Variable speed control for a hand-held electric power tool

Details Variable speed control for a hand-held electric power tool Variable speed control for a hand-held electric power tool

1996-03-28

1998-02-03

Sircus, Brian

Electricity: motive power systems

Induction motor systems

Primary circuit control

388820, 388919, 388937, 323324, H02P 700

Patent

active

057148619

ABSTRACT:
A power tool has a variable speed control circuit that comprises a user switch, a motor and a silicon controlled rectifier (SCR) connected in series between a pair of power supply terminals. A manually variable resistor is connected between a first electrode of the SCR and a circuit node. A capacitor is coupled between the circuit node and a second SCR electrode. A bleed-off resistor is connected between second electrode of the SCR and the circuit node to provide a path for discharging the capacitor when the user switch is open. A zener diode couples the circuit node to the SCR gate electrode. The circuit configuration ensures that the capacitor charges from substantially the same voltage level at the start of each half-cycle during which the SCR is to be turned on.

REFERENCES:
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patent: 3897595 (1975-07-01), Fearno
patent: 3936708 (1976-02-01), Dummer
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patent: 4052624 (1977-10-01), Hamstra
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SCR Manual, General Electric Company, 1979, pp. 252-254.
General Electric Application Note No. 200.35, p. 8.

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