EP2362711B1

EP2362711B1 - Failure detection of LEDs

Info

Publication number: EP2362711B1
Application number: EP10154657A
Authority: EP
Inventors: Heinz Telefont
Original assignee: Thales Rail Signalling Solutions GmbH
Current assignee: Thales Austria GmbH
Priority date: 2010-02-25
Filing date: 2010-02-25
Publication date: 2012-07-18
Anticipated expiration: 2030-02-25
Also published as: SI2362711T1; EP2362711A1

Description

The invention relates to an electrical circuit for powering a plurality of LEDs, with a function for indicating an LED failure,
wherein the electrical circuit comprises N legs connected in parallel, with N≥2, and with each leg comprising several LEDs and a leg resistor connected in series with the LEDs,
wherein the leg resistors of the legs have basically the same resistance, and wherein the resistance of each leg is basically the same during normal operation.
Such an electrical circuit is known from FR 2 845 559 A .
Light emitting diodes (LEDs) are widely used in railway signalling and traffic signalling. LEDs are appreciated for their high efficiency and high reliability here. However, long term use may lead a failure of an LED. A corrupted LED should be replaced quickly, in order to avoid an impairment of the railway or traffic signalling device in which it is used.
In the state of the art, it is known to equip electric circuits which power LEDs with a function indicating an LED failure. In EP 1 777 533 A1 , an array of LEDs is monitored by determining the voltage across the LED array. In EP 1 916 879 B1 , the absolute brightness provided by an array of LEDs is monitored.
The known electrical circuits are dedicated to a particular arrangement of LEDs, namely a single LED array, what limits - at a given operating voltage - the number of LEDs which can be monitored. Further, the known electrical circuits are relatively complex.
A method for supervising an electrical contact is also known from EP 1 453 072 B1 .
FR 2 845 559 A describes an electrical circuit for powering a vehicle lamp comprising several LED units, each comprising some LEDs connected in series, and each connected in series with a resistor. The voltages at the resistors are fed into a detector. If the detector recognizes an abnormal condition, a regulator cuts down the voltage at the LED units.
DE 199 29 430 A1 describes an LED rear light, with two LED groups connected in series with a resistor each. The voltages across the resistors are fed into a common operational amplifier connected to a threshold switch, which in turn powers a diagnosis LED and is connected to a potential free contact for external diagnosis.

Object of the invention

It is the object of the invention to provide an electrical circuit for powering LEDs which is simple in design, and which may power and monitor a larger number of LEDs.

