Electric Lamp Failure Indicator Circuit

The present invention relates to electric lamp failure indicators and more particularly to a warning device for providing an indication that failure has occurred, e.g. in a lamp which fulfils an important function. The invention is especially, although not exclusively, applicable to front and rear lamps of motor vehicles to provide an indication to the driver, inside the vehicle, that such lamps are functioning correctly.

Although in the following description reference is made specifically to motor vehicles, the invention is also applicable to apparatus and instruments where one or more lamps are relied upon to provide important information and where it is desired to avoid unrecognized failure of such lamps.

Lamp failure indicators for use with front and rear lamps on motor vehicles have been proposed previously. In one system, a low value electrical resistor is connected in series with a lamp being monitored; the voltage drop across the resistor which results when the lamp is switched on and working is combined with corresponding signals from the other lamp circuits to hold off a warning lamp. If any one lamp fails, the warning lamp lights. The disadvantage of this system is that the presence of the resistor reduces the output available from each lamp and the need for an expensive circuit arrangement to detect the small signals from said resistor.

In another known system, a current balancing relay is employed. The current taken by each lamp being monitored flows through a solenoid, the solenoids being arranged so that the magnetic field produced by the current of one lamp of a pair opposes the field produced by the other lamp of the pair and thus a null field exists when all lamps are operating. If a lamp fails, the magnetic field is no longer balanced and a magnetic reed switch is caused to be closed to switch on a warning lamp. For this approach to be successful, there must be a reasonable close balance in the power taken by each lamp in a pair and this causes difficulties due to the wide tolerance in lamp ratings.

It is a purpose of the present invention to overcome or minimize one or more of the disadvantages of the above lamp failure indicators.

Both known systems described above can only indicate whether a lamp is operating correctly or has failed when the lamp is switched on, but it is desirable to have an indication that the lamp has failed even when it is switched off, e.g. during the hours of daylight, in the case of a motor vehicle, when it is most convenient to replace the lamp.

The present invention provides a device for indicating an electric lamp failure, comprising (i) a core of magnetic material in the form of a closed loop, (ii) a first wire winding on the core, adapted to be connected in circuit with the lamp in such a way that electric current flowing through the lamp also flows through said winding, the number of turns on the first wire winding being sufficient to result in magnetic saturation of the core, (iii) a second wire winding on the core, adapted to be connected to an electrical circuit, which circuit is arranged to generate electrical current pulses in the second winding, thereby inducing variations in magnetic flux density in the core, and (iv) a third wire winding on the core, adapted to be connected to an electrical circuit capable of detecting the amplitude of a voltage or current induced in the third winding as a result of a change in flux density in the core due to a current pulse in the second winding, thereby providing an indication of whether or not the lamp is operating. The three wire windings may be composed of any suitable number of turns. Suitably the magnetic core is a ferrite and is of toroidal form. The magnetic material may exhibit linear characteristics or square loop characteristics.

The invention is described hereinafter with reference to the accompanying drawings in which:

FIG. 1 shows, in schematic form, one embodiment of the device of the invention,

FIG. 2 shows a typical magnetic flux density -- magnetic field strength loop for a core of linear magnetic material applied to the device of the invention, and

FIG. 3 shows another embodiment of the device of the invention.

In one embodiment, the second winding is adapted to be connected to a circuit which produces current pulses through said winding in such a direction as to increase the field strength and hence the flux density in the core, and the third winding is adapted to be connected to a pulse amplitude discriminator adapted to operate an indicating or alarm device when the amplitude of the change in flux density in the core due to a current pulse in the second winding exceeds a certain preset level. When the lamp current is flowing in the first winding and each of the second and third windings is connected with its associated circuit, the magnetic core is biased to operate in the saturated region and under these conditions, the electrical output from the third winding, due to the pulse current in the second winding, is small. However, if the lamp, or associated circuit, should fail, so that there is no longer any current flowing in the first winding, then the condition of magnetic saturation of the core, which was due to the lamp current flowing in the first winding, no longer holds. The core is then magnetically unsaturated and the electrical output from the third winding becomes of much greater amplitude. This increased amplitude is detected by the pulse amplitude discriminator which operates an indicator or alarm such as a warning lamp, buzzer etc.

