EP4598278A1

EP4598278A1 - Led luminaire with fault detection capability

Info

Publication number: EP4598278A1
Application number: EP24154564.9A
Authority: EP
Inventors: Stefan Lorenz; Mario PÖHS
Original assignee: Zumtobel Lighting GmbH Austria
Current assignee: Zumtobel Lighting GmbH Austria
Priority date: 2024-01-30
Filing date: 2024-01-30
Publication date: 2025-08-06
Also published as: WO2025162704A1

Abstract

The invention relates to an LED luminaire (10). The LED luminaire (10) comprises a number of LED strings (11a-11c) which are connected in parallel, wherein each of the LED strings (11a-11c) comprises a number of LEDs in a series connection; a number of resistors (Rab, Rbc), wherein each of the resistors (Rab, Rbc) is arranged to electrically connect two LED strings of the number of LED strings (11a-11c) with each other; and a measurement circuit (12) which is configured to sequentially measure a voltage drop across each of the number of resistors (Rab, Rbc); wherein the measurement circuit (12) is configured to detect and/or to localize a fault in the number of LED strings (11a-11c) based on the measured voltage drops.

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates the detection of faults, such as short circuits and circuit breaks, in LED luminaires.

BACKGROUND OF THE INVENTION

In LED lighting technology, several LEDs are often interconnected in a light source to achieve higher luminous fluxes. The electrical connection of the current-driven LEDs can be a series or a parallel connection, or a combination thereof.
If a single LED fails, the entire light source can be affected in different ways, depending on the interconnection of the LEDs and the exact failure type. Such individual LED failures play an important role for the safety, longevity, and stability of the light source.
Typically, multiple LEDs are connected in series to increase a required voltage. Such a series of LEDs is also called a "string" (or "LED string"). If at a given string voltage the luminous flux should be enhance via further LEDs, multiple such strings can be connected in parallel.
If an LED in a string fails due to a short circuit, the string voltage and the luminous flux decrease at a given current. In contrast, if an LED in the string fails due to a circuit break, the current goes to zero and the entire string remains dark.
When having two or more parallel LED strings, a "short circuit" causes an increased current flow through the string in which the short circuit occurs. The LEDs in this string can be overloaded in the long term. In case of a "circuit break" in a string, an increased current flows in all other strings of the light source and the LEDs in these other strings can be overloaded over time. In both cased, an individual failure may cause a larger disparity in the light source, and therefore may have more serious consequences than just the proportional loss of luminous flux.
However, the detection of an individual failure in a light source by electrical means is often inaccurate and requires a lot of effort, in particular for a light source with a large number of strings and a larger number of LEDs per string. A parallel circuit of shorter LED strings requires a precise sorting of individual LEDs by voltage to avoid individual strings being overloaded.
Thus, it is an objective of the invention to provide an improved failure detection for an LED luminaire.

