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Simple Bidirectional DC Motor Speed Controller Without a Tachometer

For motor speed control without using a tachometer, the circuit in Figure 12 shows an approach. Using the enable feature of the LT1970A, the drive to the motor can be removed periodically. With no drive applied, the spinning motor presents a back EMF voltage proportional to its rotational speed. The LT1782 is a tiny rail-to-rail amplifier with a shutdown pin. The amplifier is enabled during this interval to sample the back EMF voltage across the motor. This voltage is then buffered by one-half of an LT1638 dual op amp and used to provide the feedback to the LT1970A integrator. When re-enabled the LT1970A will adjust the drive to the motor until the speed feedback voltage, compared to the speed-set input voltage, settles the output to a fixed value. A 0V to 5V signal for the motor speed input controls both rotational speed and direction. The other half of the LT1638 is used as a simple pulse oscillator to control the periodic sampling of the motor back EMF.

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