When absolute precision isn't an issue, engineers can select PWM control and back-EMF (BEMF) sensing for a dc motor's speed control. The motor must also be protected from overload. Typical solutions use a current-sensing transformer, or sensing resistor, to measure the current through the motor. But that may not be the preferred solution when considering factors like low sensitivity, noise pickup, physical size, and price.
The circuit of Figure 1 handles speed control without a sensing transformer or resistor. Transistor Q2 acts as a differential amplifier. It takes the voltage across the motor and shifts its level. D2 blocks the reverse flow of current. D3 closes the motor circuitry for its backrush current, which is induced by the armature-coil inductance. Also, D2 and D3 are more effective than the LC noise filter commonly used to reduce EMI.
The shaded area in the upper curve of Figure 2 represents the voltage difference between a power source and the motor (V1 − V2). That also is supplied to the motor to produce the BEMF of the next cycle. Intuitively we can say that it's proportional to the current, or power, delivered to the motor.
In the first figure, a sample-and-hold amplifier catches the BEMF. A differential amplifier extracts the applied voltage, shown in the shaded area of the second figure's signal. That voltage, V5, is integrated to yield the total power supplied to the motor, then compared to a desired load threshold level. This is just a basic illustration of the concept. The circuitry was completed using two LM324s and other discrete components, including thermal-shutdown circuitry.