SUMMARY OF THE INVENTION

The object of the present invention is achieved by the solution provided in the enclosed independent claims. Advantageous implementations of the present invention are further defined in the dependent claims.
According to an aspect, the invention relates to an LED luminaire, comprising: a number of LED strings which are connected in parallel, wherein each LED string comprises a number of LEDs (light emitting diodes) in a series connection; a number of resistors, wherein each resistor is arranged to electrically connect two LED strings of the number of LED strings with each other; and a measurement circuit which is configured to sequentially measure a voltage drop across each of the number of resistors; wherein the measurement circuit is configured to detect and/or to localize a fault in the number of LED strings based on the measured voltage drops.
This achieves the advantage that a failure of individual LEDs in a string can be detected efficiently by electrical means. As a consequence, countermeasures can be taken against overloading and performance deviations in the LED luminaire.
This fault detection can be tolerant against voltage variations of individual LEDs and, at the same time, not negatively affect the overall efficiency of the LED luminaire. Furthermore, a relatively low number of additional connection lines and resistors is required for the failure detection in the LED strings. For instance, single LED failures in all strings can be detected with measurement and resistors only at a few points between the strings (less than number of the strings).
A fault in an LED string can be caused by one or more faulty LEDs in the string. For example, a fault can be a short circuit and/or a circuit break in the string.
The LED luminaire can comprise at least two parallel LED strings. The parallel LED strings can form an LED module of the luminaire. Due to the parallel arrangement, the same voltage can be applied to each string.
Each resistor can be arranged to electrically connect two LED strings with each other, in particular two adjacent or neighboring strings. For instance, a resistor can be connected to a connection point on both LED strings, wherein the respective connection point is arranged between two LEDs in the series connection. The resistors can be formed by electrical resistance elements with determined resistivity values.
The resistivity values of the resistors can be optimized with regards to a fault detection probability, and/or with regards to error tolerances of the individual LEDs or the LED luminaire.
For instance, the measurement circuit sequentially measuring the voltage drop across the resistors means that the measurement circuit measured the voltage drop successively over the individual resistors in a particular order, which can be preprogrammed.
In case there is no failure in an LED string, no or only a minimal voltage drop is detected over a connected resistor. Thus, in the typical (non-failure) state, the measurement circuit does not dissipate or waste any power in the LED luminaire (except for the evaluation electronics). Upon a failure in an LED string (e.g., short circuit or circuit break due to defect LED), an increased voltage drop occurs over the connected resistor. This voltage drop can be detected and is indicative of the failure and the location of the failure.
In an embodiment, the measurement circuit comprises a switch matrix which is configured to sequentially connect the resistors with a voltage measurement unit to measure the voltage drops.
For example, the measurement circuit also comprises the voltage measurement unit.
The measurement circuit can further comprise an evaluation unit, such as a microprocessor or an ASIC. The evaluation unit can receive measurement signals from the measurement circuit (e.g., from the voltage measurement unit) and can detect and/or localize a fault based on said signals. Upon detecting a fault, the evaluation unit can generate and forward an error message.
Knowing the state of the switch matrix (i.e., which resistor is currently connected to the voltage measurement unit) allows the evaluation unit to detect on which resistor a voltage drop occurs. In this way, a failure in an associated LED string can be localized. For instance, a comparison of two or more adjacent resistors can give the specific string with the failed LED.
In an embodiment, the switch matrix is configured to switch between the connections of the voltage measurement unit with the individual resistors of the number of resistors with a switching frequency of at least 1 Hz, preferably of at least 10 Hz. This provides the advantage of avoiding a DC coupling between resistances and measurement circuit.
