Patent GB-999201-A (GB)
Title: Semi-conductor switching circuits
IPC: H03K17/72
Filed: 1963-12-13
Abstract: 999,201. Transistor switching circuits; telegraphy. STANDARD TELEPHONES & CABLES Ltd. Dec. 13, 1963 [Dec. 14, 1962; May 7, 1963], No. 49436/63. Headings H3T and H4P] Two semi-conductor controlled rectifiers are connected to alternatively supply a load and are controlled by a respective one of two oscillators brought into operation alternatively by an input signal. Fig. 1 shows a telegraph repeater for single or double current signal and comprising two similar units U, U<SP>1</SP>. When the incoming signal operates an electronic or mechanical contact t 1  to apply a negative voltage to the incoming line TL 1  current flows through transistor T and diode D 1  but is blocked from the corresponding circuit Ul by diode D<SP>1</SP> 1 . Transistor T oscillates in blocking oscillator manner producing pulses which fire a thyristor D to correct the negative supply to the output terminal. If contact t 1  is changed over, a positive supply is applied to line TL 1  and this energizes the oscillator in unit Ul but as diode D 1  is now reversely biased, de-energizes U. Thyristor D<SP>1</SP> is thus fired connecting the positive supply 3<SP>1</SP> to the output terminal. A capacitor C 4  which was previously charged to the voltage between the supplies 3<SP>1</SP> and 5 is connected by its upper terminal to the output terminal when thyristor D<SP>1</SP> fires and as the output capacitor C 3  momentarily holds the output terminal at its previous negative value, the capacitor C4 applies a reverse voltage to thyristor D and it is therefore extinguished. When the contact t 1  is returned to the negative potential thyristor D is again fired. Capacitor C, which had previously been discharged through D<SP>1</SP> and R 2  is now, by its lower terminal, connected to thenegative supply 5 and since capacitor C 3  momentarily maintains the lower terminal of D1 at the positive value, thyristor D<SP>1</SP> is reversely biased and is extinguished. D 3  and D 4  limit the output potential swing when the load is inductive. Single current working is effected by contact t 2  acting through the dotted connections, the full line external connections between terminals 1, 2<SP>1</SP>, 11 and 2 being omitted. When t2 is open current flows through t<SP>1</SP> 2  from the positive source through unit U and R 7  to sender thyristor D conducting. When t 2  is closed, current from (1, flows through Ul, the dotted connection between 6 and 7 and line TL 2  to a negative supply so that D<SP>1</SP> is rendered conductive. The current between 6 and 7 produces a voltage across Rs such that a Zener diode Da conducts and renders a diode D b  conducting. Accordingly the blocking oscillator transformer is shortcircuited and oscillation ceases. Commutation of the thyristors is affected by C 4  in the same way as in double current working. Operation of contact t<SP>1</SP> 2  with t2 closed will operate a receiver RC 2  while maintaining the blocking oscillators T unoperative. It is stated that the thyristors may be of the " turn off " type in which case commutation may be effected by cross-coupling instead of by 0 4 . Alternatively, the thyristors may be extinguished by an auxiliary pulse source. It is also stated that the blocking oscillator could be rendered inoperative by saturating the core instead of by short-circuiting. Fig. 2 shows an alternative circuit in which one or other of the blocking oscillator transistors T, T<SP>1</SP> is operating depending upon the polarity of the supply voltage across C;, the inoperative transistor being shunted by a diode D 1 , or D<SP>1</SP> 1 . The polarity of the output voltage from the common transformer depends upon which oscillator is operating and diodes D 2  and D<SP>1</SP>, select the voltage spike so that thyristor D or D<SP>l</SP> is triggered according to the polarity of the voltage across C 1 .