Circuit forms dc-motor switch with brake


Posted on May 26, 2012

Controlling a small dc motor without speed control sounds like a trivial task; a switch or a relay should suffice. However, several problems accompany this approach. For one, the switch, because of the inductive load and the low starting resistance of the motor, tends to wear out prematurely (with all the related sparks and EMI problems). Second, when you cut the power, the motor continues to rotate for a certain time, depending on its initial speed and inertia. The circuit in Figure 1 can be useful for designs that don`t need precise control of speed and stopping position but can benefit from enhanced deceleration.



The circuit comprises two parts. Q1 plays the role of the switch. D2 protects Q2 against inductive surges. Resistor R2 keeps Q1 off as long as switch S1 is open. R1 limits the base current of Q1 when S1 is closed. S1 can be a manual switch, a relay contac




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