GB1602350A

GB1602350A - Lamp fault detector particularly for motor vehicle

Info

Publication number: GB1602350A
Application number: GB19614/78A
Authority: GB
Original assignee: Jaeger
Priority date: 1977-05-27
Filing date: 1978-05-15
Publication date: 1981-11-11
Also published as: ES470517A2; FR2392574B2; IT1108823B; DE2822482A1; MX143954A; IT7822937A0; AR213364A1; FR2392574A2

Description

(54) LAMP FAULT DETECTOR, PARTICULARLY FOR MOTOR VEHICLE (71) We, JAEGER, a French Societe Anonyme, of 2 Rue Baudin, 92303 Levallois Perret, Cedex, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a circuit enabling a fault to be detected in any one of a plurality of lamps fed with direct current and is an improvement in or modification of the invention described in our French Patent Application 7639448 (Publication No 2,376,587).

The circuit described in our French Patent Application 7639448 comprises, connected in series between a lamp to be monitored and its control switch, means capable of creating an impedance when it is fed with alternating voltage while allowing through the direct current feeding the lamp, a source of alternating voltage connected to the junction point between said lamp and said means and a device for detecting variations in the voltage at the terminals of this means.

The detection device, in the circuit of said French Application, although it is satisfactory when the current circulating in the lamp or lamps to be monitored reaches a sufficient level, is no longer sufficiently reliable when said current is low. Actually, in this situation, the inductance (used as a means of creating an impedance) is not completely saturated and the existence of parasitic capacities as well as of an imbalance in the lamp currents renders possible the appearance of a not inconsiderable signal, even in the case where no lamp is defective, which might lead to the triggering of an untimely alarm. In order to overcome this disadvantage, a capacitor may be added in parallel with the means for creating an impedance when it is a question of monitoring lamps of low power. Moreover, an attempt has been made to improve the use of the signals leaving the detection system: in the previous circuit, this was effected by simmation and finding a mean value before acting on the comparator assembly. However sometimes, in the limiting cases of operation, the ratio between the differential voltage obtained when the lamps are in working order and the differential voltage obtained when one of the lamps is defective, proves inadequate.

In accordance with the present invention, there is provided a circuit enabling a fault to be detected in any one of a plurality of lamps fed with direct current, said circuit comprising said plurality of lamps connected in parallel to receive direct current, each lamp being connected to means creating an impedance when it is fed with an alternating current, a source of alternating voltage connected to each lamp and to its associated said means, and a differential device arranged to check the lamps in pairs by detecting variations in voltage at the respective pair of means of creating an impedance, said differential device comprising, for each such pair of lamps, two transistors of which the base of one and the emitter of the other are connected to the junction point between one of the lamps and its associated said means and conversely for the junction point of the second lamp of the pair, with the base of each transistor connected to a single lamp.

Embodiments of the present invention will now be described, by way of examples only, with reference to the accompanying drawings, in which: FIGURE 1 is a diagram of a basic circuit in accordance with this invention; FIGURE 2 is a diagram of a working embodiment and FIGURE 3 is a modification of the Figure 2 embodiment.

In the circuit shown in Figure 1, two lamps 1 and 2 to be checked are connected in parallel between the negative pole of a source of direct current and a switch 3 itself connected to the positive pole of said source.

Respective inductances 4, 5 are interposed in series with the lamps, between these and the switch 3. Connected in parallel with the inductance 4 is a capacitor 6, similarly a capacitor 7 is connected across the inductance 5. The junction point Ao between the lamp I and the inductance 4 is connected to the base of a transistor 8 and to the emitter of a transistor 9, the collectors of which are connected to a detection circuit 10. Similarly the junction point A, between the lamp 2 and the inductance 5 is connected to the emitter of the transistor 8 and to the base of the transistor 9.

