MSgpio.com read only forums (Jan.15,2018)

I am intending to use PWM3 for a shift solenoid. Can I use a transistor the same as the other PWM circuits or should I stick with the mosfet? If I do stick with the mosfet do I still need the 10W resistor? Any help would be great. I am on a pretty tight deadline!

If the output doesn't change state quickly (like a PWM output), there won't be much heat generated so a resistor isn't needed. I assume that because it's for a shift solenoid that it won't be changing state rapidly. There will be a fly back pulse whenever the solenoid is deactivated so you'll need a diode connected from the MOSFETs drain to +12Volts. If you don't, the MOSFET will almost certainly fail the first time it tries to turn off.

amishmafia00 wrote:I am intending to use PWM3 for a shift solenoid. Can I use a transistor the same as the other PWM circuits or should I stick with the mosfet? If I do stick with the mosfet do I still need the 10W resistor? Any help would be great. I am on a pretty tight deadline!

Have just the same question

Use a device called a "low side switch" its packaged like a MOSFET (3 leads Gate, Source and Drain) but it has a built in flyback limiter that works closely like the low impedance injector drivers that are available on the MS2 & MS3. The difference is that it is integrated into one package. In my opinion, I think that by using one of these with a 60 Volt self limit, you can omit the diode and big resistor. Make sure that you mount it to the heat sink. There will a 2-4 Watts dissipated but it controls a PWM solenoid much better than using the flywheel diode. The higher you can allow the fly back pulse to go, the faster the solenoids response. With a 60 Volt switch, the flyback pulse is 48 Volts (60-12). This slows the solenoid current significantly faster than if we let it flywheel. Because it flywheels, the current is (on average) much higher and hence the need for the big resistor. What I'm getting at is that you want PWM solenoids to be fed similarly to a low impedance injector. The big automakers don't use big resistors anywhere!

Just a brief note: the Low side switch dissipates powe (in the form of heat) only during the "off" time while the resistor and flywheel diode dissipates during the "on" time.