EP2445318B1

EP2445318B1 - LED signal lamp with constant current operation

Info

Publication number: EP2445318B1
Application number: EP10187788.4A
Authority: EP
Inventors: Kassen Oldewurtel
Original assignee: Thales Deutschland GmbH
Current assignee: GTS Deutschland GmbH
Priority date: 2010-10-15
Filing date: 2010-10-15
Publication date: 2013-05-29
Anticipated expiration: 2030-10-15
Also published as: DK2445318T3; RS52884B; SI2445318T1; EP2445318A1; ES2426033T3; ES2426033T8

Description

The invention relates to an electrical circuit for an LED signal lamp, comprising means for switching between a low level brightness, in particular a night time operation level brightness, and a high level brightness, in particular a daylight operation level brightness, of the LED signal lamp in dependence of an AC voltage at a voltage input of the electrical circuit.
Such an electrical circuit is known from EP 1 787 886 B1 .
Signal lamps are used in a variety of fields, in particular in traffic control applications. Of particular importance are railway signal lamps. The signal lamps are used to indicate a train operator whether an upcoming part of a railway track (such as a track switch) can be entered safely.
During daytime, the light of a railway signal lamp must be bright enough for the train operator to recognize the status of the signal lamp well before arriving at the signal lamp. During night time, however, the luminous intensity of the signal lamp must be low enough so the train operator is not dazzled. This means that the luminous intensity of a railway signal lamp should be adapted in the course of a day.
Railway signal lamps of the state of the art typically use conventional light bulbs as their illuminant. In order to adapt the luminous intensity of these light bulbs, the railway control center modifies the AC input voltage of the railway signal lamps. During night time, the light bulbs are operated at a reduced voltage of about 66%, as compared to the voltage used during daytime. In the evening and in the morning, the input voltage is switched accordingly in order to have this day/night adaptation.
Recently, railway signal lamps have been equipped with power light emitting diodes (=LEDs) as their illuminant. Power LEDs have proven to me more reliable and cost-effective than conventional light bulbs.
However, the characteristic of an LED is very different from the characteristic of a light bulb, in particular as far as the correlation between input voltage and luminous intensity is concerned. When simply replacing conventional light bulbs with LEDs in a railway signal lamp, the existing equipment at railway control centers for adapting the luminous intensity by changing the input voltage is no more suitable.
EP 1 787 886 B1 discloses an electrical circuit for an LED signal lamp by means of which the luminous intensity of the LED signal lamp is switched between a daylight level and a night time level in dependence of the operating voltage. In the known electrical circuit, LEDs are supplied with power via a high resistance path for night time operation, and via a low resistance path for daylight operation. The electrical circuit behaves similar to a light bulb, and the existing equipment at a railway control center can be used for switching between daylight and night time operation mode just by altering the AC input voltage.
The known electrical circuit is significantly susceptible to tolerances of included components, in particular LEDS, such as tolerances regarding different lots and different manufactures. Therefore, mass production is difficult. Further, the electrical circuit is similarly susceptible to fluctuations of the ambient temperature, which may alter main LED parameters. Such temperature fluctuations might be caused by weather changes in many geographical regions. The brightness of the LED signal lamp is difficult to control, in particular when the input voltage varies, and the peak forward voltage at the LEDs limits the LED brightness.

Object of the invention

It is therefore the object of the invention to provide an electrical circuit for an LED signal lamp, suitable for control with a conventional railway control center, which is less susceptible to tolerances and temperature fluctuations and allows a better control over the LED brightness.

Short description of the invention

This object is achieved, in accordance with the invention, by an electrical circuit as mentioned in the beginning,
characterized in that the electrical circuit comprises

a full wave rectifier for rectifying the AC voltage supplied at the voltage input of the electrical circuit, thus providing a rectified AC voltage at two poles, and
at least one LED block circuit connected to the poles,
and that the LED block circuit comprises
an LED arrangement comprising at least one light emitting diode,
a comparator circuit switching a signal voltage at a signal output of the comparator circuit if the rectified AC voltage crosses a critical level,
a switchable constant current source, with its switching input connected to the signal output of the comparator circuit, wherein the switchable constant current source is connected in series with the LED arrangement,
and a switchable resistance, with its switching input connected to the signal output of the comparator circuit,
wherein the comparator circuit, the switchable resistance and the switchable constant current source together with the LED arrangement are connected in parallel at the poles.