Short description of the invention

This object is achieved, in accordance with the invention, by an electrical circuit as introduced in the beginning, characterized in that
that the legs are connected to a common shunt resistor,
that the electrical circuit comprises an amplifier unit amplifying a voltage at the shunt resistor with an amplification factor AF into an amplified voltage,
that the electrical circuit comprises a comparator unit for each leg, wherein the comparator units each comprise a comparator which compares the voltage at the respective leg resistor with the amplified voltage,
that the amplification factor AF is chosen such that during normal operation, the voltage at the leg resistors and the amplified voltage deviate slightly,
and that the outputs of the comparator units are connected at a common signal output unit.
According to the invention, the LEDs are distributed over a plurality of N legs, allowing a larger number of monitored (and powered) LEDs at a given operating voltage. Note that the legs may also be referred to as strings. By means of a leg resistor each, the current through each leg is indicated as a leg voltage. Each leg voltage is compared with an internal reference voltage, namely an amplified voltage originating from an amplifier unit connected to a common shunt resistor of the legs, by means of a comparator unit.
If an LED in one of the legs has a short-circuit, the current through this leg will rise. If an LED in one of the legs becomes highly resistive, the current in this leg will drop. The change of current comes along with a change in the leg voltage at the respective leg resistor, and a change in the ratio of the leg voltage and the amplified voltage. In both above failure cases, in accordance with the invention, depending on the kind (polarity) of connection of the comparator units to the leg voltage and the amplified voltage, at least one of the comparator units will switch its condition as compared to a normal (non-failure) operation. This change of condition is registered at the common signal output unit, to which all comparator unit outputs are connected.
The invention makes use of the shift of (relative) currents between the legs connected in parallel upon an LED failure. Such a shift will occur no matter what the failure type is (short-circuit or highly resistive connection). Preferably, the uniform legs are monitored by the uniform leg resistors and uniform comparator units, what leads to uniform comparator unit output signals during normal operation. In case of an LED failure, then the shift of the relative currents leads to dissenting comparator unit output signals, which is used as a failure indication at the common signal output unit.
According to the invention, the amplification factor AF is chosen such that during normal operation, the voltage at the leg resistors and the amplified voltage deviate slightly. The deviation of the voltages establishes a failure detection threshold. The deviation is chosen small enough such that the voltage shifts at the leg resistors and the shunt resistor in case of the failure of a single LED results in a voltage shift larger than the threshold at at least one of the comparators. On the other hand, the deviation is chosen high enough such that small differences in the resistance of the leg resistors or (more generally) the normally functioning legs (e.g. due to fabrication tolerances), and thus small differences in the voltages at the leg resistors, do not result in a switched comparator.
The signal output unit typically provides a binary signal, with one state indicating no LED failure, and one state indicating that at least one LED is corrupted. If necessary, the signal output unit may include a logic device. The signal output unit may be operated with a separate voltage source as compared to the voltage source powering the LEDs in order to have a potential-free signal output.
The inventive electrical circuit is particularly suited for small LED currents, such as LED currents of 10 mA or less. Typically, all LEDs of a leg are connected in series. The invention may be applied in railway security applications, in particular in railway signals, and in traffic signals.

Preferred embodiments of the invention

In a highly preferred embodiment of the inventive electrical circuit, the amplification factor AF is chosen such that during normal operation, the voltage at the leg resistors and the amplified voltage deviate about 10% or less.
In a particularly preferred embodiment, the comparator units are designed such that

during normal operation, the outputs of the comparator units are all connected to a non-earth potential, and
the output of a comparator unit, with its comparator in a switched condition as compared to its condition during normal operation, is connected to earth. In this case, a single switched comparator unit may ground the input of the common signal output unit, such that an "or"-logic function of the comparator units is realized at the input of the common signal output unit inherently.

In a preferred further development of this embodiment, the comparators are of open collector output type. This is a simple way to provide an earth potential in the switched condition.
In an alternative further development, the comparators are of push pull output type, and the outputs of the comparators are connected to the basis or gate of a comparator unit transistor. In this case, the earth potential can be provided by the comparator unit transistor in a conductive state.
Highly preferred is an embodiment wherein the common signal output unit comprises an optocoupler. The optocoupler allows a separation of the potentials of the LED power supply and at a signal output of the signal output unit ("potential-free" LED failure indication).
An advantageous further development of this embodiment is characterized in that an LED input of the optocoupler is connected to a voltage supply and to the outputs of the comparator units,
and that a signal output of the signal output unit is connected both to a voltage supply and to a transistor input of the optocoupler. The outputs of the comparator unit may, in the LED failure case, ground the voltage supply at the optocoupler's LED input and thus depower the optocoupler's LED, so that the optocoupler transistor becomes resistive. Accordingly, the signal output is powered by the connected voltage supply (which is not grounded via the optocoupler's transistor). Thus, a potential-free signal at the signal output can be provided in the failure case. It should be noted that in case the comparator units provide earth potential the during normal operation and non-earth potentials in the switched state, and the connection of the comparator outputs is done via a suitable logic device (resulting in an "or"-logic), said non-earth potential in the switched state can also be used as an LED failure indication signal.
Advantageous is further an embodiment wherein each leg comprises the same number of LEDs, with the same type of LEDs in all legs, and with the LEDs of each leg arranged in series. This design is simple results in a very similar behaviour of the legs, simplifying the identification of LED failures.
Further preferred is an embodiment wherein the number N of legs ranges from 2 to 6, and the number of LEDs in a leg ranges from 4 to 24. The relative shift of currents between the legs upon an LED failure is, in general, higher for less legs and higher for less LEDs per leg. For the indicated numbers, the LED failures can still be well distinguished from fabrication tolerances, and thus the invention works particularly fine.
In a preferred embodiment, the resistance RS of the shunt resistor is 1/N times the resistance RL of a leg resistor, and the amplification factor AF is slightly below or slightly above 2. This design can easily be adapted for an arbitrary number of legs, without significant calculation efforts. The relatively low resistance of the shunt resistor as compared to the resistance of the leg resistor causes relatively high shifts in the currents of the legs in case of an LED failure.
Advantageous is also an embodiment wherein there is an additional comparator unit per leg, an additional amplification unit for the shunt resistor, and an additional common signal output unit for the additional comparator units, thus establishing the a two channel LED failure detection. In this embodiment, the failure identification function is doubled, thus allowing a "safe" failure detection.
Another preferred embodiment provides
that the electrical circuit comprises at least two shunt subcircuits, wherein each shunt subcircuit comprises

