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

I don't see why we need a 4.7 Ohm resistor in the PC circuit. The micro controller is sending PWM so why not decrease the duty cycle?

I believe, depending get on your trans, when at full pressure solenoid is off. As you lower the pressure the valve is on more.... and regardless of pulsewidth as duty increases valve will get hot. Think of pw as hz. If you have a long pw it might go back and forth 10 times at 100%DC. If you have a short pw it could go back and forth 50 times at 100%DC. Either way valve is constantly on at 100%DC. And duty cycle is the time on so as you drop pressure duty increases. So the resistor helps with overheating. I run mine full pressure and the resistor stays cool so it's not working much, only during shifts do I drop pressure

I did some SPICE modeling of the solenoid, the resistor a diode and a switch (BJT). With the flywheel diode, the current through the solenoid doesn't stop quickly enough to provide a linear average current across the range of duty cycles. That is, it's no longer PWM because the current rise time through solenoid is much faster than the fall time. If we remove the resistor and the diode and replace the transistor with a 60Volt internally clamped low side switch, it works and the low side switch power dissipation is 60W peak but only about 3W average.