Motor controller operates without tachometer feedback


Posted on Apr 27, 2012

Speed controllers have long exploited the reverse-EMF characteristics of dc motors to control their speed. These linear driver circuits use power op amps to create a negative resistance drive to the motor that counteracts the voltage drop in the motor`s series resistance (Reference 1). The circuit in Figure 1 shows an implementation of this type of speed control using a PWM drive, which reduces power dissipation in the drive circuitry.



Control voltage VIN sets the speed, and IC1, Q1, and R3 convert VIN to a 0- to 200-µA current. The current source controls the duty cycle of the PWM driver, IC2, at Pin 3. D1, D2, and R5 prevent the circuit from pulling the control input too low, which




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