GB836435A - Protection of electronic apparatus from current and voltage surges - Google Patents

Abstract

836,435. Railway track circuit protective devices. GENERAL RAILWAY SIGNAL CO. Aug. 22, 1957 [Sept. 4, 1956], No. 26527/57. Class 105. [Also in Group XXXV] A push-pull electronic amplifying stage, particularly a push-pull transistor amplifying stage in a railway track circuit, is protected from voltage surges due, e.g. to lightning, external voltage surges and static discharges by means of a capacitor 36 connected across one diagonal of a rectifying bridge 37-40 which bridge is connected across the output 41, 42 of the push-pull amplifying stage 22-25 so as to absorb the transient voltage surges applied thereto, e.g. from the track rails 5 via the track circuit feed transformer T2, the condenser 36 absorbing the surges and having internal leakage characteristics to dissipate the excessive charge due to the surges. As shown, the capacitor 36, which may be an electrolytic condenser, has its positive terminal connected in common to all the transistor emitters of the amplifier and to the mid-point 29 of an input transformer T1 to the transistors and to the positive terminal of a transistor biasing D.C. source 13. The amplifier is fed from a transistor oscillator stage. In the railway track circuit shown, a lightning arrester 33 is connected across the rails to provide an equalizing path between the rails for relatively high voltage static discharges, the capacitor 36 absorbing the energy for relatively low voltage static discharges. Other lightning arresters 32, 34, are connected between the respective rails and ground. Specification 836,434 is referred to.