M legs connected in parallel, with M≥2, and with each leg comprising several LEDs and a leg resistor connected in series with the LEDs,
wherein the leg resistors of the legs have basically the same resistance, and wherein the resistance of each leg is basically the same during normal operation,
wherein the legs are connected to a common shunt resistor,
an amplifier unit amplifying a voltage at the shunt resistor with an amplification factor AF into an amplified voltage,
a comparator unit for each leg, wherein the comparator units each comprise a comparator which compares the voltage at the respective leg resistor with the amplified voltage,
and that the outputs of the comparator units of the at least two shunt subcircuits are connected at the common signal output unit. By means of the subcircuits, the number of monitored LEDs can be further increased in a basically unlimited way. Preferably, the comparator units are connected to earth in the switched state, thus establishing inherently an "or"-logic of the output signals of the comparator units.

Further advantages can be extracted from the description and the enclosed drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any combination. The embodiments mentioned are not to be understood as exhaustive enumeration but rather have exemplary character for the description of the invention.

Drawing

The invention is shown in the drawing.

Fig. 1: shows a schematic circuit diagram of a first embodiment of the present invention, with two legs and comparators of open collector output type;
Fig. 2a: shows a simulated measurement protocol for the electrical circuit of Fig. 1, during normal operation;
Fig. 2b: shows a simulated measurement protocol for the electrical circuit of Fig. 1, with a short-circuit in one LED of the left leg;
Fig. 2c: shows a simulated measurement protocol for the electrical circuit of Fig. 1, with a breakage in one LED of the left leg;
Fig. 3: shows a schematic circuit diagram of a second embodiment of the present invention, with two legs and comparators of push pull type;
Fig. 4: schematic circuit diagram of a third embodiment of the present invention, with four legs in two shunt subcircuits.

