GB909205A - Improvements in or relating to fault detection circuits - Google Patents

Abstract

909,205. Transistor pulse monitoring circuits. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. Jan. 30, 1959 [July 8, 1958], No. 21792/58. Class 40 (6). A monitoring circuit for a source of continuously recurring and regularly spaced pulses provides an alarm indication and a signal for disabling an external circuit controlled by the pulses when one of the pulses is omitted and also when the pulses exceed a given amplitude. In the Figure accompanying the Provisional Specification, the output of a pulse amplifier transistor TX10 is passed to a further amplifier TX1, the output of which is smoothed by a circuit L, C5, and used to render the transistor TX2 conducting. If a pulse is omitted, after a time determined by the time constant of the input circuit comprising capacitor C5, transistor TX2 becomes non-conducting so that an alarm LP lights and a signal is provided on line L2 for disabling a succeeding circuit such as a further amplifier fed from the output of transistor TX10. A thermistor TH ensures a rapid rise in voltage at the output of transistor TX2 when it is cut-off. The output voltage on a line L2 is also applied to the emitter of a transistor TX3 to render it heavily conducting so that the input circuit of transistor TX1 is virtually short circuited and the reappearance of input pulses does not terminate the alarm. The circuit may be reset by operating a key K to restore transistor TX3 to a condition in which it does not short circuit the input pulses. A diode D2 prevents the transistor TX3 being rendered conductive by the input pulses after the circuit has been re-set and a gating circuit comprising diodes D6 and D3 is provided so that if a diode D10 fails and the amplitude of the output pulses of transistor TX10 accordingly become accessive the gating circuit will conduct and render transistor TX3 conductive, causing the pulse input to transistor TX1 to be short circuited and the alarm to operate. In the Figure accompanying the Complete Specification the input pulses on line 0L are applied to a monostable transistor circuit 1TX1, 1TX2 so as to maintain it in its unstable condition. The absence of an input pulse or the cessation of the input pulses causes the mono-stable circuit to restore to its stable condition applying a pulse from the collector of transistor 1TX1 through a capacitor 1C4 to a bi-stable circuit 1TX3, 1TX4. This causes the bi-stable circuit to change its condition and thus render a transistor 1TX5 conductive to operate an alarm lamp 1LP. The changeover of the condition also provides a voltage on line L2 for disabling a circuit controlled by the input pulses. A diode circuit 1D1 prevents input pulses below a predetermined amplitude from passing to the mono-stable circuit so that a fault causing input pulses to be below this level also operates the alarm. If the input pulses exceed a predetermined level a gating circuit comprising a diode 1D5 passes the pulse to transistor 1TX3 causing the bi-stable circuit 1TX3, 1TX4 to change its state and operate the alarm and provide the control voltage on L2. The circuit is restored to its non-alarm condition by operating a key 1K. This also passes a pulse through 1D9 to the mono-stable circuit so that if the input pulses are absent the alarm condition is restored. Specification 844,907 is referred to.