According to the invention, the LED arrangement of the at least one LED block is operated via its switchable constant current source. By this means, the forward current at the LED arrangement is fixed and does no more depend on the forward voltage of the LED arrangement, which varies with the ambient temperature and/or the manufacturer and/or the lot and/or the colour of the LED(s). The brightness is proportional to the forward current of the switchable constant current source and thus mostly constant, even at varying temperature, manufacturer or lot.
The change between daylight operation mode and night time operation mode is done by setting the switchable constant current source to appropriate current values, respectively. So within each mode, a respective mostly constant brightness level may be maintained.
As a result, the control over the LED brightness is greatly increased, with tolerances and temperature fluctuations no more deteriorating the operation. Mass production and use in numerous geographical regions (with extreme weather conditions, in particular extreme temperature variations) is feasible.
As compared to a resistor based LED voltage supply of the state of the art, the "on time" of the LED arrangement within one mains cycle (which is about 20 ms assuming a 50 Hz AC voltage) is about 20% longer. Hence the brightness is increased about 20%. Further, the full wave rectification increases the "on time" of the LED arrangement by another 100% as compared to the state of the art using a single wave rectifier. So the invention also allows for an increase of the brightness of the LED arrangement and improved efficiency.
By means of the switchable resistance, the behaviour of a conventional light bulb is simulated with high precision in each operation mode. A railway control center (or other control center) typically checks whether a connected signal behaves adequately when supplying it with a particular voltage. This check typically includes measuring the current consumption. A too low current consumption indicates a torn glow filament of a light bulb, and a too high current indicates a short circuit; in both cases the signal probably does not shine as desired, and the railway control center generates an alarm message. The current consumption of the constant current source and the LED arrangement is, taking into account both operation modes, different from what is expected from a light bulb. By providing and switching a parallel resistance (also referred to as "preload resistance"), the AC voltage at the voltage input results in the expected current consumption behaviour corresponding to the simulated light bulb; thus no unnecessary alarm messages are generated just because an LED signal lamp is used instead of a light bulb. Note that the switchable resistance wastes some energy, but this is by far outweighed by the reduced equipment expenditures at the railway control center. The increased reliability and lifetime of LEDs as compared to light bulbs is still fully used with the invention.

Preferred embodiments of the invention

A preferred embodiment of the electrical circuit provides that the electrical circuit comprises several LED block circuits connected in parallel at the two poles. By this means, the overall brightness may be increased in a simple way. The LED blocks may be exchanged as a whole in case a defect is found.
In an advantageous further development of this embodiment, only part of the LED block circuits, in particular only one of the LED block circuits, comprises a switchable resistance. In this way, some of the LED blocks (preferably all but one) can be simplified. Basically only one switchable resistance is necessary to simulate the behaviour of a light bulb. The resistance values of the switchable resistance in the two operation modes should be adapted to the current consumption of the LEDs simulating the light bulb (in particular to the number of LEDs or LED blocks).
In a preferred embodiment of the inventive electrical circuit, the comparator circuit comprises a reference element, with its reference input connected to the poles via a potential divider, with one of its control contacts connected to the signal output of the comparator circuit, and with one of its control contacts connected to one of the poles. By means of the potential divider, the switching threshold at the reference element (and thus the critical level of the rectified AC voltage) can be adapted. Depending on the voltage at the poles, the control contacts are short-circuited or not, thus setting the signal output of the comparator circuit to the potential of the connected pole or not. The reference element may be of type LM431, for example. A reference element suitable for the invention is also known form EP 1 787 886 B1 , in particular Fig. 2 therein.
Further advantageous is an embodiment, wherein the signal output of the comparator circuit is connected to one of the poles via a capacity and a resistance, with said capacity and said resistance connected in parallel. By means of the capacity, variations over a cycle of the rectified AC voltage are evened out, such that unnecessary switching is avoided. By means of the resistor, the signal output of the comparator circuit is discharged such that long term changes of the rectified AC voltage have an impact on the signal at the signal output of the comparator circuit.
In a preferred embodiment, the switchable constant current source comprises a constant current source connected to one of the poles via a resistance and a switch, with said resistance and switch connected in parallel. Depending on the condition of the switch, the constant current source is supplied with power of the poles via the resistance or directly through the switch. Thus the forward current of the constant current source can be switched easily.
In a preferred further development of this embodiment, the switch comprises a transistor, with its base or gate acting as the switching input of the switchable constant current source. This is a simple realization of the switch by electronic means.
Another preferred further development of the above embodiment provides that the constant current source comprises a transistor, with its base or gate connected to one of the poles via a voltage limiting component, in particular a diode such as a Zener diode. This is a simple way to realize the constant current source. The limited voltage of the voltage limiting element such as a diode is fairly constant (even taking into account tolerances due to different manufactures or lots and temperature fluctuations), guaranteeing a constant current at the connected LED arrangement.
An advantageous embodiment of the inventive electrical circuit is characterized in that the switchable resistance comprises

a series connection of a first preload resistance and a preload switch,
and a second preload resistance,
wherein said second preload resistance is connected in parallel to said series connection of the first preload resistance and the preload switch. When the preload switch is open, only the second preload resistance is effective; this typically simulates the daylight operation with high LED current (and low resistor current). When the preload switch is closed, the first and second preload resistance are both effective in parallel; this typically simulates the night time operation with low LED current (and higher resistor current). This design is simple to realize; note that other designs resulting in an alteration of a (totally effective) resistance by closing or opening a switch may be used, such as a series connection of a first and second preload resistance, with a preload switch in parallel to one of the preload switches.