An oscillator 11 acts on the points Ao and A, with repeated low-frequency pulses. The resultant signal collected at A" and A, is different according to whether the associated lamp is working normally or not. In fact, each inductance and its associated capacitor form a resonant circuit which has a high coefficient of overvoltage when it is not damped by the resistance of the lamp (that is to say when this is not working) and low in the opposite case. The signal at the respective point Ao or A1 is therefore greater in the first case than in the second. Therefore, when one of the lamps is defective, a differential signal appears between Ao and A1; this signal is used by a detection circuit 10 which controls an alarm means 12. A storage means 13 enabling the alarm to be preserved may be interposed between the detection circuit 10 and the alarm means 12.

In the case where it is desired to check n lamps actuated by the same switch and particularly when n is an odd number, a modification of the system consists in grouping the transistors in a ring, but while retaining the principle of the differential system explained above. Thus the first lamp is compared with the nth, the second lamp with the third and so on... in this manner, it is possible to detect the defectiveness of (n - 1) lamps out of the n lamps pecked. Figure 2 illustrates a circuit using this solution: the transistors associated with the three lamps 14, 15 and 16 are connected in such a manner that thejunction point B0 between the lamp 14 and the associated inductance 20 is connected on the one hand to the emitter ofthe transistor 19 and on the other hand to the base of the transistor 17, while the junction point B,, between the lamp 15 and the inductance 21, is connected to the base of the transistor 18 and to the emitter of the transistor 17 and so on.... thus each intermediate junction point is connected on the one hand to the emitter of the transistor which precedes it and to the base of the transistor which follows it, forming a ring.

The lamps 23 and 24 as well as 25 and 26, which go in pairs, are checked by means of a differential system as described in Figure 1, composed of transistors 27 and 28 so far as the lamps 23 and 24 are concerned and transistors 29 and 30 for the lamps 25 and 26.

The source of alternating current consists of an oscillator with inverter circuits 31, 32, 33 followed by transistors 34 and 35; it is connected to the junction point between each of the lamps to be checked and the associated inductance. The three transistors in a ring, 17, 18 and 19, have their collectors connected to the base of a transistor 44 which is followed by a resistance-capacitor circuit 5253; integration of the signals emitted by the ring circuit is effected in capacitor 53, an inverter 45 changing over when a certain predetermined threshold is reached, to control a transistor 46 and, through it, an alarm means 47. In a similar manner, a circuit composed of transistors 40 and 42, a resistance-capacitor circuit 5051 and an inverter 41, connected to the collectors of the transistors 27 and 28, controls an alarm means 43 which checks the lamps 23 and 24.

An alarm means 39 which checks the lamps 25 and 26 by means of a transistor 36, a resistance-capacitor circuit 48--49 and an inverter 37 also comprises a thyristor 38 to maintain or effect a storage of the alarm.

This storage is useful, particularly when it is a question of checking the satisfactory operation of brake indicator lamps; these lamps, actually are only lit up when the driver of the vehicle actuates the brake pedal and, without storage, there is the risk that the alarm would only be set off at a moment when the driver's attention was attracted elsewhere. A modification of the circuit (Figure 3) is likewise useful in this case: it is a question of providing an alarm which is different according to whether the fault effects the left-hand or right-hand brake indicator, always taking into consideration the fact that these lamps only work when the brake pedal is actuated.

In order to do this, the collectors of transistors 55 and 56 are separated and connected to individual integration circuits for the control of a different alarm means: thus the transistor 55 controls an alarm 60 which thus corresponds to a fault found in lamp 58, while lamp 57 is monitored by an alarm 59 by means of the transistor 56; two thyristors 61 and 62 are provided to effect a storage of the alarm, the rest of the circuit being unchanged in comparison with the previous case.

Reverting to Figure 2, it may happen, sometimes, that parasitic pulses reach a sufficient level to cause the untimely control of the input transistors 36, 40 and 44 of the circuit controlling the alarm means. There would thus be a risk of the integration capacitors being charged up to the threshold for setting off the alarm which would then be actuated at the wrong moment. In order to avoid this, the operation of these input transistors is only activated during a minimum time, in phase with the probable appearance of the useful pulses, the synchronisation being ensured by the pulses emitted by the oscillator and received at the base of a transistor 54 which then controls said input transistors. This device is likewise applicable to the modification illustrated in Figure 3.