In an embodiment, the measurement circuit is configured to localize a fault in the number of LED strings based on the switching state of the switch matrix when detecting a fault.
In an embodiment, the measurement circuit is configured to detect a fault in the number of LED strings based on: a comparison between a measured voltage drop with one or more reference values, a comparison between at least two voltage drops measured across different resistors, and/or a comparison between at least two voltage drops measured across the same resistor in two different measurement cycles.
Furthermore, the measurement circuit can detect a type of error based on the comparison (s). The Type of error can be short circuit or a circuit break in a string.
In an embodiment, the measurement circuit is at least partially integrated in a control circuit of the LED luminaire, or the measurement circuit is an external circuit.
In an embodiment, the measurement circuit is configured to vary a measurement time and/or a number of measurement cycles for measuring the voltage drop across the respective resistors. This achieves the advantage that a higher interference resistance of the measurement can be achieved.
In an embodiment, the measurement circuit comprises a timer which is configured to control the measurement time(s) and/or the number of measurement cycles.
In an embodiment, each of the LED strings is connected to two further LED strings of the number of LED strings by a respective resistor of the number of resistors.
For example, each two LED strings of the number of LED strings which are arranged directly adjacent to each other are connected by a respective resistor of the number of resistors. Furthermore, the first and last LED string of the number of strings (which only have one direct neighbor) can be connected to each other via a resistor.
In an embodiment, the resistors are connected to each other in a ring circuit. This achieves the advantage that a failure detection in all LEDs can be improved. Furthermore, the current through the resistances can be minimized in case of voltage tolerances/fluctuations.
In an embodiment, each of the LED strings is connected to only one further LED string of the number of LED strings by a respective resistor of the number of resistors.
For example, each second pair of adjacent LED strings can be connected to each other via a resistor, wherein the LED luminaire comprises an even number of LED strings. This achieves the advantage that the number of resistors and connection lines to the switch matrix can be minimized.
In an embodiment, at least one, preferably each, resistor of the number of resistors is arranged to electrically connect to two of the LED strings at a respective connection point which is located between the penultimate and the last LED of each LED string.
Thereby, the LEDs of one string can be counted along their series connection. For instance, all LED strings of the number of LED strings comprise the same number of LEDs. Two adjacent LED strings can form a pair of LED strings.
In an embodiment, at least one, preferably each, resistor of the number of resistors is arranged to electrically connect to two of the LED strings at a respective connection point which is located after exactly half of the LEDs of each LED string or at a respective connection point which is directly before or after a middle LED of each LED string.
For instance, the connection point is located after exactly half of the LEDs if the string comprises an even number of LEDs and at a connection point directly before or after the middle LED, if the LED string comprises an odd number of LEDs. In an example, all LED strings of the number of LED strings comprise the same even or odd number of LEDs.
In an embodiment, the number of LED strings comprises a first, a second and a third LED string, and wherein the number of resistors comprises a first and a second resistor; wherein the first resistor is arranged to electrically connect to the first LED string and to the second LED string at a respective first connection point which is located after n LEDs of the first and the second LED string, respectively; and wherein the second resistor is arranged to electrically connect to the second LED string and to the third LED string at a respective second connection point which is located after n+1 LEDs of the second and the third LED string, respectively.
For instance, each of the first, second and third LED string comprises the same number N of LEDs, wherein N ≥ 3, and wherein 1 < n < N.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in the followings together with the figures.

