Electrically-powered tool

Claim 1

An electrically-powered tool of the type powered from an alternating current source, comprising: an electric motor (M1) for driving said tool; a motor housing (15) mounted within said tool for enclosing said electric motor (M1); a main switch (SW1) mounted within said electrical motor (M1) and electrically connected in series with said alternating current source and said electric motor (M1); a switching element (TRI1) also connected in series with said electric motor (M1) for controlling the phase of current flow to said electric motor (M1); a revolution directing circuit (33) for generating pulses proportional to the speed of said electric motor (M1); a monostable multivibrator (Q3) for generating a fixed length of pulse responsive to the pulse outputs from said revolution detecting circuit (33); a smoothing circuit (34) for feeding the pulse outputs from said monostable multivibrator (Q3) to a capacitor (C3) and for generating a d.c. voltage proportional to the varying pulse frequency in proportion to the speed of said electric motor (M1); a first transistor (Tr1) for varying on resistance thereof in inverse proportion to the magnitude of the d.c. output voltage fed from said smoothing circuit (34); a second transistor (Tr2) for substantially varying on resistance thereof when the magnitude of the d.c. output voltage fed from said smoothing circuit (34) fails below the d.c. output voltage of said electric motor (M1) in an overload condition; a first and second series circuits in parallel: said first series circuit including said first transistor (Tr1) and a resistor (R9), said second series circuit including said second transistor (Tr2) and a resistor (R10); a capacitor (C5) in parallel with said series circuits and charging only when said main switch (SW1) is turned on, and a bias resistor (R8) in parallel with said capacitor (C5); a third transistor (Tr3) for varying on resistance thereof through the base voltage dependent on the voltage dividing portion of said parallel circuit and a resistor (R14); a capacitor (C4) charged through said third transistor (Tr3) during each half cycle of the a.c. voltage fed from said alternating current source; a programmable unijunction transistor (PUT1) adapted to conduct when the charged voltage of said capacitor (C4) becomes higher than the gate voltage set by resistors (R16 and R17), thereby developing a discharge voltage from said capacitor (C4); and a trigger circuit (36) for receiving the output voltage from said programmable unijunction transistor (PUT1) and for generating a trigger signal to the gate terminal of said switching element (TRI1).