Variable speed trigger switch

For a power tool having a motor and means for producing a feedback signal related to the actual speed of the motor, a variable speed trigger switch that is adapted to be connected to an AC power line and to the motor of the tool for controlling the application of power to the motor, comprising:

• a housing defining a trigger aperture, a pair of stationary contacts one of which is electrically connected to the AC power line, a strip of electrical resistance material, a spring return trigger slidably mounted in said housing through said aperture and carrying a bridging contact adapted to electrically interconnect said stationary contacts as said trigger is retracted and a wiper contact adapted to contact along said strip of electrical resistance material as said trigger is retracted, and an electrical motor speed control circuit mounted on a printed circuit board located within said housing and including a semiconductor control device connected in series with said motor across the AC power line for controlling the application of power to the motor, and control circuit means for controlling the conduction angle of said semiconductor control device relative to the AC power line signal in accordance with the position of said trigger;
• said control circuit means including:
• (i) an integrated circuit adapted to provide conventional closed loop motor speed control and comprising a first input port adapted to receive a voltage signal related to the desired motor speed, a second input port connected to receive said feedback signal, error circuit means for comparing said voltage signal to said feedback signal and producing an error signal in accordance with said comparison, a first output port connected to the output of said error circuit means, and timing circuit means responsive to said error signal for producing a firing signal at a second output port in accordance with said error signal, (ii) a voltage supply circuit for providing a fixed voltage signal to said first input port of said integrated circuit corresponding to a predetermined motor speed, and (iii) clamping circuit means connected between said wiper contact and said first output port of said integrated circuit for limiting the voltage of said error signal to a value related to the voltage at said wiper contact, such that for voltage signals at said wiper contact below the value of said fixed voltage signal, said control circuit means functions as an open loop controller by providing a firing signal to said semiconductor control device in accordance with the value of the signal at said wiper contact and allowing the speed of the motor to vary in accordance with the load applied to the power tool, and for voltage signals at said wiper contact above the value of said fixed voltage signal, said control circuit means attempts to maintain said predetermined motor speed when the power tool is loaded by increasing the conduction angle of said semiconductor control device to a maximum conduction angle related to the value of the signal at said wiper contact.