Control circuit arrangement for an electromagnetically operated power tool

Claim 2

A control circuit arrangement for an electromagnetically operated power tool, comprising: a primary circuit for connection to an AC voltage supply and including a solenoid and a thyristor in series; said thyristor being capable of being triggered into a conducting state upon application of a trigger signal from a trigger control circuit for an adjustable part of a half-cycle of the AC supply voltage; said trigger control circuit being actuated upon closure of a main trigger switch connected thereto; said main trigger switch being connected in series with said solenoid and said thyristor in said primary circuit for supplying current to the solenoid; a delay circuit in said trigger control circuit for delaying the generation of a trigger signal for a predetermined period after closure of the main switch so as to delay firing of the thyristor to prevent arcing at the contacts of the main trigger switch; and said trigger control circuit including a ramp generator and phase shift circuit for generating a trigger signal for enabling energisation of the solenoid in response to actuation of the main trigger switch.

Claim 3

The control circuit arrangement as claimed in claim 1, wherein the ramp generator circuit is adjustable by means of a capacitor, and wherein the firing angle of the phase shift circuit is adjusted by means of a potentiometer thereby predetermining the amount of energy which is passed by the thyristor for energising the solenoid.

Claim 4

A control circuit arrangement for an electromagnetically operated power tool, comprising: a primary circuit for connection to AC main power voltage and including a solenoid and a thyristor in series; said thyristor being capable of being triggered into a conducting state upon application of a trigger signal from a trigger control circuit; said trigger control circuit being actuated upon closure of a main trigger switch connected thereto; said main trigger switch being connected in series with said solenoid and said thyristor in said primary circuit for supplying current to the solenoid; said trigger control circuit including a delay circuit for delaying the generation of a trigger signal for a predetermined period after closure of the main switch so as to delay firing of the thyristor to prevent arcing at the contacts of the main trigger switch; safety circuit means provided in the trigger control circuit to prevent the generation of a second trigger pulse for as long as the main trigger switch is held closed, said safety circuit means being reset upon release of the main trigger switch; said safety circuit means comprising a D-flip-flop one input of which is provided by the delay circuit associated with the main trigger switch and a further input of which is provided by a zero crossing detector connected to the main power voltage; and said trigger control circuit including a ramp generator and phase shift circuit for generating a trigger signal for enabling energisation of the solenoid in response to actuation of the main trigger switch.

Claim 5

A control circuit arrangement for an electromagnetically operated power tool, comprising: a primary circuit for connection to AC main power voltage and including a solenoid and a thyristor in series; said thyristor being capable of being triggered into a conducting state upon application of a trigger signal from a trigger control circuit; said trigger control circuit being actuated upon closure of a main trigger switch connected thereto; said main trigger switch being connected in series with said solenoid and said thyristor in said primary circuit for supplying current to the solenoid; said trigger control circuit including a delay circuit for delaying the generation of a trigger signal for a predetermined period after closure of the main switch so as to delay firing of the thyristor to prevent arcing at the contacts of the main trigger switch; safety circuit means provided in the trigger control circuit to prevent the generation of a second trigger pulse for as long as the main trigger switch is held closed, said safety circuit means being reset upon release of the main trigger switch; said safety circuit means comprising a D-flip-flop one input of which is provided by the delay circuit associated with the main trigger switch and a further input of which is provided by a zero crossing detector connected to the main power voltage; and said trigger control circuit including a ramp generator and phase shift circuit for generating a trigger signal for enabling energisation of the solenoid in response to actuation of the main trigger switch.

Claim 6

The control circuit arrangement as claimed in claim 3, wherein the ramp generated by the ramp generator circuit is adjustable by means of a capacitor, and wherein the firing angle of the phase shift circuit is adjusted by means of a potentiometer thereby predetermining the amount of energy which is passed by the thyristor for energizing the solenoid.