WO2008061301A1

WO2008061301A1 - A fault detector and a fault detection process for lighting

Info

Publication number: WO2008061301A1
Application number: PCT/AU2007/001785
Authority: WO
Inventors: John Albert Montagnat; Balu Jeganathan
Original assignee: Lednium Technology Pty Limited
Priority date: 2006-11-20
Filing date: 2007-11-20
Publication date: 2008-05-29

Abstract

A fault detector configured to compare a plurality of signals representative of the distribution of electrical power in corresponding portions of an array of interconnected light generating components, and to generate a fault signal only if the comparison indicates at least one change in the relative distribution of electrical power in the portions of the array, the change being indicative of a fault in at least one of the light generating components.

Description

A FAULT DETECTOR AND A FAULT DETECTION PROCESS FOR LIGHTING

FIELD The present invention relates to a fault detector and a fault detection process, these being operative to detect failure of one or more light emitting components in a lighting component including an array of interconnecting light emitting components.

BACKGROUND

In recent years, incandescent and fluorescent light sources have been increasingly replaced with solid state light sources (SSLs), most commonly with light emitting diodes (LEDs). Perhaps the most significant advantage of LEDs over more traditional forms of lighting is their enhanced longevity which reduces or can even eliminate the need to replace malfunctioning light sources, and thus greatly reduces labour costs in particular.

One increasingly important application for SSLs is automotive lighting, and it is now common to find LED light sources replacing incandescent bulbs in automobiles for use as exterior brake lights, reversing lights, turn signals, side markers, daytime running lights, and even some forms of interior lighting. It is expected that future improvements in LED technology will also result in their use as headlights and fog lights. The adoption of SSL devices in the automotive industry is not only due to their long effective life, but also their relatively robust construction.

In terms of effective life, an LED compares extremely favourably to all other types of lamp in general use. For example, the lifetime of a single LED is typically some tens of thousands of hours, compared to only some thousands of hours for incandescent lamps in stationary situations, and perhaps only hundreds of hours in automotive applications. SSL light sources are particularly suitable for automotive applications because they are largely unaffected by the vibration and mechanical stresses that are produced during normal operation of a vehicle. SSL devices may well outlive the useful life of the vehicle itself. When used as turning signal lamps, SSL light sources give rise to a difficulty not encountered when incandescent bulbs are used. It is a requirement, statutory and/or otherwise desirable, that a vehicle driver be alerted when a turning signal is no longer functioning correctly. Incandescent lamps are powered by a substantially constant voltage source, and consequently the current that is drawn from the constant voltage source can be monitored to provide feedback on the presence, absence or relative magnitude of the lamp current, usually in the form of a visual and/or audible signal. In an automobile, a slave lamp on the dashboard is configured to flash in sympathy with the indicator lamps. Since front and rear turning signal lamps are mandatory, and in the case of a large vehicle additional multiple lamps may also operate together with the front and rear turning signal lamps, when any one lamp fails to draw current, the change in total current is used to increase the rate at which all the vehicle's indicator lamps flash. In the most common failure mode, the filament of an incandescent lamp breaks, thereby causing an open-circuit, as a result of which a driver of the vehicle is alerted to the failure by an increase in the flash rate and/or corresponding audible signal.

In contrast to incandescent lamps driven by constant voltage sources, SSL devices are driven by constant current sources, and therefore the standard feedback system based on changes in current cannot be used with SSL devices. Moreover, whereas an incandescent lighting unit is traditionally in the form of a glass bulb containing only one light generating component (usually a heated tungsten filament), a single SSL lighting unit or module usually contains a plurality of electrically interconnected but otherwise independent light generating components (e.g., 15, 25 or more LEDs). Consequently, whereas a single failure prevents a traditional lighting unit from operating at all, an SSL lighting unit will usually continue to function with one or more of its constituent light emitting components (usually LEDs) no longer operating at all. Nevertheless, a failure of any one light source within such a lighting unit may appear visually unacceptable, and/or may contravene statutory requirements. Accordingly, it is important in some applications to detect such failures and produce an alarm signal. The lighting industry uses the word "luminaire" to refer to a complete lighting unit or module that includes one or more light generating components (e.g., LEDs or filaments), together with components for positioning and protecting those light generating components, distributing the resulting light, and for connecting the light generating components to a source of electrical power (e.g., reflectors, diffusers, a housing, a socket or other form of electrical connector(s), internal wiring, ballast, etc). It would greatly simplify the adoption of new forms of lighting if a luminaire incorporating a plurality of light-emitting components could be compatible with the standard form of fault detection circuitry described above as a drop-in replacement for a luminaire driven by a standard incandescent bulb..