A second embodiment enables two lamps to be monitored using a pair of first windings on a single magnetic core. Each of the windings is adapted to be connected in circuit with a lamp to be monitored and arranged so that, when the lamp is working, the current passing through its associated winding is such as to result in magnetic saturation of the core, the pair of first windings being arranged so that, when both lamps are operating, the magnetic field produced by the current in one winding is opposed by that produced by the current in the other winding. The result is zero magnetic field due to the pair of first windings, when both lamps are operating. The current pulses passing through the second winding are arranged to be alternately negative pulses and positive pulses. When both lamps are operating, the amplitude of the output from the third winding is large; if one lamp fails, then the condition of opposing magnetic fields no longer exists and the core becomes magnetically saturated. This saturation is in either a negative or positive sense depending upon which lamp of the pair fails. When this occurs, the amplitude of the output from the third winding decreases. This change in amplitude is detected by the circuit associated with the third winding which then operates an indicator or alarm.

The device of the invention is able to monitor the condition of a lamp in its unenergized or energized state. In the case of the first embodiment referred to above, the addition of a capacitor connected in parallel with the first winding and lamp is necessary to allow the monitoring of the lamp when this is unenergized; the second embodiment referred to above enables monitoring of an energized or unenergized lamp to be effected without any additional circuit components.

Referring to FIG. 1 of the drawings, a core comprising a toroid 1 of a magnetic material composed of a ferrite is provided with a first wire winding 2, a second wire winding 3 and a third wire winding 4. The first winding 2 is connected in circuit with a lamp 5 which is to be monitored, a battery 6 which powers the lamp and a switch 7 which controls the lamp. The second winding 3 is connected to a current pulse generator 8 which provides current pulses in the second winding and thereby induces variations in magnetic flux density in the core 1. The third winding 4 is connected to a pulse amplitude discriminating circuit 9, comprising a voltage comparator, which triggers an alarm or indicator 10, such as a warning lamp or buzzer.

The operation of the device is best understood by reference to FIG. 2 of the drawings. In FIG. 2 there is shown a typical B-H loop for a linear magnetic material composed of a ferrite from which the toroidal core 1 of FIG. 1 is made, where B represents the magnetic flux density and H represents the field strength. In the region 11, the flux density B is an approximately linear function of the field strength H and the material is said to be unsaturated. Outside this region, e.g. at regions 12, the magnetic material of the core is said to be saturated and increasing field strength results in only a very small increase in flux density. The field strength H necessary to operate the material at any point on the B-H loop is provided by the current passing through the first winding 2 when the lamp 5 is operating. The field strength produced is proportional to the product of the number of turns of wire in the winding 2 and the current flowing through the winding. If the field strength H is varied, the flux density B also varies and the amplitude of the variations of flux density depend on the variations in field strength and on the steady state operating point on the B-H loop. If the steady state operating point is in the linear region 11, then any variations in field strength will produce larger changes in flux density than would variations of the same amplitude if the operating point is in one of the regions 12 of magnetic saturation. In the embodiment shown in FIG. 1, variations in field strength of constant amplitude and rate of change are applied to the magnetic material by passing current pulses from generator 8 through the second winding 3 on the core 1. When the lamp being monitored is operating, the current flowing through the first winding 2 of the core results in magnetic saturation of the core and the steady state operating point on the B-H loop is at S. The amplitude of the corresponding change in flux density at this operating point is small (reference numeral 13 in FIG. 2). If the lamp 5 fails, then no current flows through the first winding 2 and the core 1 is no longer magnetically saturated and the steady state operating point changes to 0 in FIG. 2. At this point, the amplitude of the change in flux density due to the current pulses passing through the second winding is significantly greater than when the operating point was at S (see reference 14 of FIG. 2). The amplitude of the change in flux density is sensed in the third winding 4 on the core voltage pulses appear in the winding 4 and the amplitude of these pulses depends upon whether the operating point is in the magnetically saturated or unsaturated region of the material of core 1. A voltage comparator 9 is connected to the winding 4 and when the lamp 5 fails, the increased amplitude of the voltage pulses in winding 4, compared with the small amplitude of the pulses when the lamp is operating, is detected by the comparator and this causes an alarm 10, e.g. a warning lamp, to be operated.

