GB1531217A - Proximity switch - Google Patents

Abstract

1531217 Proximity switching systems SQUARE D CO 31 Oct 1975 [22 Nov 1974] 50629/74 Heading G1N A proximity switch actuated by the presence of a metallic object to connect a load to a supply and which is energized from the same supply includes energy storage means for supplying the switch during the time it is actuated. A load 10 such as a motor or counter is connected in series with A.C. supply 11 through proximity switch 15 which is actuated when a metallic object is in a predetermined position relative to coil 51 of oscillator 50. In the absence of the object oscillations from the oscillator are coupled via capacitor 53 to the base of transistor 41 so that the latter is rendered conducting during positive half cycles to provide a discharge path for capacitor 45. As a result SCR 31 remains non-conducting and supply 11 remains unconnected from load 10. When a metallic object approaches coil 51 the oscillations are damped and transistor 41 remains cut off. Capacitor 45 thus continues to charge through resistor 42 until it reaches the breakdown threshold of diode 46 when the latter breaks down and capacitor 45 discharges through the gate of SCR 31 to switch the latter on and connect the supply through rectifier 20 to load 10. Since the supply to the proximity switch 15 is now shunted through the SCR in order to keep the circuit energized a storage capacitor 75 is connected as shown which charges up during the period that the SCR is non-conducting and supplies the circuit when the SCR conducts. In order to prevent the SCR being switched on during the initial energization of the switch transistor 64 is connected to the base of transistor 41. When the supply is first switched on transistor 64 is non-conducting and the bias on transistor 41 is such that this transistor remains conducting and cannot switch off in response to decreases in the oscillator output. When the charge on capacitor 66 reaches a predetermined level transistor 64 is rendered conducting and the bias on the base of transistor 41 is such that this transistor may be switched on by the positive cycles of the oscillations and off when these decrease. The characteristics of circuit 40 are such that the SCR turns on only after the positive voltage in any half cycle of the supply reaches a minimum value to make capacitor 75 to have charged to a sufficient level. LED 81 lights when SCR 31 is fired.