It is desired, therefore, to provide a fault detector and a fault detection process that alleviate one or more of the above difficulties, or at least provide a useful alternative.

SUMMARY

In accordance with the present invention, there is provided a fault detector configured to compare a plurality of signals representative of the distribution of electrical power in corresponding portions of an array of interconnected light generating components, and to generate a fault signal only if the comparison indicates at least one change in the relative distribution of electrical power in said portions of said array, said change being indicative of a fault in at least one of said light generating components.

The present invention also provides a fault detection process, including: comparing signals representative of the distribution of electrical power in respective portions of an array of interconnected light generating components; and generating a fault signal only if said comparing indicates at least one change in the relative distribution of electrical power in said portions of said array, said change being indicative of a fault in at least one of said light generating components. It is envisaged that embodiments of the present invention can include replacable components of the lighting systems described herein, in addition to as a complete system. In particular, it is expected that an array of light generating components (most probably in the form of a luminaire) can be provided as a replaceable component of a complete lighting system so that, if a light generating component of an array develops a fault, the array can be replaced without needing to replace the fault detector, for example. In terms of the described embodiment, where the signals representative of power distribution in portions of the array are in the form of voltages generated from current flowing along corresponding electrical pathways of the array, the resistors that generate those voltages can either be part of the replaceable array, or alternatively can be provided externally to the array, with the array component providing external connections to the (incomplete) electrical pathways so that when the replaceable component is connected with the other component(s) of the system, the currents flow out of and back into the array through those connectors, allowing the relevant currents to be monitored.

The present invention also provides a luminaire, including: an array of interconnected light generating components, including resistors to generate voltages representative of the distribution of electrical power in corresponding portions of said array; and electrical connectors for supplying electrical current to said light generating components and for providing said voltages to a fault detector configured to compare said voltages and to generate a fault signal only if the comparison indicates a fault in at least one of said light generating components.

Preferably, said light generating components are electrically interconnected as an array of columns and rows, wherein respective electrodes of the light generating components of each column are each interconnected in parallel.

The present invention also provides a luminaire, including: an array of interconnected light generating components; and electrical connectors configured to supply electrical current to said light generating components and to conduct electrical currents from and to selected nodes of said array via a fault detector configured to compare said currents and to generate a fault signal only if the comparison indicates a fault in at least one of said light generating components.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein: Figures 1 and 2 are circuit diagrams representing prior art lighting devices consisting of an array or matrix of light-emitting diodes (LEDs), shown connected to a constant current source;

Figure 3 is a graph of the current flowing through a typical prior art LED as a function of the voltage across it; Figure 4 is a circuit diagram of a first preferred embodiment of a luminaire connected to a constant current source;

Figure 5 is a circuit diagram of a second preferred embodiment of a luminaire connected to a constant current source; and

Figure 6 is a circuit diagram of a fault detecting portion of a further preferred embodiment of a luminaire.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 is a circuit diagram of a prior art solid state lighting device, in which many light- emitting diodes (LEDs) DI l to D33 are electrically interconnected, and no provision is made for monitoring the operation of the individual LEDs DI l to D33. Although there is no set way in which the LEDs in a prior art lighting unit are electrically interconnected, one common arrangement is generally as shown, where the LEDs DI l to D33 are electrically arranged into subsets or groups, with the members of each group interconnected in series, and the groups themselves being interconnected in parallel. In Figure 1, the LEDs Dl 1 to D33 are shown as a two-dimensional array or matrix, where the reference numeral assigned to each LED indicates its row and column position.

For example, LED 11 is located at the top left position (column 1, row 1), and LED 23 is located at column 2, row 3, and so on. Figure 1 actually shows the most complex form of interconnecting the LEDs DIl to D33, where like electrodes of all the LEDs in a column are connected together and simultaneously the LEDs in each row are connected in series. Thus the (p-type) anodes of LEDs DIl, D 12, and D 13 are connected together, and their (n- type) cathodes are also connected together, so that these three LEDs DIl, D 12, and D 13 are in parallel. The same connection arrangement applies to LEDs D21, D22, and D23, and also to LEDs D31, D32, and D33. The cathodes of the first group of LEDs (Dl 1, D12, and D 13) are then connected to the anodes of the second group of LEDs (D21, D22, and D23), and the cathodes of the second group are connected to the anodes of the third group of LEDs (D31, D32, and D33). This configuration provides the maximum number of possible paths into which the circuit current can divide, and thereby eliminates some failure modes that are possible if simpler interconnections are used, as described below.

