I find your circuit a bit confusing - you have the opto driven MOSFET
in parallel with exverything accros the power supply. No wonder it
gets hot!

If you are having trouble with the high Rds-on of logic level fets you
may need to go to "standard" MOSFETs. However, these will not switch
effectively with only a 5V drive from the PIC so you need to add a
driver circuit. There are examples in the archives. (Russell has
provided a good one). The other thing to watch is that the switching
speed is not too high. MOSFET gates are very capacitive so it takes an
appreciable time period for a PIC pin to turn  them on fully. Again,
if this is the problem, slow down the PWM frequency or use a driver.
The project I am just about completing drives contactors with up to 3A
of coil current and operates at ~30kHz., At this frequency the
risetime, even with a driver, is becoming noticable.
When driving contactors & solenoids note that the inductance changes
as they pull in (the magnetic circuit closes). This can also create
strange effects if you are trying to figure out what's going on.
And if you do use a driver circuit - make sure it is properly decoupled.

You are better off measuring coil current than trying to measure coil
volatge due to the waveforms involved. I get very widley varying
voltage readings even with a "true RMS" meter but the coil current is
very stable. If you can't get a reading I suspect your meter is
damaged.

RP

On 23/09/05, Damon Hamm <damonhamm@gmail.com> wrote:
> I am attempting to fire 8 solenoids from a 16F628 and am inexperienced
> at playing with solenoids.
>
> The setup:
> One 16F628 connected to an optoisolator (necessary?) then to 8 power
> MOSFETs (with necessary resistors in tow).  8 mini 12V push solenoids
> (with flyback diodes) on the high side of the MOSFETs.
>
> Upon a trigger, I want to fire the solenoids (sometimes all 8 at once)
> and hold for 2 seconds or so.
>
> 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.
>
> Has anyone had any experience with solenoids that they can offer?  Any
> resources, circuit critique, and component suggestions would be
> greatly appreciated!
>
> I have looked at DRV101T solenoid drivers (PWM), but would rather save
> $100 and use my solution.  My multimeter always reads 0 current
> flowing when I attempt to see what the solenoid is using (can one not
> read current in series with inducors or is my MM fried? - had a 120V
> short with it years ago but everything else works fine).
>
> Thanks!
>
> - Damon
>
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