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

1994-05-27

1996-04-30

Shoop, Jr., William M.

Electricity: motive power systems

Induction motor systems

Primary circuit control

388820, 388917, 388919, 323324, H02P 700

Patent

active

055128106

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

REFERENCES:
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patent: 4157491 (1979-06-01), Werner et al.
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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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