Although it is possible for a single LED to become short circuited, it is quite unusual for this to occur; it is much more common for an LED to become open circuited. However both conditions are possible. If all of the LEDs in a SSL lighting unit of the general form of Figure 1 were instead electrically interconnected in series only or in parallel only, then a single LED failure could prevent the device from operating effectively, and in some failure modes can completely prevent the device from operating at all. For example, if the LEDs are interconnected in series and one LED becomes short-circuited, that particular LED will fail but the other LEDs can continue to operate normally because the constant current supply 102 will maintain circuit current at a normal level. On the other hand, if a single LED fails and produces an open circuit, then the circuit current is reduced to zero because the only current path is blocked, and thus all the LEDs in the lighting unit cease operating. Conversely, if all the LEDs in a lighting unit are electrically interconnected in parallel only, and a single LED fails by short circuit, the total current in the circuit will remain unchanged, but all of this current will pass into the short circuit and all of the LEDs will therefore cease to operate because insufficient voltage will be developed across the short circuit to enable the other LEDs to be appropriately biased. On the other hand, if the failure causes an open circuit condition, then that particular LED will cease to draw current and be extinguished, but all other LEDs can continue to operate normally. The total circuit current will be unchanged and will be divided among the LEDs that continue to operate.

Finally, for a simple series/parallel connection within an array, the effect of a single LED failure is more limited but still significant. A simple series/parallel array is one where the LEDs in each of the rows are connected in series and these rows are connected in parallel, as shown in Figure 2. This light device is similar to the device shown in Figure 1 in that it includes a matrix of the same nine LEDs DI l through D33 shown in Figure 1 - with LEDs DI l, D21 and D31 connected in series, as are D 12, D22 and D32, and D 13, D23 and D33, the anodes of DI l, D12 and D13 connected together and the cathodes of D31, D32 and D33 connected together. However, the intermediate connections on LEDs D21, D22 and D23 are omitted so that there are only three current circuits created by the parallel connections of the anodes of DI l to D21 and to D31. Consequently, this connection arrangement is described as 'simple'.

When any LED fails and results in an open-circuit condition, the series connection in which it lies cannot pass current and that row therefore ceases operating; however, the total circuit current remains unchanged because the current is shared by the remaining rows. For example, if D22 becomes open circuited then row 2 cannot pass current and D21, D22 and D23 will be extinguished. If a single LED fails to a short-circuit condition, then the remaining LEDs in that series connection (row) will continue to operate, but all the other rows will cease operating because the voltage developed across the row with a faulty LED, by the constant current supply, will be insufficient to appropriately forward bias the series connection of LEDs in the remaining rows, but there is no change to the total circuit current. Thus for the circuit of Figure 2, if D22 becomes short circuited then D 12 and D32 will continue to operate but D22 and the remaining six LEDs will be extinguished. This could lead to serious overloading of LEDs D12 and D32, because the entire circuit current would be passing through them, being three times the normal current passing through each LED when operating satisfactorily. This could permanently damage LEDs D 12 and D32.

The more complex series/parallel arrangement of Figure 1, where like electrodes of the LEDs in each column are connected together and the LEDs in each row are connected in series, is less sensitive to an individual LED failure. When any LED fails with an open circuit, only that particular LED is extinguished because there is a parallel current path through the remaining LEDs in that column and the forward bias voltage will still appear across all the LEDs. Should an LED fail by short-circuit, then all LEDs in the same column will cease operating because no voltage can appear across the short circuit, and therefore that column, and the undamaged LEDs in that column, will not be appropriately forward biased. In both of these instances, the total circuit current will remain unchanged.

A significant conclusion from the analysis above is that in only one of the failure modes discussed is the total circuit current changed by the event under discussion. This is for the case where a simple series connection is employed and an open circuit develops, and there being no alternate (parallel) path for the circuit current, the current falls to zero. This is the only failure mode of an SSL or other form of composite lighting unit (i.e., one containing multiple light generating components) that can be detected by the standard circuit current monitoring method.

As shown in Figure 3, the relationship 302 between voltage and current in a typical light- emitting diode (LED) is non-linear (note the x-axis offset of about 70%). When the value of the voltage applied across an LED is below a certain threshold, very little current flows through the LED. However, when the applied voltage exceeds about 80% of the voltage corresponding to the rating of the device, the voltage - current relationship 302 becomes linear. This means that when the LED is passing more than approximately 20% of its rated current, the relationship between changes in applied voltage and the resultant changes in current are similar to those of a resistor. A typical value of the equivalent resistance of an LED operating in the linear region is approximately one Ohm. Because this is a low value of equivalent resistance, and because 80% of the rated voltage produces around 20% of rated current, small changes of applied voltage produce correspondingly large changes of current. This feature makes this type of device more suited for use in a circuit where the current is held fixed at a particular value, and the voltage developed across it is determined by the slope of the voltage - current curve 302.