Fig. 1: shows a schematic diagram of an LED luminaire according to an embodiment;
Figs. 2A and 2B: show a control of a switch matrix by a timer according to an embodiment;
Fig. 3: shows results of voltage drop measurements during a switching cycle for different error states of an LED according to an embodiment.
Fig. 4: shows an arrangement of resistors in an LED luminaire according to an embodiment;
Fig. 5: shows an arrangement of resistors in an LED luminaire according to an embodiment;
Fig. 6: shows an arrangement of resistors in an LED luminaire according to an embodiment; and
Fig. 7: shows an arrangement of resistors in an LED luminaire according to an embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 shows a schematic diagram of an LED luminaire 10 according to an embodiment. The LED luminaire 10 comprises a number of LED strings 11a, 11b, 11c which are connected in parallel, wherein each of the LED strings 11a-11c comprises a number of LEDs connected in series.
The LED luminaire 10 further comprises: a number of resistors Rab, Rbc, wherein each of the resistors Rab, Rbc is arranged to electrically connect two LED strings of the number of LED strings with each other; and a measurement circuit 12 which is configured to sequentially measure a voltage drop across each of the number of resistors Rab, Rbc; wherein the measurement circuit 12 is configured to detect and/or to localize a fault in the number of LED strings 11a-11c based on the measured voltage drops.
Thereby, a voltage drop across one of the resistors Rab, Rbc is indicative of a failure state of an LED in the respective LED string connected to the resistor. In case there is no failure, no current (or only minimal current) flows through the resistors.
The exemplary luminaire 10 shown in Fig. 1 comprises three strings 11a, 11b, 11c with four LEDs a1-a4, b1-b4, c1-c4 each. However, the luminaire 10 can comprise any number N of strings with N > 1, each string comprising two or more LEDs.
The LED strings 11a-11c can be arranged in columns, such that the individual LEDs are arranged in a matrix configuration, wherein each column of the matrix is formed by an LED string. For instance, each LED string 11a-11c has the same number of LEDs. The LEDs and/or the resistors can be arranged on an LED board of the LED luminaire 10. The LED luminaire 10 can be an LED light source.
A failure in one of the LED strings 11a-11c can be a short circuit or a circuit break, e.g., caused by one or more faulty LEDs in the string.
The resistors Rab, Rbc can be formed by electrical resistance elements with determined electrical resistivity values. For instance, the resistivity values of the different resistors Rab and Rbc may vary. The resistivity values of the resistors Rab, Rbc can be optimized with regards to a fault detection probability, and/or with regards to error tolerances of the individual LEDs or the LED luminaire 10.
The resistors Rab, Rbc can be connected to the respective LED strings 11a-11c via short conductive lines. The conductive lines can contact the LED strings at connection points between two LEDs. Besides the number of resistors Rab, Rbc, which are used for voltage drop detection, the LED luminaire 10 can of course comprise further resistors and resistive elements with different functionalities.
The measurement circuit 12 may comprises a voltage measurement unit 15 which is capable of detecting a voltage drop across each of the resistors Rab, Rbc, and a switch matrix 14 which is configured to sequentially (e.g., successively in a particular order) connect the voltage measurement unit 15 with the individual resistors Rab, Rbc. The voltage measurement unit 15 can comprise a sensing element for detecting the voltage drop, e.g., via a current measurement of a cross-current flowing across a resistor.
For instance, the measurement circuit 12 or more specifically the voltage measurement unit 15 detects a voltage drop by sequentially measuring if a current flows through the resistors Rab, Rbc. In case the measurement circuit 12 detects such a cross current between two connected LED strings 11a, 11b, 11c, it can deduct that there is a failure in one of the strings.
This approach has several advantages, in particular compared to performing individual current measurements on each LED. Firstly, it allows to monitor the LEDs of the luminaire 10 with only a few electrical measurements. Thereby, a low number of measurement points, electrical lines (e.g., conducting paths, wires etc.) and elements (e.g., one resistor per string) are required for these measurements, which leads to a simple routing on the module of the luminaire 10. By measuring the voltage drop across the resistors sequentially, only one voltage measurement unit 15 is required in the measurement circuit 12.
Furthermore, the voltage drop monitoring does not reduce the efficiency of the LED luminaire 10 (in contrast to a current measurement in each string). Connecting two respective LED strings via a single resistor further avoids the disadvantages of a full matrix connection between the LEDs of the luminaire and allows for tolerances in the LED voltage.
The measurement circuit 12 can at least be partially integrated in a control circuit of the LED luminaire 10, e.g. in an LED driver of the LED luminaire 10. In this case, only a few additional connection between the LED strings 11a, 11b, 11c and the control circuit are needed.
Separating the monitoring between the module side electronics (e.g., the resistors and their connection to the strings) and the external components (e.g., the measurement circuit 12 in the driver) allows for a simple analysis of a large number of strings and modules by a single measurement circuit 12 in the driver.
Alternatively, the measurement circuit 12 can be an external circuit in a separate module (e.g., between LED module and driver) .