Fig. 1 illustrates a first embodiment of an inventive electrical circuit 1. The electrical circuit 1 comprises a main voltage supply 2 providing an operating voltage (see check mark A). The voltage supply 2 powers via a series resistor RV two legs 3, 4, each comprising an array of (here) six LEDs and a leg resistor R1, R2 (see check mark B for the voltage supplied to the legs 3, 4). All LEDs are of the same type here, and the two leg resistors R1, R2 have the same resistance RL, such that the current I is divided equally between the two legs 3, 4 into currents J, K. The two currents J, K are led into a common shunt resistor R3 (with resistance RS), which is connected with earth. In the example shown, RS is 1/2*RL.
The voltage at the shunt resistor R3 is amplified with an amplification unit 5, which applies an amplification factor AF (here) slightly below 2, such as about 1.91 (in the amplification unit 5 shown, the amplification factor AF is determined by the resistors R4 and R5, with AF=1 +R5/R4, and thus may be altered by altering the resistors R4, R5).
By means of comparator units 6, 7 (here comprising only a comparator 6a, 7a, also marked with IC1 and IC2, with an open collector output each), the voltages ("leg voltages") at the leg resistors R1, R2 (see check marks C, D) are compared with the amplified voltage (see check mark E) of the amplification unit 5. Since the amplification factor AF is slightly below 2, the voltage at check marks C, D (i.e. at the non-inverting inputs of the comparators 6a, 7a) is slightly larger than at check mark E (i.e. at the inverting inputs of the comparators 6a, 7a) during normal operation. This deviation (or defined comparator offset voltage) represents a switching threshold for the failure detection in the electrical circuit 1.
During normal operation, i.e. when all LEDs in the legs 3, 4 operate normally, the outputs 6b, 7b of the comparator units 6, 7 are at a non-earth potential. In this case, an LED input 8 (see check mark F) of an optocoupler OK1 is powered by a voltage supply V1 via a series resistor R6. The optocoupler OK1 has a bright LED then, and the transistor of the optocoupler OK1 is conductive. As a result, the voltage of a voltage supply V2 supplied via a resistor R7 at a signal output 9 is grounded via the transistor input 11, and the signal output 9 (see check mark G) of the signal output unit 10 is at a "low" signal.
If an LED e.g. in the left leg 3 fails and becomes highly conductive ("short circuit"), the current J in leg 3 becomes larger as compared to current K in leg 4 ("shift of the relative currents"). More specifically, the voltage at leg resistor R2 (at check mark D) will drop below the amplified voltage (at check mark E), which has slightly risen. As a result, comparator 7a switches and sets the output 7b to earth potential. This means that the voltage supplied by voltage supply V1 is grounded via the comparator unit 7, and the LED input 8 (at check mark F) is at earth potential. The LED of the optocoupler OK1 becomes dark, and the transistor of the optocoupler OK1 becomes resistive. The signal output 9 (at check mark G) switches to a "high" signal, since it is supplied with a voltage by the voltage supply V2 via resistor R7.
A similar situation evolves when an LED in leg 3 becomes highly resistive (such as 1 MOhm). In this case, the current J in leg 3 drops sharply (to practically zero). Accordingly, the voltage at leg resistor R1 also drops sharply (at check mark C), and comparator 6a switches and sets its output 6b to earth potential. Then again the LED of optocoupler OK1 becomes dark, and the signal output 9 switches to a "low" signal.
It should be noted that any one of the comparator units 6, 7 may switch the failure indication signal at signal output 9 to "low" by connecting its output 6b, 7b to earth, thus realizing inherently an "or" logic.
Figs. 2a , 2b and 2c illustrate simulated measurement protocols of currents I, J, K and voltages at check marks A, B, C, D, E, F G of the electrical circuit 1 of Fig. 1 for the three situations described above, i.e. normal operation, a short circuited LED in leg 3, and a highly resistive LED (1 MOhm) in leg 3. The simulations were done with Cadence analog workbench software. Resistances were chosen with RV=50Ohm, R1, R2=20Ohm, R3=10Ohm, R4=9.1kOhm, R5, R6, R7=10kOhm, V2 was constantly at 5V DC, and the voltage at V1 was chosen identical to check point B. For the simulated measurement, the operating voltage at voltage supply 2 was ramped over in total 400 ms (see right hand axis). The scale/division-factor of the different channels A - G (along the upward axis) in the figures are A: 40V, B: 30V, I: 200mA, J: 200mA, K: 200mA, C: 5V; D: 5V, E: 5V, F: 5V, G: 20V. For better orientation, the current and voltage values at the 250ms-point are also included in the figures.
During normal operation ( Fig. 2a ), as soon as the supply voltage at A is high enough to make the LEDs conductive, the failure indication signal at G becomes low (see step at about 90 ms, with values immediately after the step of A=9.5V, B=9.2V, I=4mA, J=2mA, K=2mA, C=82.4mV, D=82.4mV, E=80.8mV, F=1.04V, and G=0.24V). Both leg voltages at C and D are above the amplified voltage at E, and the voltage at F is not grounded and therefore is high (at about 1 V) thanks to the voltage supply V1. At the 250ms-point, C=2.00V, D=2.00V, and E=1.91V.
In case of an LED short circuit in leg 3 ( Fig. 2b ), the voltage at check point F stays basically at earth potential, and the failure indication signal at G stays high. This is because the voltage at D (1.67V at the 250ms-point) has dropped below the voltage at E (2.15V at the 250ms-point), and the corresponding comparator unit 7 grounds the voltage at F.
The voltage at check point F stays basically at earth potential, and the failure indication signal at G stays high also in case of an LED becoming highly resistive in leg 3 ( Fig. 2c ). In this case, however, the voltage at C (0.84V at the 250ms-point) drops below the voltage at E (1.60V at the 250ms-point), and the corresponding comparator unit 6 grounds the voltage at F.
Fig. 3 illustrates a second embodiment of an inventive electrical circuit 1, similar to the one shown in Fig. 1. Therefore, only the differences are explained.
In the embodiment of Fig. 3, the comparator unit 6 (and analogue comparator unit 7, not explained in detail) comprises a comparator 6a of push-pull output type with a supply voltage +VC/-VC of +15V/-15V. In order to provide an earth potential at the comparator unit output 6b in the switched state, the comparator output is connected to the base of a comparator unit transistor 13 of npn type, the emitter of which is connected to earth. However, the low voltage of about -14V supplied by the comparator 6a in the switched state would damage the transistor 13 for reasons of a too large negative base-emitter voltage (which is typically about -5V), therefore the connection includes a diode 14 and a current limiting resistor R7, limiting the base-emitter voltage (here) to about -0.6V.
When including too many legs or too many LEDs per leg in the electrical circuit 1 of Fig. 1, then the shift of currents might become too low to be safely detectable, in particular in case of short-circuited LEDs. Fig. 4 illustrates a third embodiment of an inventive electrical circuit 1 (only the differences of which as compared to Fig. 1 are explained) which grants the possibility to monitor an in principle unlimited number of LEDs.
This is done by means of shunt subcircuits 15, 16, each connected to the same signal output unit 10. Within each shunt subcircuit 15, 16, the failure detection is realized as described in Fig. 1. In particular, currents within each of (here) two legs are compared to a current across a shunt resistor after an amplification, and comparator units 6, 7, 17, 18 switch their outputs 6b, 7b, 17b, 18b to earth potential in case of a detected failure. Note that in the example shown, the comparator units 6, 7, 17, 18 have a comparator 6a, 7a, 17a, 18a of push pull output type, and are connected to a comparator unit transistor 13 each, here of N-MOSFET type. All comparator unit outputs 6b, 7b, 17b, 18b are connected to the LED input of the optocoupler OK1, using the inherent "or" logic of the grounding upon LED failure, since one comparator unit outputs 6b, 7b, 17b, 18b switched to earth potential is enough to ground the LED input 8.
It should be noted that a basically unlimited number of further shunt subcircuits may be added, i.e. connected to the LED input 8 at connection 19, thus allowing the monitoring of an unlimited number of LEDs by means of the invention.

