Speed control for power tools having a DC motor.

摘要

This invention is suitable for use in a power tool having a DC motor and a manually operated mechanism for controlling the speed of the motor. An astable multivibrator (26) having a variable duty cycle is coupled to a FET (22) connected in series with the motor (12) to function as a series switching device. A potentiometer (21) coupled to said control mechanism is used to control the duty cycle of the series switching transistor (22) thereby controlling the average current and hence speed of the motor. A first circuit (28) may be connected between the transistor (22) and astable (26) to control the turn off time of the transistor (22) to minimize transient voltage spikes.