DC Motor Controller

A DC motor controller.

Here is a simple, practical application for an operational amplifier. Some DC motors come with a built-in tachometer. This can be used in the feedback loop of an op-amp circuit to keep the motor running at a constant speed.

Assume that the tachometer produces 1 volt for each 100 RPM. Connect the output of the tachometer to the inverting input of an op-amp. Then, connect a reference voltage to the non-inverting input. In the circuit above we use a potentiometer connected to a reference voltage. This reference voltage could come from a zener diode or another stable voltage. If we adjust the potentiometer to +2 volts, the op-amp will adjust its output to whatever it takes to put +2 volts on the inverting input. In this case, that voltage goes to the base of a transistor that can deliver the current needed to run the motor. When the motor is turning at 200 RPM, the tachometer is delivering +2 volts to the inverting input.

What's the voltage at the op-amp's output? I have no clue. All I can say is that it is about 0.7 volts higher than the voltage that will cause this particular motor to run at 200 RPM. What transistor do you use? Whatever transistor you can find that can handle the current. If you need a lot of current you may need a Darlington pair (see transistor signal gain). The op-amp will adjust its output voltage until it sees +2 volts at the inverting input. That voltage is whatever it takes.