Lamp monitoring circuits

#This invention relates to electrical monitoring circuits, and more particularly but not exclusively to circuits for monitoring the operation of the lighting circuits of a motor vehicle.

#It has already been proposed, for example in our British Patent No. 1,342,248, to monitor the lighting circuits of a motor vehicle, in order to detect any lamp failures which may occur, by monitoring the potential difference across a low value resistor inserted in series with each lamp. Since the nominal voltage of the electrical systems of many vehicles is only 12 volts, it is obviously desirable that the potential difference across the series resistors should be kept as small as possible, to avoid reducing excessively the voltage supplied to the lamps. There are also other factors which make it desirable to keep this potential difference as small as possible; in particular, if the potential difference is small, the resistance of the series resistor will be correspondingly small, and the circuit fuse will be better able to protect the circuit against short-circuit faults. One method which may be used for sensing such a small potential difference is to establish two reference currents, of slightly different magnitudes. The larger current is then supplied as the input current to a common base comparator, which is similar to an ordinary current mirror arrangement of two transistors, but with the base-emitter voltages of the transistors unbalanced by the amount of the potential difference, if any, across the series resistor, in such a way that the presence of such a potential difference reduces the output current produced by the comparator. This output current is then compared with the smaller of the two reference currents, as for example by connecting both currents to a summing node; if the comparison indicates that the comparator output current is greater than the smaller reference current, this means that the comparator is more or less balanced, and hence that the lamp is drawing no current, and a warning signal should be given.

#The two reference currents may be established by two circuits of similar configuration, fed from a common stabilized-voltage supply line; to produce the slightly different magnitudes of current, the values of one or more of the components in the circuits are made slightly different. Various configurations of constant-current circuit may be used, but such circuits usually exhibit an appreciable temperature-dependence. This is somewhat undesirable, since, if the absolute values of the two reference currents change, without their ratio also changing, the difference between the two currents will also change, so that the threshold at which a warning signal is given will also change, and therefore the value of the series resistor has to be chosen to provide a potential drop during normal operation which is greater than the threshold value, even at the greatest value of the threshold which can occur as a result of temperature variations. It follows from this that, at most temperatures, the potential drop will be unnecessarily large.

#The invention may be carried into practice in various ways, but several specific embodiments will now be described by way of example, with reference to the accompanying drawings, of which:

#FIG. 1 is a circuit diagram of an arrangement for monitoring the lamps of the lighting circuits of a motor vehicle;

#FIG. 2 is a circuit diagram, similar to that of FIG. 1, and showing an arrangement for monitoring fuses and other components in series with the lamps;

#The circuit of FIG. 1 is arranged to monitor the operation of the lamps of a motor vehicle. Two of these lamps are shown at 10, while a single switch controlling both these lamps is shown at 12, and the battery of the vehicle is shown at 14. These items are all entirely conventional. The circuit also includes, in series between each lamp 10 and its controlling switch 12, a low value resistor 16, whose function is to generate a small potential difference between its ends when the associated lamp 10 draws current. Connections 18, 20 and 22 extend from the junctions between the resistors 16 and the lamps 10 and between the resistors 16 and the switch 12 to three terminals T14, T15 nd T16 of an integrated circuit 24, whose limits are shown in broken line. The function of the integrated circuit 24 is to detect the presence or absence of the small potential differences across the resistors 16, and to provide a warning signal if one or both of these potential differences should be absent at a time when the switch 12 is closed. In the circuit of FIG. 1, the warning signal takes the form of a warning lamp 26.

#The arrangement and operation of the integrated circuit 24 will now be described in greater detail.

#Since the lighting circuits of a motor vehicle will normally include more than one lighting control switch, each controlling a group of lamps, the integrated circuit 24 is arranged to be able to monitor three such groups of lamps. More particularly, the integrated circuit 24 includes three similar, although not identical, sections 2, 4 and 6, each of which is concerned with monitoring a single group of lamps. For simplicity, only one group of lamps 10, connected to the terminals T14, T15 and T16 of the circuit section 2, are shown in FIG. 1; it should be clear that a second group (of up to four lamps, each with its own series resistor) can be connected in similar manner to the terminals T6 to T10 of the circuit section 4, and a third group of two lamps can be connected to the terminals T11, T12 and T13 of the circuit section 6. The integrated circuit 24 also includes a fourth, common, section 8, which cooperates with all three sections 2, 4 and 6, and controls the warning lamp 26.