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# Is this circuit capable of PWM??

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#### strokedmaro

##### New Member
Attached is a design that was intended to simply turn the 5 ohm solenoid on and off. Would it be possible to pulse this with a PIC? I'm trying to get information on the duty cycle and frequency but I'm unsure if the hardware I used to build it is capable? What would you say is the maximum possible frequency this circuit could reliably pulse at? Currently I'm guessing no more than 100Hz is required but again I'm trying to find out an exact number....THANKS!

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It'll all depend on how fast the solenoid plunger can move. Most can't go at 100 Hz.

What will the solenoid be used for?

thats the thing...its a transmission solenoid (from a transmission that's not designed to go in my car). The new one has a solenoid designed to pulse but I'm not sure of the specifics...The old one is just constantly on. So Im trying to take a signal thats "always on" and pulse it at the rate required for the new one. The question is how much can this hardware take? Not that I cant use other stuff but Ive build this already in with the other logic required for the new tranny. Thanks!

P.S. Everything else works perfectly...This new solenoid also works but Im afraid that being "on" all the time will kill it quickly

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The circuit will PWM beyond 25 kHz as long as the solenoid doesn't have to follow it. I guess the objective is to induce a current from 0 up to 2.4A (12V/5ohms) into the coil, and have the coil do the smoothing. Should work.

To get the "exact" frequency, you need the inductance of the coil, the mass of the solenoid plunger, and the spring constant of the return spring (or likely in this case the hydraulic pressure). At higher frequencies, eddy currents in the solenoid frame and plunger are noticeable. Using all of the constants, variables, and parameters above, solve for the resonant frequencies and associated damping factors. Then decide if you want to operate at mechanical resonance, the electrical resonance, or more likely, to avoid both.

My guess is it's a highly damped system and the frequency doesn't matter, within reason. Try 100 Hz and if it chatters move up till it doesn't. If you get into kHz and it stars to sing, you may be higher than necessary.

I asked around and found that its probably more in the range of 50-60Hz...much easier to figure out. I also just received a meter in the mail that I can use to find the Hz and duty cycle....now just have to find someone with this transmission to let me grab the numbers Thanks again for all the help!

Automotive solenoids that receive PWM don't turn on & off rapidly. The purpose of using PWM is to allow the solenoid to open a varying amount from fully closed to fully open. The solenoid will be damped and spring loaded, so applying 50% duty cycle (for example) would have the solenoid open up appx 50%.

Some solenoids in transmissions do nothing more than turn on or off (to change fluid flow for a shift) while others are PWM (controlling line pressure based on engine torque).

Why did you change the solenoids in your transmission? What type of tranny is this, what car and what are you trying to do?

The transmission is a 4L80E..the pwm solenoid in this transmission is designed to pulse the torque converter clutch at varying speeds to help with the loss in efficiency of the converter. It was not designed to lock the clutch completely as my old 4L60E transmission was designed to do. Ive already completed and tested the control logic (they are different) and this is basically the last part.

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