The 4.7 ohm resistor

A forum for discussing applications and implementations of the MegaShift transmission controller code for the GPIO from B&G. This can control up to 8-speeds and 6 shift solenoids (plus a 16x9 table for controlling a PWM line pressure valve). It has manual and fully automatic modes (16x9 load x speed table), with under and over rev-limit protection, and full data logging of all inputs and outputs (among many other abilities). A TransStim to test your completed board is also available.
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roarin_mouse56
Posts: 7
Joined: Wed Aug 23, 2017 9:47 am

The 4.7 ohm resistor

Post by roarin_mouse56 »

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?
Rukavina
Posts: 47
Joined: Fri Aug 15, 2014 7:58 pm

Re: The 4.7 ohm resistor

Post by Rukavina »

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
roarin_mouse56
Posts: 7
Joined: Wed Aug 23, 2017 9:47 am

Re: The 4.7 ohm resistor

Post by roarin_mouse56 »

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.
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