Patent GB-1466319-A (GB)
Title: Current-monitoring apparatus
IPC: G01R19/165
Filed: 1975-10-27
Abstract: 1466319 Lamp failure alarms SMITHS INDUSTRIES Ltd 27 Oct 1975 [14 Oct 1974] 44483/74 Heading G4N Current-monitoring apparatus, e.g. the current through a vehicle lamp, comprises a magnetizable core member 15 in which a magnetic flux is induced by the current to be monitored, pulse generating means for supplying a train of flux-inducing current pulses to the core member, the train comprising a first group of pulses for inducing flux pulses of one sense and a second group for inducing flux pulses of the opposite sense and interposed among the pulses of the first group, the magnetic field strength of each flux pulse being sufficient to cause change in the direction of magnetization of the core member only if the magnetic field strength due to the current to be monitored is less than a predetermined value, and sensing means 18, 31 for providing a signal in response to a change of direction of magnetization, wherein the pulse generating means supplies the current pulses in one of the groups through a warning device 30 and is arranged to vary the duration of those pulses in response to said signal to change the state of the warning device 30. As shown, the side lamps 11 of a vehicle are connected to a supply line 13 via lines 12 which pass through respective torroidal ferrite cores 15 and a switch 14. The pulse generating means comprises an oscillator 19 which provides a pulse train as shown in Fig. 2(b) to a bi-stable circuit 20 and a pair of AND gates 21, 22, the outputs Q, Q of the bi-stable shown in Figs. 2(c) and 2(d) being applied respectively to second inputs of the AND gates 21, 22. Only when the switch 14 is closed does an amplifier 25 connected to the output of AND gate 21 supply a first group of pulses (SET pulses), see Fig. 2(e), along a line 16 threaded through the cores 15, and an amplifier 29 connected to the output of AND gate 22 supplies the second group of pulses (RESET pulses), see Fig. 2(f), along a line 17 threaded through the cores 15 and through the warning lamp 30, these pulses being sufficiently well spaced and of sufficiently short duration not to cause lamp 30 to be lit. Should one of the lamps fail, the flux in the associated core 15 is caused to oscillate due to the alternate SET and RESET pulses and pulses as shown in Fig. 2(g) are supplied over lines 18 to the amplifier 31. This amplifier receives STROBE pulses, Fig. 2(h), from oscillator 19 via a delay circuit 23, and provides an output, Fig. 2(i), upon occurrence of a positive pulse on line 18 in the presence of a STROBE pulse. The output of amplifier 31 is supplied to a switch 24, the switch 24 being turned on by an output pulse of amplifier 31 to supply a pulse, Fig. 2(j), to the amplifier 29, this pulse lasting until the occurrence of the next reset pulse. Thus the amplifier 29 supplies a pulse train having a pulse duration equal to the sum of the durations of the output pulse of switch 24 and the reset pulse to cause the lamp 30 to be energized intermittently. A second amplifier 26 may be coupled to the output of gate 21 for supplying SET pulses to the brake lights. In the embodiment of Fig. 4 (not shown), this feature is described, the lines 17, 18 passing through all the cores of the sidelights and the brake lights and the line 16 passing through the cores of the brake lights being energized only when the brakes are applied. In this embodiment capacitors are provided for supplying a pulse to the sidelight liner 12 and to the oscillator 19 when the brakes are applied for checking the sidelights, the lamp 30 providing a single flash if one of the lights 11 has failed. The sidelights may then be switched on to confirm this.