The measurement circuit 12 can further comprise an evaluation unit which evaluates the measurement by the voltage measurement unit 15 and detects a fault. For instance, the evaluation unit can be a microprocessor or an ASIC (e.g., in an LED driver of the luminaire 10). For example, the evaluation unit can be a component of the voltage measurement unit 15 or it can be connected to the voltage measurement unit 15 by a signaling line.
For instance, the evaluation unit receives measurement signals from the measurement circuit (e.g., from the voltage measurement unit) which can comprise information on the detected voltage drops and/or the resistor at which the voltage drop was detected. Based on this information the evaluation unit can detect and/or localize a fault. Upon detecting a fault, the evaluation unit can generate and forward an error message.
For example, the measurement circuit 12, in particular the evaluation unit, is configured to detect a fault in the number of LED strings 11a-11c based on: a comparison between a measured voltage drop with one or more reference values (e.g., a reference voltage of 0 V), a comparison between at least two voltage drops measured across different resistors (especially in case of a symmetrical arrangement of the resistors), and/or a comparison between at least two voltage drops measured across the same resistor in two different measurement cycles (i.e., changes of the voltage drop over time).
For instance, the measurement circuit 12 or more specifically the evaluation unit is configured to detect a fault if a measured voltage drop across a resistor exceeds a threshold value and/or if a difference between two voltage drops measured across different resistors or across the same resistor in different measurement cycles exceeds a threshold value.
Furthermore, the measurement circuit 12 or more specifically the evaluation unit can detect the type of error (circuit break or short circuit) based on the comparison(s) and/or based on the measured current or voltage.
If a fault is detected, the measurement circuit 12 can issue an error signal. The criteria for the detection of a failure (e.g., the reference or threshold values) can be module dependent and/or configurable for different LED modules (e.g., different modules of the luminaire 10, each module having a different number/arrangement of strings).
The measurement circuit 12 can further comprise a timer 16. The timer 16 can be configured to control the switching operation of the switch matrix 14. The voltage measurement unit 15 can then detect the forwarded voltage drop.
For instance, the timer 16 can control the switch matrix 14 to vary a measurement time and/or a number of measurement cycles for sequentially measuring the voltage drop across the number of resistors Rab, Rbc.
This approach allows for a simple realization of a fault detection in the LED luminaire 10. For instance, the timing of the timer 16 is not time critical, the switch matrix 14 requires only a low number of switches, only one voltage measurement can be performed for each string, an error message can be send by a single signal, and/or error messages of different modules of the luminaire 10 can be logically assigned.
Figs. 2A and 2B show the control of the switch matrix 14 by the timer 16 according to an embodiment.
As shown in Fig. 2A, the timer 16 can control the switch matrix 16 via control signals to sequentially connect the resistors Rab, Rbc to the voltage measurement unit 15. Fig. 2B shows the corresponding switching states of the switch matrix 14 for the first two switching states (connection of Rab and Rbc, respectively).
As can be seen in Fig. 2A, the timer 16 can control the switch matrix 14 to first connect resistor Rab to the voltage measurement unit 15 for a time interval of t_AB (corresponding to the left image in Fig. 2B). Subsequently, the timer 16 can control the switch matrix 14 to connect resistor Rbc to the voltage measurement unit 15 for a time interval t_BC (corresponding to the right image in Fig. 2B).
In case the luminaire has an additional resistor Rca which connects strings 11c and 11a (not shown in Fig. 1), the timer can control the switch matrix 15 to subsequently switch to this resistor, as indicated in Fig. 2A. When all resistors have been connected one after the other to the voltage measurement unit 15, the timer can control the switch matrix 14 to start a new cycle.
In this way, the voltage drop across each resistor can be measured sequentially. In particular, each switching state of the switch matrix 15 can be correlated to a voltage measurement, which allows localizing a possible fault.
Fig. 3 shows the results of voltage drop measurements during a switching cycle for different error states of an LED of the luminaire 10 shown in Fig. 1 according to an embodiment. Hereby, the effect of different error states of LED a1 in string 11a on the voltage measurements is shown as an example. The same measurement principle is valid for all other LEDs in the strings 11a-11c.
As shown in the second chart of Fig. 3, the current through Rab is close to zero if LED a1 is OK (no fault), a positive current through Rab occurs if LED a1 is open (circuit break) and a negative current occurs through Rab if LED a1 is shorted.
Thus, if all LEDs of the luminaire 10 (including a1) operate without fault, then Vab = Vbc ~ 0, with Vab being the voltage across Rab and Vbc being the voltage across Rbc (see third chart in Fig. 3).
If there is a circuit break in string 11a (LED a1 open), then the voltage drop (Vdrop) across Rab is increased, which is measured during intervals t_AB (see fourth chart in Fig. 3). Also, Vbc can be somewhat increased, with Vab > Vbc > 0V.
If there is a short circuit in string 11a (LED a1 short) then Vdrop across Rab is reduced below zero voltage (see fifth chart in Fig. 3). Also, Vbc can be somewhat reduced, with: Vab < Vbc < 0V.
In general, a fault in one of the strings can be detected in one of the following ways:

