> I need to drive a solenoid on and off from a logic level output.  The
> solenoid has 28ohms of coil resistance, and a 500mA operating spec.  The
> only available power I have to drive it must come from the 80VDC supply.

> Priority of tradeoffs:
> 1) Reliability (room temp. operation only)
> 2) Needless Power loss
> 3) Number of components (complexity)
> 4) Price (don't want to spend more than $30)
> 5) Size (not much of a factor)
>
> Thank you in advance for your help,
> Jay

Wagner has already said that PWM is the answer.. he is right but I
would add that if this is being driven by a PIC then the PIC should
have little difficulty providing the PWM.
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).
Been there ... blown up that by not thinking of this.

_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

www.psych.auckland.ac.nz

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