According to the principles to be exploited in the embodiments of the present invention, when the current entering an array of LEDs (or other form(s) of light generating components) is fixed to a constant value, the operation of each LED in the array can be assessed continuously by monitoring the absolute value of the voltages appearing within the lamp circuit and comparing this to the voltage appearing across other LEDs in the array across which a similar absolute value of voltage will appear when equivalent LEDs are operating satisfactorily.

Two typical operating points are shown in Figure 3. Firstly with 50% of rated current passing through a typical LED, the voltage appearing across this LED is about 87.5% of the rated forward voltage, as shown by the dotted line 304. Secondly, if the current is increased to 75% of rating, then the voltage appearing across the device is shown to increase to about 93.75%, as shown by the dashed line 306. Although the voltage developed across various LEDs can vary from around 2 Volts up to about 4 Volts, in the linear region of LED operation, the change in the absolute value of voltage will be in excess of 100 millivolts for a change of current from 50% to 75% of the nominal current rating. The magnitude of an error signal such as this voltage is suitable for conventional processing circuitry, and a relatively simple and inexpensive alarm system can be used to compare the voltages appearing across the LEDs. Each group of LEDs in parallel has a voltage appearing across it according to the current carried by each LED and the corresponding I-V relationship, such as that shown in Figure 3. If the voltage of each parallel group is compared to the voltage of each other group, then if an open circuit appears, the voltage across the group that contains the open circuit will increase because the current is now shared by fewer parallel paths. Similarly, a short circuit will cause the voltage across the corresponding group of parallel LEDs to fall to a value near zero. In the case of a short circuit, this is a satisfactory solution because it is a condition that is simple to detect. However, an open circuit, which is a much more probable outcome, should be discernable from other events that occur normally and that could otherwise potentially generate false alarms. For example, for a given value of current, the voltage across an LED falls with increasing temperature, which could cause false alarms if a group voltage falls sufficiently because of external heating or internal self-heating. Consequently, a more sophisticated monitor is required.

If the initial design value of constant current flowing into the circuit of Figure 1 was set to 150% of the rating of an individual LED, then the average design value of current in each LED would be 50% of the rated value. This is a conservative, yet quite realistic, design value that would be arrived at after making allowances for manufacturing variables, operating temperatures and other general unquantified variables. If, for example, LED DI l subsequently becomes open-circuited, then LEDs D12 and D13 share the current that normally would have been passing through DI l, and the current in D 12 and Dl 3 increases from 50% to 75% of their current rating. The SSL could continue to operate like this indefinitely, even though LED DI l is inoperative. Similar conditions would apply for any other single diode failure that causes an open circuit, and even for a condition where one diode of each parallel path becomes open-circuited. Thus it is possible for an SSL of this design to continue to operate without any change to the electrical supply system in spite of failures to 1/3 of the devices in the array. The effectiveness of such a situation optically would be seriously deficient, it would be cosmetically unacceptable and may even contravene the statutory regulations covering luminaires in automotive applications. Similarly, if DI l were to fail by short circuit, the SSL would continue to operate by virtue of unchanged conditions to six of the nine LEDs. D21 through D33 would not experience any change, even though DI l, D12 and D13 no longer operate because all of the circuit current is passing through the short circuit. It follows logically that a short circuit to any of the LEDs in an array such as that shown in Figure 1 will extinguish the LEDs operating in parallel with it, and yet will allow the other LEDs to operate normally. These failures to this type of SSL would not cause any external indication of an abnormal condition, if such SSL devices were to be substituted for traditional automotive lamps, and no other measures to generate an alarm were provided.

As shown in Figure 4, a lighting system includes an array or matrix 401 of nine standard LEDs designated DI l through D33, electrically interconnected in a series/parallel, three column and three row array or matrix. The anode sides of the three LEDs DI l, D12, and Dl 3 in the first column are connected to the output of a constant current source 402 via a transistor Ql. Also in the first column, a first resistor Ra is connected between the cathodes of the first and second row LEDs DIl and D12, and a second resistor Rb is connected between the cathodes of the second and third row LEDs D 12 and Dl 3. The cathodes of all the first column LEDs Dl 1, D 12, and D13 are also connected to respective inputs of a first column voltage comparator 404 that compares the cathode voltages of the first column LEDs Dl 1, D 12, and Dl 3 to detect any imbalance.

The cathode sides of the first column LEDs DI l, D 12, and Dl 3 are also respectively connected to the anode sides of the LEDs D21, D22, and D23 defining the second column.