|Vab| or |Vbc| exceed a threshold value (positive or negative).
|Vab - Vbc| exceed a threshold value.
The voltage measurement unit 15 stores a measurement value from a previous measurement and compares a current measurement value with the stored value. If the difference between both values exceeds a threshold, a fault is detected.

The switch matrix 14 can switch between the resistors Rab, Rbc with a high switching frequency of at least 1 Hz, in particular at least 10 Hz (e.g., tab, tbc < 100ms). The voltage measurement unit 15 and/or the evaluation unit can evaluate the frequency components (e.g., AC filtering) to improve a signal to noise ratio.
The switching states of the switch matrix 14 (and, thus, the correlation to the measurements) can be encoded digitally, in such a way that the measurement time and/or the number of measurement cycles (i.e., the number of measurements in a certain time) is different for each resistor.
For instance, for two resistors Rab, Rbc the time duration of a sampling for measuring a possible voltage drop can be different, or the number of measurement cycles in each measurement phase can be different. For example, resistor Rab can be sampled once in a measurement phase und resistor Rbc can be sampled two or more times in the same measurement phase. This is advantageous over an evaluation of signal levels, because this digital coding is more resistant to faults and interferences.
Figs. 4-7 show different possible arrangements and interconnections of resistors Rab, Rbc, Red in the LED luminaire 10.
In the example shown in Fig. 4, each LED strings 11a-11d is connected to two further LED strings 11a-11d of the number of LED strings by a respective resistor Rab, Rbc, Rcd, Rda. Thereby, each LED strings 11a-11d can be connected to its two direct neighbors via a resistor Rab, Rbc, Rcd, while the first and the last LED string 11a, 11d (which only have one direct neighbor) are connected to each other via a resistor Rda.
Thereby, the resistors are connected to each string at the same "height" (i.e., after the same number of LEDs in the series connection), resulting in the resistors being connected to each other in a ring circuit.
For instance, the resistors are connected to each string in the "middle" (i.e., after half of the LEDs in the series connection, in case of an even number of LEDs per string).
Typically, this arrangement is applicable for a luminaire 10 with at least two strings and at least two, preferably between four and 100, LEDs per string. For instance, when driving the LED luminaire with safety extra-low voltage (SELV), a string can have up to 30 LEDs, and when driving with mains voltage, up to 100 LEDs can be connected in series.
This leads to a fully symmetric arrangement of the resistors Rab, Rbc, Rcd, Rda and therefore to a robust fault detection for all LEDs, an even distribution of an overload in the event of a fault, and a minimal current through the resistors in case of voltage tolerances/fluctuations.
In the example shown in Fig. 5, a resistor Rab is arranged to electrically connect at least two LED strings 11a, 11b at a respective connection point which is located between the penultimate and the last LED of each string (i.e., above the last LED of the series connection).
Typically, this arrangement is applicable for a luminaire with at least two strings and at least two LEDs per string.
This arrangement allows a simple measurement of the voltage drop at the resistor Rab and a simple routing, since no ring circuit is necessary (less interconnections). However, in case of two parallel rings, it might be more difficult to detect a defect of LEDs with are further away from the resistor Rab. For instance, the voltage measurement from the low side of the strings is especially easy, as the voltage across the last LED (a1, b1) is only slightly above the low side level.
In the example shown in Fig. 6, each LED string 11a-11d is connected to only one further LED string 11a-11d by a respective resistor Rab, Rcd.
Thereby, the resistors can be connected to each string in the "middle" (i.e., after half of the LEDs in the series connection, in case of an even number of LEDs per string).
Typically, this arrangement is applicable for a luminaire with at least two strings and at least two LEDs per string.
This arrangement allows for a robust fault detection for all LEDs with a low number of measurement points (e.g., only two measurements for monitoring four strings), and a low number of conducting paths.
In the example shown in Fig. 7, the resistors Rab, Rbc, Red are arranged in a "stair" configuration. Thereby, each of the resistors Rab, Rbc, Red connects two adjacent strings, wherein the resistors are connected in step-wise increasing "heights" to the strings.
For example, a first resistor Rab connects to a first and a second LED string 11a, 11b at a respective first connection point located directly after n LEDs in each of the strings 11a, 11b (in Fig. 1, after the first LED); a second resistor Rbc connects to the second and a third LED string 11b, 11c at a respective second connection point located directly after n+1 LEDs in each of the strings 11b, 11c; and a third resistor Rcd connects to the third and a fourth LED string 11c, 11d at a respective third connection point located directly after n+2 LEDs in each of the strings 11c, 11d.
Typically, this arrangement is applicable for a luminaire with at least three LED strings and at least as many LEDs per string as there are LED strings.
This stair configuration allows to determine which of the LED strings 11a-d has a faulty LED based on the absolute value of the voltage measurements on the resistors. This leads to an improved sequential measurement.
Individual LEDs can be protected by varying resistance values of the resistors (e.g., Rab and Rcb >> Rbc).
Although the invention has been illustrated and described with respect to one or more implementations, equivalent alternations and modifications will occur to those skilled in the art upon the reading of the understanding of the specification and the annexed drawings. In addition, while a particular feature of the invention may have been disclosed with respect to only of the several implementations, such features may be combined with one or more other features of the other implementations as may be desired and advantage for any given or particular application.

