DC Motor Speed Controller using a LM317

A very simple DC motor speed controller circuit can be constructed using a LM317 voltage regulator integrated circuit. This DC motor speed controller can be used for speed control  of mini drills or for other small DC motors. This motor controller circuit will provide a large output current. The maximum output current from the secondary turns of the transformer shout provide 1.5 times of the maximum DC output current. The output voltage and the speed (rpm) is set by the P2 variable resistor.

DC Motor Speed Controller using a LM317: 

DC Motor Speed Controller using a LM317:

As soon as the current drawn exceeds a certain value, T2 will be switched on. This results in a base current for T3 so that R5 is in parallel with R6. This automatically raise the output voltage to counter a threatened drop in rpm.  The moment at which this action occurs is set by P1.

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