The cathode sides of these second column LEDs D21, D22, and D23 are connected in the same manner described above for the first column LEDs DI l, D12, and D13. That is, a third resistor Rc is connected between the cathodes of the first and second row LEDs D21 and D22, and a fourth resistor Rd is connected between the cathodes of the second and third row LEDs D22 and D23, with the cathodes of all the second column LEDs D21 , D22, and D23 also connected to respective inputs of a second column voltage comparator 306 that compares the cathode voltages of the second column LEDs D21, D22, and D23 to detect any imbalance. The cathode sides of the LEDs D21, D22, and D23 of the second column are also respectively connected to the anode sides of LEDs D31, D32, and D33 defining the third (and last) column of the array. The cathode sides of these last column LEDs D31, D32, and D33 are connected to a common current return path via further respective resistors Re, Rf, and Rg. The voltages across these three resistors Re, Rf, and Rg is monitored by a third column voltage comparator 408 to detect any imbalance.

The outputs of the three voltage comparators 404, 406, 408 are provided as input to a signal generator 410. Whenever any substantial voltage imbalance appears at the input of any of the three voltage comparators 404, 406, 408, a signal appears at the output of the corresponding comparator to trigger the signal generator 410. The signal generator controls a further component, shown here as a transistor Ql, that modifies the operation of the luminaire. The transistor Ql can be used to modify the operation of the lighting system in different ways. In the described embodiment, the transistor Ql is used as a controlled series element that reduces the value of constant current by imposing a voltage drop across Ql, that the source is incapable of supplying, thereby reducing the circuit current. As known to those skilled in the art, although a hypothetical ideal constant current source is able to supply a constant current regardless of the voltage across its output terminals, in reality a constant current source can only supply a constant current up to some maximum output voltage. For example, the power supply available in a standard automobile is a 12 volt wet battery, and in some trucks a 24 volt wet battery. Accordingly, by biasing the transistor Ql to develop a sufficiently large voltage across itself (i.e., V_CE), the voltage provided across the array 401 of LEDs (and hence the current drawn by the array 401) is correspondingly reduced.

The operation of the lighting system is as follows. In the following description, a voltage across any one of the resistors Ra through Rg is referred to by a capital V with subscripts corresponding to the identifier of the corresponding resistor so that the voltage across resistor Ra is designated Vra, Vrg refers to the voltage across resistor Rg, and so on. When all the LEDs DI l to D33 are operating normally, the seven resistors Ra through Rg contribute little to the operation of the device. In an ideal embodiment, where the LEDs DI l to D33 have identical I-V characteristics, and Ra through Rd are of equal value, the cathode voltages of all nine LEDs DI l to D33 are identical and consequently no current will flow through the resistors Ra through Rd until, and unless, any one of the LEDs DI l to D33 fails and forms either an open-circuit or a short-circuit. In practice, small currents are expected to flow through at least some of the resistors Ra through Rd due to small differences in the I-V characteristics of the LEDs DI l through D33. However, these voltages are small compared to the voltages that appear when an LED failure occurs. In any case, it will be apparent to those skilled in the art that any quiescent imbalances can be offset in the corresponding voltage comparator.

In normal operation, row current resistors Re through Rg, which are also ideally of equal value, equally share the constant current supplied by the constant current source 402, have substantially equal voltages across them, and are preferably selected to have a sufficiently low value of resistance to have a negligible effect on the overall operation of the lighting system.

If, for example, LED DI l fails and effectively becomes an open circuit, then the constant circuit current is shared by LEDs D 12 and D13. However, the circuit is no longer symmetric, and resistor Ra will now conduct a substantial portion of the current to supply

LED D21, causing the potentials Vra and Vrb to differ substantially, and this will be detected by the first column voltage comparator 404. Additionally, because the current supplied to the second column first row LED D21 will be reduced relative to that during normal operation, the corresponding current through the third column first row LED D31 - and hence the first row resistor. Re ~ will also be reduced, causing a corresponding reduction of the voltage Vre. Consequently, the voltages across the second and third row resistors Rf and Rg will be correspondingly increased, and the third column voltage comparator 408 will detect this. Thus the first and third column voltage comparators 404, 408 will independently cause the signal generator 410 to announce an abnormal circuit condition. The signal generator 410 preferably emits a tone burst of a suitable attention grabbing low frequency, which causes the visible warning lamp to flash in sympathy, or simply reduces the circuit current to a value which a conventional monitor can detect. In an alternative embodiment, the signal generator 410 provides a fault signal to an external dedicated fault alarm device that operates a warning lamp, steady state or flashing, and/or an audible fault signal.

If, rather than becoming an open-circuit, LED DI l becomes a short circuit, then essentially all the constant circuit current will pass through it. The current then divides among the three available current paths (through the three rows), causing a larger current to pass through Ra than through Rb. Consequently Vra and Vrb are no longer equal, Vre becomes greater than Vrf and Vrg, and a fault alarm signal is generated in the manner described above.

