *current*, and
it will be closer to 17%.  It will certainly need to be "tuned" though.
And a suitable diode across the coil, plus a 100V capacitor on the
supply are essential.  Maybe a small decoupling resistance before the
capacitor also.

  It would be far more elegant to get the PIC to do this than invoke the
trusty 555.  In addition, you can use a higher initial duty cycle to
make the solenoid pull in and then when it is either detected, or
expected after a certain time to have done so, drop the duty cycle
(current) to half or less.
--
  Cheers,
        Paul B.