> I am having somewhat success using PWM to control the initial current
> then drop it down once the coil is saturated to avoid burnout.
> Schematic:
> http://www.damonhamm.com/other/pic_solenoid_schematic1_pwm.gif
> (Initial tests do not utilize the #1 RC 'decoupling' circuit.)
>
> But, the MOSFET (1.5A? in a TO-220 case) heats up to be quite hot to
> the touch, which I wouldn't consider viable.  Using 5V at the MOSFET
> gate (full on or PWM at 100%) doesn't produce enough current to fire
> the solenoid, while using 12V with PWM at 20% for 100ms, then at %1 to
> hold for 2s, still causes everything to heat up much too much after
> several cycles.  I am still surprised that a duty cycle of 1:255 (~1%)
> still provides too much current!  Is there something wrong with my
> setup or are the MOSFETs just too wimpy?  I have some 6Watt DIP
> MOSFETs on the way to test out.

I have serious doubts this is switching the way you think.

1. I doubt the opto switches the FET fast enough.

2. I doubt the current in the solenoid changes fast enough to get the FET
out of "linear" mode - i.e. the current rise and fall times in the FET are
significant resulting in the FET being in the linear region between full off
and full on. I would doubt that a 1% duty cycle would do anything for you

3. You first need to sort out why the 100% duty cycle does not produce
enough current through the FET to pull in the solenoid. It sounds like the
FET is not a logic level one, in which case it probably needs 10 to 15V at
the gate to turn on. Then look at the other problems.

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