In an advantageous further development of the above embodiment, the preload switch comprises a transistor, in particular with its base or gate acting as the switching input of the switching resistance. With a transistor, the switching function may be realized electronically by simple means.
Further preferred is an embodiment wherein the electronic circuit comprises a diode connected in series with a pole. The diode ensures the voltage polarity beyond the full wave rectifier, in particular blocking unintended voltage fluctuations/interferences.
In another preferred embodiment, the AC voltage has a peak voltage of about 12 V, and the critical level is between 9.5 V and 11.5 V, in particular about 11 V. Said AC voltage level of 12 V is valid for the daylight operation mode; during night time operation mode, the AC voltage has a peak voltage of about 8 V. These values are suitable for most railway signals and traffic lights.
A highly preferred embodiment provides that the LED arrangement comprises several light emitting diodes, in particular two light emitting diodes connected in series. A plurality of light emitting diodes of an LED arrangement, typically connected in series, may generate a higher brightness, as compared to a single LED, and beneficially all LEDs of the LED arrangement may be controlled in common.
Finally, in another preferred embodiment, the electrical circuit is designed such that the current through the LED arrangement changes by a factor of between 10 and 100, preferably between 20 and 60, most preferably about 50, when switching between low level brightness and high level brightness. These current changes lead to brightness adaptations found useful in practice. Typically, in night time mode, due to low current required by the LED arrangement, considerable current is directed through the switchable resistance to simulate the behaviour of a filament light bulb. In contrast, during daylight mode, typically basically all current is consumed by the LED arrangement, and none or only few current flows through the switchable resistance.
Also within the scope of the present invention is the use of an inventive electrical circuit in a railway signal or in a traffic light signal. The inventive electrical circuit may act as an LED signal lamp, replacing a conventional light bulb for generating light signals in a daylight and a night time operation mode, wherein the operation mode is determined by the AC voltage level fed into the signal, such as a 66% voltage level at night time operation mode as compared to daylight operation mode.
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 block diagram of an inventive electrical circuit;
Fig. 2: shows a schematic block diagram of an LED block circuit of an inventive electrical circuit;
Fig. 3: shows a circuit diagram of an embodiment of an LED block circuit of an inventive electrical circuit.

Fig. 1 shows in a schematic block diagram an inventive electrical circuit 1 for an LED signal lamp.
The electrical circuit 1 comprises a voltage input 2, with two input contacts, for inputting an AC voltage. This AC voltage is provided by a railway control center (or other signal control center) which typically switches the AC voltage between:

About 12 V (peak voltage) for a daylight operation mode;
About 8 V (peak voltage) for a night time operation mode; and
Zero (for switching off the signal).

Typically, the AC voltage has a frequency of 50-60 Hz and originates from transforming down a 230-240 V mains voltage.
The AC voltage provided at the voltage input 2 is fed into a full wave rectifier 3, which rectifies the AC voltage into immediately subsequent sine-shaped voltage half-waves of identical (positive) polarity. The rectified voltage is supplied to two poles 4a, 4b. Note that the lower pole 4b is typically at mass potential.
At the poles 4a, 4b, in the example shown, one LED block circuit 5 is connected. Note that alternatively, a plurality of LED block circuits 5 may be connected in parallel at the poles 4a, 4b.
The LED block circuit 5 comprises an LED arrangement 6 powered by a switchable constant current source 7; for the details of the LED block circuit 5 compare Figs. 2 and 3.
In Fig. 2 , the basic principle of an LED block circuit 5 is illustrated. The rectified AC voltage provided at pole 4a (with pole 4b at mass potential), after having passed diode 8, is fed into a comparator circuit (or voltage comparator) 9. The comparator circuit 9 analyses whether the rectified AC voltage is below or above a critical level Vcrit of voltage, here with Vcrit=10,5 V. If the rectified AC voltage falls below or rises above Vcrit, a connected preload switch S2 and the connected switchable constant current source 7 are switched.
The switchable constant current source 7, powered via the poles 4a, 4b, is connected in series with an LED arrangement 6, which comprises two LEDs connected in series here. Depending on the switching state of the switchable constant current source 7, the LED arrangement 6 receives a high constant current (in daylight operation mode, when the rectified AC voltage is above Vcrit) or a low constant current (in night time operation mode, when the rectified AC voltage is below Vcrit).
The comparator circuit 9 also switches the preload switch S2. In night time operation mode, the preload switch 10 is closed, such that preload resistors 11, 12 are both effective, and a relatively high current passes through the two preload resistors 11, 12 in summary. Note that in night time operation mode, the current consumption of the LED arrangement 6 is relatively low. In daylight operation mode, the preload switch S2 is open, such that the first preload resistor 11 is disconnected and only the second preload resistor 12 is effective; then only a relatively low current passes through the preload resistor 12. Note that in daylight operation mode, the current consumption of the LED arrangement 6 (together with the constant current source 7) is relatively high. By means of the current through the preload resistors 11, 12, the current consumption of a conventional light bulb is simulated. The entirety of preload switch S2 and the preload resistors 11, 12 is also called a switchable resistance.
The switchable constant current source 7 provides precise current conditions over a wide range of temperatures and component tolerances, in particular LED tolerances. The luminous intensity is highly predictable in both operation modes, and mass production of the electrical circuit, and more specifically of an LED block circuit of such an electronic circuit, is feasible.
It should be noted that the LED block circuit 5 may comprise further switchable constant current sources and connected LED arrangements, connected in parallel to the switchable constant current source 7 and the LED arrangement 6 shown in Fig. 2, in order to increase the number of powered LEDs or the brightness of the LED signal lamp as a whole, respectively.
Fig. 3 illustrates in more detail an embodiment of an LED block circuit 5 by way of example in a detailed circuit diagram.
The LED block circuit 5 is connected between two poles 4a, 4b providing a rectified AC voltage. The LED block circuit 5 comprises basically

