# Using PWM to control Variable Force Solenoids

#### Bryan Denmark

Joined Mar 29, 2019
3
Hello, I have a small tractor using 12V DC and a 120A alternator. Movement is controlled by a hydraulic motor that is controlled by variable force solenoids(VFS). There are two VFSs, one for forward, and one for reverse, controlled by two POTs. Originally, 5V DC fed into each POT and the variable output went to a computer that then controlled the VFSs with variable 0-12V DC. The computer has died and cost $1200 to replace. My main question is, could I use a DC motor speed controller that uses pulse width modulation to control my VFSs, or would that just cause the solenoids to open and close rapidly, rather than a smooth gradual opening? I was considering trying two of the ones in the image below by connecting my POT instead of the one provided. I don't know exactly how much current the solenoids require, but I do know 20A is more than enough. I am open to building a circuit if I must, I just don't want to spend$1200. I can still use the original 5V or a 12V input for my POTs. They just don't output enough current to open the solenoids. I have plenty of amps available from the battery/alternator, but need to be able to control it smoothly using the POTs as they are attached to forward/reverse foot pedals.

#### MisterBill2

Joined Jan 23, 2018
4,583
How well the control works depends a lot on the frequency of the PWM switching. So it is something that you will need to experiment with. The hydraulic servo packages that I have worked with used linear DC control currents, I think up to 100mA, but possibly it was less than that. A suggestion to help with the experimenting is to add a 12 volt light in parallel with the solenoid valve coil to show what is happening.You will also need some arrangement to switch between valves with the one controller module.
But it should work. I suggest adding a "kill switch" so that you can quickly stop the engine if things start to go wrong.