Claims

An electrical circuit (1) for powering a plurality of LEDs, with a function for indicating an LED failure,
wherein the electrical circuit (1) comprises N legs (3, 4) connected in parallel, with N≥2, and with each leg (3, 4) comprising several LEDs and a leg resistor (R1, R2) connected in series with the LEDs,
wherein the leg resistors (R1, R2) of the legs (3, 4) have basically the same resistance, and wherein the resistance of each leg (3, 4) is basically the same during normal operation,
characterized in
that the legs (3, 4) are connected to a common shunt resistor (R3),
that the electrical circuit (1) comprises an amplifier unit (5) amplifying a voltage at the shunt resistor (R3) with an amplification factor AF into an amplified voltage,
that the electrical circuit (1) comprises a comparator unit (6, 7, 17, 18) for each leg (3, 4), wherein the comparator units (6, 7, 17, 18) each comprise a comparator (6a, 7a, 17a, 18a) which compares the voltage at the respective leg resistor (R1, R2) with the amplified voltage,
that the amplification factor AF is chosen such that during normal operation, the voltage at the leg resistors (R1, R2) and the amplified voltage deviate slightly,
and that the outputs (6b, 7b, 17b, 18b) of the comparator units (6, 7, 17, 18) are connected at a common signal output unit (10).
An electrical circuit (1) according to claim 1, characterized in that the amplification factor AF is chosen such that during normal operation, the voltage at the leg resistors (R1, R2) and the amplified voltage deviate by about 10% or less.
An electrical circuit (1) according to claim 1 or 2, characterized in that the comparator units (6, 7, 17, 18) are designed such that
- during normal operation, the outputs (6b, 7b, 17b, 18b) of the comparator units (6, 7, 17, 18) are all connected to a non-earth potential, and

