Achieving a constant speed for a DC motor with a varying load is a difficult task. For miniature DC motors, this was a common issue faced by manufacturers of tape players – the motor would see a range in torque as the tape moved between spindles.
The problem is often solved with some form of sensor of the motor shaft, like an encoder, and a control loop. However, it’s not a suitable solution for all designs – extremely small motors often don’t have space for external sensors. There used to be lots of discrete ICs designed for this application, sadly since the demise of tapes these are difficult to find.
In this application bulletin we explain how it is possible to build a control loop with electronics by measuring the back EMF of the motor to determine its speed, and adjust the applied voltage accordingly. Whilst we cover some existing ICs, there are also example circuits based on transistors and op-amps too. Use the link below to access latest in our series of application bulletins: