Patent GB-1144422-A (GB)
Title: Improvements in or relating to cycling electric time control apparatus
IPC: H03K17/292
Filed: 1967-07-11
Abstract: 1,144,422. Semi-conductor switching circuits. BENDIX CORP. 11 July, 1967 [18 July, 1966], No. 31891/67. Heading H3T. [Also in Division G3] Electric time control apparatus comprises a switch for connecting and disconnecting from an electric supply source two timing networks interconnected by an energy storage device, the discharge of the stored energy of which is controlled by one timing network, the switch having an actuator energized from the electric supply source, one timing network determining the time duration of a state of a controlled device, and the other timing network and said energy storage device determining the time duration of another state of said controlled device. The illustrated system is for the control of a solenoid actuator 18 of a valve regulating the inflation and deflation of de-icer bags on aircraft. When the switch 61 is closed the solenoid actuator 18 is energized via contacts 16e, 16b and 16a, 16f and power is supplied via contacts 16e, 16b to the first timing network 12 comprising a relaxation oscillator including a unijunction transistor 25 and providing triggering pulses for a thyristor 36. Charging current also flows to a capacitor 20 through a circuit comprising a resistor 40, the actuating member 24 of the switch 16, a Zener diode 56, and a resistor 58. This charging current is insufficient to operate the actuating member 24, but when the output from the timing network 12 renders the thyristor 36 conducting, the current flow through the member 24 is increased and the changeover switch 16 is operated to de-energize the network 12 and the solenoid actuator 18, and to energize the second timing network 14 comprising a relaxation oscillator including a unijunction transistor, 42. The output pulses from the relaxation oscillator 14 render a thyristor 54 conductive to discharge the capacitor 20. The charge on the capacitor 20 so biases the anode 54a of the thyristor 54 that it conducts only whilst receiving pulses from the unijunction transistor 42. The capacitor 20 is thus discharged intermittently. The thyristor 36, however, remains conducting when the network 12 is de-energized, and when the voltage across the capacitor 20 and the thyristor 54 falls to a level such that the thyristor 36 and Zener diode 38 are effectively short-circuited, the current through the actuator member 24, drops and changeover switch 16 is operated either mechanically or by transistor circuitry, and the next timing cycle is commenced. The solenoid actuator 18 is thus energized for a period determined by the time necessary for the unijunction transistor 25 to become conductive on energization of the timing network 12, and is de-energized for a period dependent upon the frequency of the output pulses from the timing network 14 and the time constant of the discharge circuit of the capacitor 20. If desired a switch 72 and a resistor 60 may be placed in parallel with the capacitor 20, so that when the switch 72 is connected to the resistor 60 an alternative mode of operation is effected in which the solenoid actuator 18 is de-energized for periods dependent upon a time of charging of the capacitor 20 via the thyristor 54, and is energized for periods dependent upon a discharging time of the capacitor 20.