Fail-safe voltage-limiting circuit for an audio frequency overla

Details Fail-safe voltage-limiting circuit for an audio frequency overla Fail-safe voltage-limiting circuit for an audio frequency overla

1985-05-16

1987-02-24

Caldwell, Jr., John W.

Railway switches and signals

Block-signal systems

Automatic

246122R, B61L 2502

Patent

active

046451480

ABSTRACT:
A fail-safe voltage-limiting circuit for an audio frequency-controlled railway track circuit includes a pair of NPN transistors arranged in the manner of a darlington pair, wherein the base connector of the first transistor receives an amplitude-modulated signal from a filter and detector circuit connected to the rails. A first pair of resistors arranged in series between the positive and negative system battery lines, includes a first fail-safe resistors also connected to the collector of the first transistor. The ratio of the first and second resistors are selected to limit the positive excursion of the input signal, thus clipping excess noise at this limit. A second pair of resistors is coupled to the second transistor to limit the negative excursion of the amplitude-modulated input signal. The second pair of resistors includes a second fail-safe resistor connected to the emitter of the second transistor and a third resistor connected in series to the second fail-safe resistor at this junction as well. An amplitude-limited sinusoidal signal is taken from the emitter of the second transistor and fed to a demodulator and driver circuit to operate a fail-safe relay as a function of the presence or absence of the amplitude-modulated portion of the input signal.

REFERENCES:
patent: 3345512 (1967-10-01), Failor
patent: 3715669 (1973-02-01), LaForest
patent: 4109205 (1978-08-01), Moorey
patent: 4179744 (1979-12-01), Lowe
patent: 4320880 (1982-03-01), Sibley
patent: 4560943 (1985-12-01), Negus et al.

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