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 .
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 .
