GB1480198A - Electronic switching circuit - Google Patents

Abstract

1480198 Switching circuits; registers INTERNATIONAL STANDARD ELECTRIC CORP 8 May 1975 [13 May 1974 3 June 1974] 19382/75 Headings H3T and H4K An electronic telephone call storage arrangement includes a plurality of identical switching circuits, each of which includes a transistor and a Zener diode, and means whereby the switches can be reset to their rest state simultaneously. In a first embodiment each switch includes a transistor, such as T1, Fig. 3, which is made to conduct, thereby applying earth potential N to a terminal F1 to which a load can be connected, by applying a negative battery potential to the terminal S1. The Zener provides protection against reverse voltages resulting, for example, from inductive loads. In a second embodiment, in which each switch once put on, stays latched on, each switch includes an SCR, such as SCR1, Fig. 6, and a protective Zener Z1. In the quiescent state a terminal M1 which functions as both the input and the output terminal, is floating. The switch is turned on by earthing terminal M1, (as by closing a switch E1, Fig. 5, not shown), causing current to flow via a resistor RF1 to charge a capacitor C1, and the direction of the current flow is such as to make the gate-cathode potential of the SCR negative. When, subsequently, earth is externally removed from the terminal M1, the capacitor C1 discharges via resistor RF1 causing current flow in the opposite direction. This makes the gate-cathode potential of the SCR positive so that the SCR fires, current passing from earth T via the SCR and resistors RD1 and R to negative battery -B, thereby again applying earth to the terminal M1. The SCR remains on until the earth supply at T is removed.