I am trying to figure out how to variably reduce the voltage running to a small DC motor. The motor is powered by a mosfet sending between 0.5v and 3v, so it seems a typical LM317 and potentiometer setup wouldn't work because of the dropout. I'd like to find a way to reduce the voltage by as little as 0.2v, without needing to manipulate the mosfet driver or other parts of the circuitry. Maybe this isn't possible in a simple way.

Obviously I am new to this sort of thing. Thanks for any advice.

 

What is the voltage rating of the motor?
It is not necessary to supply the exact motor voltage when it is controlled by PWM, it is common t0 use a supply at least 10% above the motor rated voltage.
It is only important where there is no controller between supply and motor.
Max..

 

Depending on the power needed by the motor a simple rheostat might do the job.

 

If the motor is currently being controlled by a Mosfet then I would think this will offer variable control?
What is before the Mosfet?
Max.

 

The OP says he doesn't want to "manipulate the mosfet driver or other parts of the circuitry".

 

I tried a rheostat but wasn't successful. I think because the current is around 1 amp. It was simply stopping the motor until the last few degrees of rotation toward zero resistance.

There is a mosfet driver before the mosfet, so I could intercept the PWM signal from the microcontroller to the driver, and run it through an Arduino so that I can add or subtract the PWM value needed, then send the new value to the driver. I don't have access to the firmware.

But I'm hoping to not have to do it this way.

 

What value rheostat did you try? You probably need a much lower value.

1 amp times 3 volts is only 3 watts on the motor, so a rheostat of the proper value should work.

You're probably looking for something in the 5 ohm range. (or less even)

 

A circuit is needed so we can see what you have I think.
And the motor will not run all the way down on whatever the control system you are using as the motor requires some power to start. So maybe you will have to go to at least 10 or 20% power before the motor will start to turn. And the mechanical load on the motor will change this greatly.
PWM control will be better than variable resistance. And do you have a suitable diode reverse connected across the motor?
And to get good motor speed control, some form of speed sensing is needed and a feedback system to control the PWM or applied volts incorporated.