GB875247A - Transistor relay circuit - Google Patents

Abstract

875,247. Transistor switching circuits. STANDARD TELEPHONES & CABLES Ltd. June 19, 1959 [June 25, 1958], No. 21117/59. Class 40(6) [Also in Group XL(b) ] A switching circuit for converting single current signals to double current signals comprises two transistors of the same conductivity type arranged in series between the positive and negative terminals of a supply, an output circuit connected between the junction of the transistors and the centre tap of the supply, a resistor connected between the transistors and arranged so that the transistors conduct alternatively, an input circuit connected to one of the transistors comprising a transistor of opposite conductivity type and a positive feedback circuit between an output terminal and the input transistor. The Figure shows two transistors TS1, TS2 connected between the positive and negative terminals u of a supply source and output terminals a, b connected to the junction of the transistors and the centre tap of the source. Transistor TS1 is controlled by the current through R6 so that when transistor TS1 is conducting transistor TS2 is cut off and vice versa. Input telegraph signals are applied to transistor TS1 through a transistor TS3 of opposite conductivity type so that when the input transistor is conducting transistor TS1 is also conducting and TS2 cut off the output terminal a thus being connected to the positive battery terminal. When the input transistor is cut off transistor TS1 is cut off and TS2 conducting so that the output terminal is connected to the negative source. Positive feedback via a resistor R1 speeds the changeover between the conducting conditions. In a further embodiment (Fig. 2, not shown) temperature stability is improved by connecting a temperature sensitive resistance network between the emitter transistor TS1 and a resistor in the emitter circuit of transistor TS3. In addition, the transistors are protected against excessively high voltages which may be produced if the load is inductive or if the output line is faulty by connecting a capacitor and a voltage dependent resistor across the output terminals. The resistors R4, R6 and R8 may be reduced or eliminated if they are supplemented or replaced by diodes (Fig. 3, not shown).