GB927372A - Railway track circuits - Google Patents

Abstract

927,372. A.C. track circuits. WESTING- HOUSE AIR BRAKE CO. Oct. 25, 1960 [Oct. 28, 1959], No. 36564/60. Class 105. An A.C. track circuit, which in addition to normal operation also releases its track relay for a limited period when the track rails are subjected to a relatively high resistance shunt, comprises an insulated track section 10 which is connected at one end to an A.C. source 9 and at its other end to a saturable transformer 31 which supplies track relay 55 via the emittercollector path of transistor 43, connected in series with non-saturable transformer 25, which supplies an output for controlling the conduction of the emitter-collector path of transistor 43 via a second transistor 81. The base circuit of transistor 43 is biased from the rectified output of transformer 31 via a voltage dividing network including thermistor 63 and resistor 65, shunted by condenser 59, and the emittercollector path of transistor 81 and the base circuit of transistor 81 are biased from the rectified output of transformer 25 via a voltage divider circuit which includes resistor 77 and resistors 89 and 73, shunted by a large value condenser 71. In normal operation, both transistors conduct and relay 55 is energized. On application of a low-resistance shunt across the rails, the output of transformer 31 reduces to a sufficiently low value to release the relay. On application of a relatively high resistance shunt the whole of the voltage reduction is applied to transformer 25, since saturated transformer 31 tends to maintain a constant input voltage and this decrease produces a reversal in the polarity of the voltage drop across resistor 77 during the discharge time of condenser 71. Transistor 81 cuts-off and the base potential of transistor 43 rises to a positive value, with respect to its emitter, which cut-off the transistor and releases relay 55 in its collector circuit. Relay 55 remains released for a period determined by the discharge time of condensers 71 and 59 and its own inherent time delay.