IE41228L - Gaurd circuit for relay - Google Patents

Abstract

1472275 Transistor switching circuits STANDARD TELEPHONES & CABLES Ltd 18 April 1974 17011/74 Heading H3T A guard circuit for a bi-stable relay RLY comprises a switch circuit TR4, TR5 having a pair of terminals which are connected in series with the relay coil between the terminals of a power supply V2, V0; a control terminal (junction of diode D2 and collector of transistors TR3) which can be clamped to inhibit current flow through the switch; and a capacitive feedback path C2 which prevents current flow through the switch when the voltage at the junction between the switch is driven beyond the voltage V 0 of the terminal to which the relay coil is connected. To ensure that relay RLY is initially in the state in which contact NC is made supply V2 is connected before supply V1 so that if contact NO is made current will pass through the switch TR4, TR5 to reset the relay to the state in which NC is made. To change the state of the relay supply V1 is connected and a positive signal is applied to input 11 to turn on transistors TR1, TR3 and discharge capacitor C1 through relay RLY thus setting it to the state in which contact NO is made. During the discharge pulse of capacitor C1, capacitor C2 hold the base of transistor TR5 and the emitter of TR4 at the same potential. At the termination of the pulse, diode D2 clamps the base of transistor TR5 to V 0 , so that in neither case can switch TR4, TR5 pass current. In order to reset the relay, the positive signal is removed from the input 11 so that transistors TR1, TR3 turn off and transistor TR2 turns on to charge capacitor C1 producing a pulse which will switch relay RLY back to its other state. In place of transistors TR4, TR5 a single bi-polar transistor, a single field effect transistor, or a combination of both may be used. [GB1472275A]