Small DC motor electronic speed stabilization circuit

Figure WS-1 is a typical circuit for electronic speed stabilization of a commonly used small DC motor. BG1 is a speed regulating tube, BG2 is a comparison amplifier tube, and the comparison voltage is obtained from points A and B at both ends of the bridge composed of R1, R2R3 and the motor M. W is a speed fine-tuning resistor. When the circuit is in a parallel state, the voltages at points A and B are zero, no current flows through W, and BG2 is not conducting. Since the motor generates a reverse electromotive force when it rotates, the magnitude of this reverse electromotive force increases with the increase of the motor speed. The higher the reverse electromotive force, the smaller the current flowing through the motor, which is equivalent to the increase of the internal resistance of the motor; the lower the reverse electromotive force, the greater the current flowing through the motor, which causes the potential at point A to increase. This increased potential is coupled to the emitter of BG2 through D1 and D2, causing the emitter potential of BG2 to increase. Relatively speaking, the base potential of BG2 decreases, so the collector current flowing through BG2 decreases, the base current of the speed control tube BG1 decreases, the internal resistance of BG1 increases, the output current decreases, and the motor speed decreases.

On the contrary, when the motor speed decreases, the internal resistance of the motor becomes smaller, the potential of A decreases, and the emitter potential of BG2 also decreases. This is equivalent to the base potential of BG2 increasing, the collector current of BG2 increasing, the internal resistance of BG1 decreasing, the output current increasing, and the motor speed increasing.