The speed of a de motor is proportional to the applied voltage, but the torque diminishes at low voltages. Low speed performance is usually erratic when analog controllers are used, especially under changing load conditions. Pulse-width-modulated (PWM) controllers offer superior control and operate efficiently at low speeds.
The PWM controller shown in Figure 7-35 uses complementary half-H peripheral drivers (SN75603, SN75604) with totem-pole outputs rated at 40 V and 2.0 A. These drivers effectively place the motor in a full bridge configuration which has the ability to provide bidirectional control.
Timer U1 operates in the astable mode at a frequency of 80 Hz. The 100Ω discharge resistor results in an 8 µS trigger pulse which is coupled to the trigger input of timer U2. Timer U2 serves as the PWM generator. Capacitor Cl is charged linearly with a constant current of 1 mA from the IN5297, which is an FET current regulator diode.
Motor speed is controlled by feeding a de voltage of 0 to 10 V to the control input (pin 5) of U2. As the control voltage increases, the width of the output pulse (pin 3) also increases. These pulses control the on/off time of the two motor drivers. Note that the trigger pulse width of timer U1 limits the minimum possible duty cycle from U2. Figure 7-36 illustrates the analog control voltage versus drive motor pulse width.
Figure 7-37 illustrates the output waveforms of U1 with respect to the output of U2. The maximum duty cycle that may be achieved is about 98% at which time the control voltage is 12.5 V.
CAUTION: Careful grounding is required to prevent motor-induced noise from interfering with the proper operation of the Ul and U2 timer circuits. Supply lines must be bypassed and decoupled at each timer to prevent transients from causing circuit instability. Separate power supplies should be used for the motor and the timer circuits.