It will be apparent from the symmetry of the array that the same logic applies to a short circuit or an open circuit occurring in LED D21, LED D 13, or LED D23. However, if a failure occurs in either of the first and second column, middle row LEDs D12 or D22, the symmetry of the circuit indicates that there will be no change to the difference between the voltages appearing across Ra and Rb, or across Vrc and Vrd. However, the current through the middle row resistor Rf will no longer be equal to the current through the first row resistor Re and the third row resistor Rg. If the failure causes an open circuit, then Vrf will be less than Vre and Vrg, and if the failure causes a short circuit, then Vrf will be greater than Vre and Vrg. Clearly, any of these events will cause a fault alarm signal via comparator 408.

An open circuit at any one of the third column LEDs D31, D32 or D33 causes the voltage across the corresponding series resistor Vre, Vrf or Vrg in the same row to fall to zero while the voltages increase across the series resistors in the other two rows. A short circuit at any one of the third column LEDs D31, D32 or D33 will obviously greatly increase the current through - and hence the voltage across - the series resistor in the same row while the voltages across the series resistors in the other two rows fall to zero. There are eighteen possible first contingency failure modes (i.e., where only one LED fails and effectively becomes either a short circuit or an open circuit), and all of these first contingent events trigger an alarm.

Similarly, it can be shown that of the possible seventy two (thirty-six combinations and two failure modes) possible second contingent events, where two LEDs fail simultaneously, all of these events will trigger an alarm in the circuit of Figure 4.

Third contingent events will result in complete failure (i.e., no LEDs will be operative) of the lighting system if three failures cause open circuits in the same column of the array. Three simultaneous short circuits in the same column will not be detected by the circuit of Figure 4.

Figure 5 shows a second preferred embodiment of a lighting system, including an array or matrix 501 of twenty four LEDs DI l through D46 in six rows and four columns. First, second, and third column resistors Ra, Rb and Rc are of equal value, and third and fourth row series resistors Rd and Re are of equal value, although this value may be different the value of the column resistors Ra to Rc. The arrangement of the column resistors Ra to Re means that, in practice, and when operating normally without any fault condition, the current through the circuit matrix divides into two major portions that ultimately pass through the third and fourth row series resistors Rd and Re, and that the matrix operates as two sections, where each section is comprised of four series-connected sub-groups or half- columns of three LEDs connected in parallel. Because the three LEDs in each half-column are connected in parallel, failure of any one of the three LEDs in any half-column lias the same effect as a similar fault in either of the other two LEDs of that half-column. For example, failure of LED DI l has the same effect as the same failure mode in either D 12 or D 13, and failure of LED D34 has the same effect as the same failure mode in LED D35 or D36, and so on. It will be apparent from the symmetry of the circuit that, under quiescent conditions with all LEDs operating normally, the three column resistor voltages Vra, Vrb and Vrc will be approximately equal and near zero in magnitude because little or no current will flow through the resistors Ra, Rb and Rc. Similarly, Vrd and Vre will be practically equal in value because each of row series resistors Rd and Re will carry approximately 50% of the total circuit current, and consequently there will not be any alarm signal.

If an open circuit appears at the first column, second row LED D 12, for example, the current previously carried by that LED D 12 will be shared by the five remaining LEDs D11, D13, D14, D15, and D16 in the same column, which are connected in parallel with the failed LED D12. The current subsequently passes into the next column of six parallel LEDs, D21 to D26, and thus divides into the six available current paths. However, the breaking of symmetry changes the current passing through the first column resistor Ra, and the resulting changes in the voltage inputs to the first comparator 504 trigger an alarm signal.

Similarly, it can be shown that an open circuit at any one of the first, second, or third column LEDs DI l to D36 triggers an alarm signal. If an open circuit occurs at any one of the fourth column LEDs D41 to D46, then since only five current paths remain, the currents through the two series resistors Rd and Re will be in the approximate ratio of 3 :2, which produces an imbalance in the input voltages Vrd and Vre at the second comparator 506 and thus triggers an alarm signal.

If a short circuit occurs at any one of the LEDs DI l to D46 in the array, then all of the remaining five LEDs in the same column will also cease operating and a substantial current will pass through one of the column resistors Ra₅ Rb or Rc, thus triggering an alarm signal.

It will be apparent that the number of LEDs in such a lighting device or system is not limited to the numbers shown but can be varied as desired. The lighting systems described above can not only detect failures of one or more individual light generating components within a single device, luminaire, or lighting system, it can also work in concert with the standard turning signal monitoring system used in automobiles. This is particularly advantageous as it allows traditional (e.g., incandescent) lamps to be replaced by solid state lighting components or luminaires, because the traditional monitor is also the means by which the turn signal lamps are made to operate intermittently. Thus an indication of a fault is transmitted to an attention raising indicator in the vicinity of the driving position in the normal way.

