GB1499676A - Contactless switching circuit - Google Patents

Abstract

1499676 Switching circuits for A.C. loads HONEYWELL Ltd 8 Jan 1975 704/75 Heading H2H A switching circuit for controlling the supply of A.C. to a load comprises a rectifier 31, a thyristor 32, a sensor circuit 33 including a high frequency oscillator which starts oscillating in response to the proximity of a piece of metal, a supply circuit 34 which energizes the sensor circuit only during the initial rising portions of the half cycles of the A.C. supply and a sensitivity circuit 35 which increases the sensitivity of the sensor circuit during a half-cycle following a half cycle in which the oscillator was operative. The load is connected to input terminals of the rectifier 31 and during the initial rising portions of the half cycles of the A.C. supply, a transistor Q1 is conductive to supply power to the sensor circuit. When the piece of metal approaches a coil L2, the inductance of the coil L2 is decreased and a differential amplifier formed by transistors Q6, Q7 enables the starting of the oscillator. An amplifier transistor Q10 turns on a transistor Q11 which switches on a transistor Q3 to fire the thyristor 32. A transistor Q4 is also switched on and is maintained on for the half cycle by the charge stored on a capacitor C3 if, during the preceding half cycle, the thyristor 32 was conductive. The output of the circuit 35 turns on a transistor Q12 which increases the current through a transistor Q8. The gain of the differential amplifier is increased thereby increasing the sensitivity of the circuit 33 so that the oscillator remains operative even if the piece of metal moves further from the coil L2 during the next half cycle.