It is possible to monitor two lamps using a single core of magnetic material and such an embodiment is illustrated in FIG. 3 of the drawings.

A core, comprising a toroid 15 of a magnetic material composed of a ferrite, is provided with a pair of first wire windings 16 and 17 connected together at one end, a second wire winding 18 and a third wire winding 19. Winding 16 is connected via a lamp 20 which is to be monitored to one terminal of battery 22 and winding 17 is also connected to this terminal of the battery via another lamp 21 which is also to be monitored. The junction of windings 16 and 17 is connected to the other terminal of battery 22 through a switch 23. Switch 23 operates to control the lamps 20 and 21. The second winding 18 is connected to a current pulse generator 24 which provides alternating negative pulses and positive pulses in the second winding and thereby induces variations in magnetic flux density in the core 15. The third winding 19 is connected to a pulse amplitude discriminating circuit 25 which triggers an alarm 26. When switch 23 is closed, lamps 20 and 21 operate if they are sound and current flows through the windings 16 and 17. These windings are arranged so that the current in one is opposed by the current through the other. The result is zero magnetic field due to these windings in the core 15. This means that the operating point on the B-H loop is at O in FIG. 2. In this condition the amplitude of output voltage pulses from winding 19 is large. The number of turns on each of the windings 16 and 17 is arranged so that in the absence of a current flowing through one of them, the current through the other would result in magnetic saturation of the core, this saturation being in a positive sense in the case of one winding and in a negative sense in the case of the other winding. If one or the other of lamps 20, 21 fails, current ceases to flow through the winding 16 or 17 associated with the failed lamp; the core 15 then becomes magnetically saturated and the operating point on the B-H loop for the material is at a point such as S in FIG. 2. When this occurs, the amplitude of output voltage pulses from winding 19 decreases to a low level. This change in amplitude is detected by the circuit 26 which operates alarm 26 to indicate that one of lamps 20, 21 has failed.

Instead of using a single core 15 for the pair of lamps, two cores could be used with windings as in FIG. 1, but a single current pulse generator and pulse discriminating circuit could be used for both wound cores.

The arrangements shown in FIGS. 1 and 3 can be used to provide an indication that a lamp has failed even when the lamp is not energized, i.e., when switches 7 and 23 are in the open position. To achieve this with the circuit of FIG. 1, a capacitor 27 must be connected in parallel with winding 2 and lamp 5. When switch 7 is open, the resistance of the unenergized lamp 5 is low; the core 1 with its windings thereon functions as a transformer with the result that when the current pulses from generator 8 pass through winding 3, they result in output voltage pulses from the winding 2 and these produce a current in the winding 2 and through the lamp 5, the capacitor 27 allowing this current to pass. This current is of sufficient amplitude to reduce the flux density in the core 1 (due to the field produced by the current in winding 3) to a low value and hence the output from the winding 4 is of low amplitude. If the lamp 5 fails, the current through the winding 2 ceases and since the field produced by the current pulses in winding 3 is now no longer opposed by the "transformer action" current in the lamp circuit, the flux change in the core 1 becomes large, resulting in an output of greater amplitude from the winding 4. This change in amplitude can be detected by the circuit 9 and used to operate alarm 10.

In the case of the arrangement of FIG. 3, there is no need to employ a capacitor in the circuit when used to detect failure of an unenergized lamp. In FIG. 3, the windings 16 and 17 and lamps 20 and 21 form a closed circuit even when switch 23 is open. When current pulses from generator 24 pass through winding 18, they result in output voltage pulses from the windings 16 and 17 which add and produce a current in the windings 16 and 17 and through lamps 20 and 21. In the same way as with the arrangement of FIG. 1, this current reduces the flux density of the core 15 to a low value and hence the output from the winding 19 is of low amplitude. If either of lamps 20, 21 fails, the current through the windings 16, 17 ceases and the flux change in the core 15 becomes large, resulting in an output of greater amplitude from the winding 19. This change in amplitude can be detected by the circuit 25 and used to operate alarm 26.

Although the present invention is described herein with particular reference to specific details, it is not intended that such details shall be regarded as limitations upon the scope of the invention except insofar as included in the accompanying claims.