The values of resistance chosen for resistors Ra, Rb, Rc, Rd, and Re shown in Figure 5, are determined by estimating the voltage that would appear across these resistors under a fault condition. When all the LEDs operate normally, the current through the resistors is ideally zero; however, some small leakage or balance current may flow because of normal differences between individual LEDs. This potential leakage current imposes an upper limit on the resistance value. The resistance value is chosen so that the product of the resistance value and the current through the resistors is less than the threshold or trigger value of the voltage comparators that monitor each resistor. Conversely, the resistance value is selected to produce a voltage difference above the threshold when a component becomes an open-circuit or short-circuit, as described above.

For example, if the design value of current through each of the LEDs in an array, such as that shown in Figure 5, is 100 milliAmperes (mA), then an open circuit at D22 means that in theory the LEDs D21, D23, D24, D25 & D26 share an additional 100mA between them. The current in each of these components will rise from 100mA to 12OmA. Resistor Ra will carry the additional current passing through D24, D25 & D26 - a total of 60mA. Resistor Rb will also conduct 6OmA; however the sense (direction) of the voltage appearing across Rb (Vrb) will be opposite to that of Vra. The minimum value of resistance for Ra can then be determined from the threshold or trigger voltage of the comparator 504, 506 divided by the current flowing in Ra when a fault condition exists. If the threshold voltage is 10 millivolts (mV), then the value of Ra + Rb would be required to be more than 1O x 10^"3 divided by 60 x 10^"3 i.e., 0.167 Ohms. The minimum value of Ra would be 50% of this value since the voltage difference at the input of the comparator is effectively the sum of Vra and Vrb. Preferably, the resistance value of all such resistors will be higher than the minimum value so determined, but will be in a similar range of values. For the example above, a suitable resistance value chosen from the preferred resistor table might be 0.1 Ohms. If a much higher resistance value is chosen, 10 Ohms for example, then to develop a voltage of 10 mV requires a current of only 0.5 mA. This value of current might be similar to the balance current which may flow through Ra in a practical circuit which is operating without any fault, and thus cause false triggering of the detector,

In a further preferred embodiment, the lighting device of Figure 5 is modified by replacing the comparators 504, 506 and signal generator 510 with the components shown in Figure 6. The electrical connections 512 to 526 correspond to those shown in Figure 5.

The voltages appearing across resistors Ra to Re are compared to each other with respect to the zero or ground voltage of the circuit, by first testing if VI l is larger or smaller than V15, and by similarly testing V12 against V16, V13 against V17 and V14 against V18. If the two results of each test are negative, then the outputs of the gates 604 will all appear as positives, which will satisfy the condition at the input of the gate and the transistor 612 will be switched on, allowing the LED array 501 to be energised. If any single one of the eight

' comparators 602 returns a positive output, then the corresponding one of the NOR gates

604 will have a zero output, and the AND gate 606 will also have zero output, thereby switching off the transistor 610 and extinguishing the LEDs. A positive output at the comparators 602 can only result when a significant voltage appears across one of the resistors Ra to Re. An additional function is provided to prevent the circuit from resetting. Without some form of control, the circuit may oscillate quickly between on and off states because whenever the LED array is de-energised, the logic circuit will reset, and allow the transistor to energise the LEDs. Introducing a capacitor 612 between the transistor gate and ground provides a delay time during which the LEDs remain energised until the capacitor 612 discharges, and also giving a delayed start-up until the capacitor 612 is charged. This facilitates a flashing operation, in which the flash rate can be determined by manipulating the capacitor value and its corresponding charge and discharge times.

In the preferred embodiment shown in Figure 6, the specific components shown are for example only and have been selected from semiconducting devices which are readily available commercially. Many other combinations of components will be apparent to those skilled in the art.

Although the preferred embodiments have been described above in terms of lighting systems including a plurality of LEDs, it will be apparent that the principles of the invention can also be applied to other types of light-generating components (even incandescent light sources), and even to combinations of different types of light- generating components. Moreover, it will be apparent to those skilled in the art that the invention can be applied to lighting systems powered by constant voltage power supplies rather than constant current supplies, wherein, as in the described embodiments, portions of the total current drawn by the light emitting components are monitored by comparing voltages across resistors located in current paths of the lighting system. Thus, in general, embodiments of the present invention operate by comparing signals (which can be voltages or currents) that are in any case representative of the distribution of electrical current to corresponding portions of an array of interconnected light generating components. It will also be apparent that the comparison detects changes in the relative (rather than absolute) distribution of electrical power so that, for example, should the supply of electrical power to the entire array change for some reason, the comparison will not be affected, since any pair of compared signals will both change by the same corresponding amount. Hence if the supply of electrical power to the entire array drops (e.g., a car battery loses charge and consequently supplies less current to the array), a false alarm will not be generated. Additionally, although there has been an emphasis on automotive applications, the invention is not limited to such applications but can be equally applied to applications where there may be a need or desire to detect one or more failures of one or more light- generating components of a lighting device or system having many such light-generating components.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as hereinbefore described with reference to the accompanying drawings.