WHAT WE CLAIM IS: 1. A circuit enabling a fault to be detected in any one of a plurality of lamps fed with direct current, said curcuit comprising said plurality of lamps connected in parallel to receive direct current, each lamp being connected to means creating an impedance when it is fed with an alternating current, a source of alternating voltage connected to each lamp and to its associated said means, and a differential device arranged to check the lamps in pairs by detecting variations in voltage at the respective pair of means of creating an impedance, said differential device comprising, for each such pair of lamps, two transistors of which the base of one and the emitter of the other are connected to the junction point between one of the lamps and its associated said means and conversely for the junction point of the second lamp of the pair, with the base of each transistor connected to a single lamp.

2. A circuit as claimed in claim 1, comprising n lamps connected in parallel, each with an associated transistor and with the transistors connected in a ring, the junction point between the first lamp and its associated said means being connected to the base of the first transistor and to the emitter of the nth and the junction points between the (n - 1) other lamps and their associated said means each being connected to the base of the transistor of the same rank and to the emitter of the transistor of immediately lower rank.

3. A circuit as claimed in Claim 1 or 2, in which the transistors have their collectors connected together and to a control means of an alarm device.

4. A circuit as claimed in Claim 1 or 2, in which the collectors of the transistors are connected to individual alarm device control means.

5. A circuit as claimed in Claim 3 or 4, in which the or each control means includes an input transistor having its current path connected to the collector of a further transistor, a control terminal of which is controlled by said source of alternating voltage.

6. A circuit as claimed in Claim 5, in which a storage device is included in the control means of the alarm device.

7. A circuit as claimed in any preceding Claim, in which a capacitor is connected in parallel with each said means.

8. A circuit enabling a fault to be detected in any one of a plurality of lamps, substantially as herein described with reference to Figure 1, 2 or 3 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

avoid this, the operation of these input transistors is only activated during a minimum time, in phase with the probable appearance of the useful pulses, the synchronisation being ensured by the pulses emitted by the oscillator and received at the base of a transistor 54 which then controls said input transistors. This device is likewise applicable to the modification illustrated in Figure 3.

WHAT WE CLAIM IS: 1. A circuit enabling a fault to be detected in any one of a plurality of lamps fed with direct current, said curcuit comprising said plurality of lamps connected in parallel to receive direct current, each lamp being connected to means creating an impedance when it is fed with an alternating current, a source of alternating voltage connected to each lamp and to its associated said means, and a differential device arranged to check the lamps in pairs by detecting variations in voltage at the respective pair of means of creating an impedance, said differential device comprising, for each such pair of lamps, two transistors of which the base of one and the emitter of the other are connected to the junction point between one of the lamps and its associated said means and conversely for the junction point of the second lamp of the pair, with the base of each transistor connected to a single lamp.
2. A circuit as claimed in claim 1, comprising n lamps connected in parallel, each with an associated transistor and with the transistors connected in a ring, the junction point between the first lamp and its associated said means being connected to the base of the first transistor and to the emitter of the nth and the junction points between the (n - 1) other lamps and their associated said means each being connected to the base of the transistor of the same rank and to the emitter of the transistor of immediately lower rank.
3. A circuit as claimed in Claim 1 or 2, in which the transistors have their collectors connected together and to a control means of an alarm device.
4. A circuit as claimed in Claim 1 or 2, in which the collectors of the transistors are connected to individual alarm device control means.
5. A circuit as claimed in Claim 3 or 4, in which the or each control means includes an input transistor having its current path connected to the collector of a further transistor, a control terminal of which is controlled by said source of alternating voltage.
6. A circuit as claimed in Claim 5, in which a storage device is included in the control means of the alarm device.
7. A circuit as claimed in any preceding Claim, in which a capacitor is connected in parallel with each said means.
8. A circuit enabling a fault to be detected in any one of a plurality of lamps, substantially as herein described with reference to Figure 1, 2 or 3 of the accompanying drawings.