- the output (6b, 7b, 17b, 18b) of a comparator unit (6, 7, 17, 18), with its comparator (6a, 7a, 17a, 18a) in a switched condition as compared to its condition during normal operation, is connected to earth.
An electrical circuit (1) according to claim 3, characterized in that the comparators (6a, 7a, 17a, 18a) are of open collector output type.
An electrical circuit according to claim 3, characterized in that the comparators (6a, 7a, 17a, 18a) are of push pull output type, and the outputs (6b, 7b, 17b, 18b) of the comparators (6a, 7a, 17a, 18a) are connected to the basis or gate of a comparator unit transistor (13).
An electrical circuit (1) according to one of the preceding claims, characterized in that the common signal output unit (10) comprises an optocoupler (OK1).
An electrical circuit (1) according to claim 6, characterized in that an LED input (8) of the optocoupler (OK1) is connected to a voltage supply (V1) and to the outputs (6b, 7b, 17b, 18b) of the comparator units (6, 7, 17, 18),
and that a signal output (9) of the signal output unit (10) is connected both to a voltage supply (V2) and to a transistor input (11) of the optocoupler (OK1).
An electrical circuit (1) according to one of the preceding claims, characterized in that each leg (3, 4) comprises the same number of LEDs, with the same type of LEDs in all legs (3, 4), and with the LEDs of each leg (3, 4) arranged in series.
An electrical circuit (1) according to any one of the preceding claims, characterized in that the number N of legs (3, 4) ranges from 2 to 6, and the number of LEDs in a leg (3, 4) ranges from 4 to 24.
An electrical circuit (1) according to one of the preceding claims, characterized in that the resistance RS of the shunt resistor (R3) is 1/N times the resistance RL of a leg resistor (R1, R2), and that the amplification factor AF is slightly below or slightly above 2.
An electrical circuit (1) according to one of the preceding claims, characterized in that there is an additional comparator unit per leg (3, 4), an additional amplification unit for the shunt resistor (R3), and an additional common signal output unit (10) for the additional comparator units, thus establishing the a two channel LED failure detection.
An electrical circuit (1) according to one of the preceding claims,
characterized in
that the electrical circuit (1) comprises at least two shunt subcircuits (15, 16), wherein each shunt subcircuit (15, 16) comprises
- M legs (3, 4) connected in parallel, with M≥2, and with each leg (3, 4) comprising several LEDs and a leg resistor (R1, R2) connected in series with the LEDs,
wherein the leg resistors (R1, R2) of the legs (3, 4) have basically the same resistance, and wherein the resistance of each leg (3, 4) is basically the same during normal operation,
wherein the legs (3, 4) are connected to a common shunt resistor (R3),
- an amplifier unit (5) amplifying a voltage at the shunt resistor (R3) with an amplification factor AF into an amplified voltage,

- a comparator unit (6, 7, 17, 18) for each leg (3, 4), wherein the comparator units (6, 7, 17, 18) each comprise a comparator (6a, 7a, 17a, 18a) which compares the voltage at the respective leg resistor (R1, R2) with the amplified voltage,
and that the outputs (6b, 7b, 17b, 18b) of the comparator units (6, 7, 17, 18) of the at least two shunt subcircuits (15, 16) are connected at the common signal output unit (10).