Claims

CLAIMS:

1. A fault detector configured to compare a plurality of signals representative of the distribution of electrical power in corresponding portions of an array of interconnected light generating components, and to generate a fault signal only if the comparison indicates at least one change in the relative distribution of electrical power in said portions of said array, said change being indicative of a fault in at least one of said light generating components.

2. The fault detector of claim 1, wherein the fault detector includes one or more comparators to compare said signals.

3. The fault detector of claim 2, wherein said array includes resistors arranged in electrical pathways of said array to generate said signals from the flow of electrical current along said electrical pathways.

4. The fault detector of any one of claims 1 to 3, including electrical pathways distributing electrical power to said light generating components and interconnecting said light generating components to form said array.

5. The fault detector of any one of claims 1 to 4, wherein said fault detector is configured to generate said fault signal only if the comparison indicates that the electrical powers in said corresponding portions of said array are not substantially equal, said portions of said array being such that said electrical powers in said corresponding portions are substantially equal when said light generating components are operating normally.

6. The fault detector of any one of claims 1 to 5, wherein at least some of the electrical pathways interconnecting said light generating components include resistors to generate voltages representative of the distribution of electrical power in corresponding portions of said array, said fault detector including one or more voltage comparators arranged to receive and compare voltages across said resistors; said resistors being located at symmetrical locations of said array such that corresponding pairs of said voltages are substantially equal when said light generating components are operating normally, failure of one or more of said light generating components causing a substantial difference in at least one of said pairs of voltages, the at least one voltage comparator being responsive to the substantial voltage difference to generate said fault signal.

7. The fault detector of any one of claims 1 to 6, including a component responsive to said fault signal to change a mode of operation of said array of light generating components so that failure of one or more of said light generating components causes a change in said operating mode of said array of said light generating components.

8. The fault detector of claim 7, wherein said change in operating mode includes a reduction in total electrical current flowing through said array of light generating components.

9. The fault detector of claim 8, wherein said change in said current is compatible with a standard turning signal circuit of an automobile such that when the array of light generating components is used as a turn signal indicator, the failure of one or more of said light generating components causes a change in the rate of flashing of the remaining others of said light generating components.

10. The fault detector of any one of claims 1 to 9, wherein said light generating components are solid-state devices.

11. The fault detector of claim 10, wherein said solid-state devices are light-emitting diodes.

12. A lighting system incorporating the fault detector of any one of claims 1 to 11.

13. A lighting device incorporating the fault detector of any one of claims 1 to 11.

14. A luminaire incorporating the fault detector of any one of claims 1 to 11.

15. An automobile luminaire incorporating the fault detector of any one of claims 1 to 11.

16. A fault detection process, including: comparing signals representative of the distribution of electrical power in respective portions of an array of interconnected light generating components; and generating a fault signal only if said comparing indicates at least one change in the relative distribution of electrical power in said portions of said array, said change being indicative of a fault in at least one of said light generating components.

17. The fault detection process of claim 16, wherein the electrical powers in each of one or more corresponding pairs of said portions of said array are substantially equal when said light generating components are operating normally, a substantial inequality in the electrical powers in a corresponding pair of said portions being indicative of a fault in at least one of said light generating components.

18. A fault detector having components for executing the process of claim 16 or 17.

19. A luminaire, including: an array of interconnected light generating components, including resistors to generate voltages representative of the distribution of electrical power in corresponding portions of said array; and electrical connectors for supplying electrical current to said light generating components and for providing said voltages to a fault detector configured to compare said voltages and to generate a fault signal only if the comparison indicates a fault in at least one of said light generating components.

20. The luminaire of claim 19, wherein said light generating components are electrically interconnected as an array of columns and rows, wherein respective electrodes of the light generating components of each column are each interconnected in parallel.

21. A luminaire, including: an array of interconnected light generating components; and electrical connectors configured to supply electrical current to said light generating components and to conduct electrical currents from and to selected nodes of said array via a fault detector configured to compare said currents and to generate a fault signal only if the comparison indicates a fault in at least one of said light generating components.