With an integrated voltage regulator connected reverse to a mini drill can be adjusted speed within certain limits so that it remain constant, independently of load.
In the scheme presented below the electronic speed control is made by a voltage regulator, allowing use of motor which require a supply voltage from 2.5 V to 12 V at a maximum current absorbed by an 1A.
Output voltage is determined by the ratio of resistances R1 and R2. Uout = (R1 + R2) Uc/R2.
Uc is equal to -2.23 V for 79GU voltage regulator.
Integrated controller is designed to keep constant the value of -2.23 V, the voltage from common terminal .
With the P2 potentiometer can be adjusted the degree of coupling (coupling the output voltage increases with increasing output current). Thus remains constant preset speed.
Maximum voltage measured when the P2 is turned up must be about 20% lower than the maximum allowable voltage of the motor (if is not, the value of R1 should be reduced or enlarged accordingly).
Voltage regulator is designed with thermal protection, but it must be mounted on a properly dimensioned radiator.