Claims

An LED luminaire (10), comprising:
a number of LED strings (11a-11c) which are connected in parallel, wherein each of the LED strings (11a-11c) comprises a number of LEDs in a series connection;

a number of resistors (Rab, Rbc), wherein each of the resistors (Rab, Rbc) is arranged to electrically connect two LED strings of the number of LED strings (11a-11c) with each other; and

a measurement circuit (12) which is configured to sequentially measure a voltage drop across each of the number of resistors (Rab, Rbc);

wherein the measurement circuit (12) is configured to detect and/or to localize a fault in the number of LED strings (11a-11c) based on the measured voltage drops.
The LED luminaire (10) of claim 1,
wherein the measurement circuit (12) comprises a switch matrix (14) which is configured to sequentially connect the resistors (Rab, Rbc) with a voltage measurement unit (15) to measure the voltage drops.
The LED luminaire (10) of claim 2,
wherein the switch matrix (14) is configured to switch between the connections of the voltage measurement unit (15) with the individual resistors of the number of resistors (Rab, Rbc) with a switching frequency of at least 1 Hz.
The LED luminaire (10) of claim 2 or 3,
wherein the measurement circuit (12) is configured to localize a fault in the number of LED strings (11a-11c) based on the switching state of the switch matrix (14) when detecting a fault.
The LED luminaire (10) of any one of the preceding claims,
wherein the measurement circuit (12) is configured to detect a fault in the number of LED strings (11a-11c) based on: a comparison between a measured voltage drop with one or more reference values, a comparison between at least two voltage drops measured across different resistors (Rab, Rbc), and/or a comparison between at least two voltage drops measured across the same resistor (Rab, Rbc) in two different measurement cycles.
The LED luminaire (10) of any one of the preceding claims,
wherein the measurement circuit (12) is at least partially integrated in a control circuit of the LED luminaire (10), or wherein the measurement circuit is an external circuit.
The LED luminaire (10) of any one of the preceding claims,
wherein the measurement circuit (12) is configured to vary a measurement time and/or a number of measurement cycles for measuring the voltage drop across the respective resistors (Rab, Rbc) .
The LED luminaire (10) of claim 7,
wherein the measurement circuit (12) comprises a timer (16) which is configured to control the measurement time(s) and/or the number of measurement cycles.
The LED luminaire (10) of any one of the preceding claims,
wherein each of the LED strings (11a-11c) is connected to two further LED strings of the number of LED strings (11a-11c) by a respective resistor of the number of resistors (Rab, Rbc).
The LED luminaire (10) of claim 9,
wherein the resistors (Rab, Rbc) are connected to each other in a ring circuit.
The LED luminaire (10) of any one of claims 1 to 8,
wherein each of the LED strings (11a-11c) is connected to only one further LED string of the number of LED strings (11a-11c) by a respective resistor of the number of resistors (Rab, Rbc) .
The LED luminaire (10) of any one of the preceding claims,
wherein at least one, preferably each, resistor of the number of resistors (Rab, Rbc) is arranged to electrically connect to two of the LED strings (11a-11c) at a respective connection point which is located between the penultimate and the last LED of each LED string (11a-11c).
The LED luminaire (10) of any one of claims 1 to 11,
wherein at least one, preferably each, resistor of the number of resistors (Rab, Rbc) is arranged to electrically connect to two of the LED strings (11a-11c) at a respective connection point which is located after exactly half of the LEDs of each LED string or at a respective connection point which is directly before or after a middle LED of each LED string.
The LED luminaire (10) of any one of claims 1 to 10,
wherein the number of LED strings comprises a first, a second and a third LED string (11a-11c), and wherein the number of resistors comprises a first and a second resistor (Rab, Rbc);

wherein the first resistor (Rab) is arranged to electrically connect to the first LED string (11a) and to the second LED string (11b) at a respective first connection point which is located after n LEDs of the first and the second LED string (11a, 11b), respectively; and

wherein the second resistor (Rbc) is arranged to electrically connect to the second LED string (11b) and to the third LED string (11c) at a respective second connection point which is located after n+1 LEDs of the second and the third LED string, respectively.