The LM3524, LM2904 in an arrangement which controls the speed of a motor without requiring the usual tachometer or other speed pick-off. The output transistors, paralleled in the common emitter configuration, drive the 2N5023 and the motor turns. The LM3524 output pulse is also used to drive a 1000 pF-500 kW differentiator network whose output is compared to the LM3524 internal 5V reference. The 10k-4k divider at the motor output insures the LF398 output will always be within the common range of the LM3524’s input. The 10k 1 μF combination provides filtering during the time the LF398 is sampling. Here is a schematic drawing:
The diode associated with this time constant prevents any possible LF398 negative output from damaging the LM3524. The 10 MOhm resistor paralleling the 0.01 μF sampling capacitor prevents the servo from “hanging up” if this capacitor somehow manages to charge above the motor’s back EMF value. The 39k 100 μF pair sets the loop frequency response. The maximum pulse width modulator (PWM) duty cycle is clamped by the 2k-2k divider and diode at 80%, thus avoiding overshoot and aiding transient response at turn-on and during large positive step changes. The 60k-0.1 μF values at pins 6 and 7 set the pulse modulation frequency at 300 Hz.
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