a comparator circuit 9,
a switchable constant current source 7 (connected in series with an LED arrangement 6), and
a switchable resistance 13.

These three subcircuits 9, 7, 13 are connected in parallel to the poles 4a, 4b, but in series with a diode 8. Pole 4b is at mass potential in the example shown.
The comparator circuit 9 comprises a reference element 14 with a reference point of 2.5 V, with its reference input RI connected to the poles 4a, 4b via a voltage divider. Said voltage divider comprises two resistors R1, R2, the ratio of which is chosen such that a desired critical voltage level Vcrit between the poles 4a, 4b results in a voltage of 2.5 V at the reference input RI. For example, if Vcrit is 10.5 V, then R2 is about 0.31*R1 (such as R1=100 kOhms and R2=31 kOhms; note that relatively high resistances such as above 10 kOhms are preferred at the voltage divider in order not to waste power here). Connected in parallel to resistor R2 is a capacitor C1 for filtering out interferences.
The reference element 14 comprises two control contacts CC1, CC2. CC1 is connected to the lower pole 4b directly, whereas CC2 is connected to the upper pole 4a via a resistor R3 here. If the voltage at reference input RI is above 2.5 V (i.e. if the rectified voltage is above Vcrit), then CC2 is short-circuited with CC1 (i.e. with mass potential); else CC1 and CC2 are electrically isolated with respect to each other.
Accordingly, at a signal output SO of the comparator circuit 9, wherein the signal output SO is connected to CC2 via a resistor R4 and a diode 15, the signal is "high" as long as the voltage at reference input RI is below 2.5 V (night time operation mode), and the signal is "low" as long as the voltage at RI is above 2.5 V (daylight operation mode). Note that the signal at signal output SO is smoothed over the sine half waves by means of a capacitor C2, with a resistor R5 in parallel for discharging purposes.
The signal output SO is read out by a signal input SI1 the switchable constant current source 7. The switchable constant current source 7 comprises a constant current source 7a and a switching subcircuit 7b. The constant current source 7a comprises a transistor T1 (here a bipolar transistor) with its emitter connected to the switching subcircuit 7b via a resistance R6, its collector connected to an LED arrangement 6, and its base connected to the upper pole 4a via a voltage limiting component, here a Zener diode 16, so a fixed maximum voltage (namely the limited voltage of the voltage limiting component) is present at the base (or across resistor R7, respectively). The current through the constant current source 7a ― and thus through the LED arrangement 6 - is determined by said fixed maximum voltage and is thus highly constant, resulting in a high control over the LED brightness in the LED arrangement.
By means of the switching subcircuit 7b, the supplied voltage at resistor R6 of the constant current source 7a can be altered, what in turn alters the current through the LED arrangement 6. When the signal at signal output SO is "low", then switch S1 (here realized as a field effect transistor T2) is closed (i.e. the transistor T2 is conductive), such that at R6 there is the potential of pole 4a. When the signal at signal output SO is "high", then switch S1 is open (i.e. the transistor T2 is non-conductive), such that at R6 there is a potential reduced by the effect of resistor R8 as compared to the pole 4a.
The signal output SO of the comparator circuit 9 is further read out by the signal input SI2 of the switchable resistance 13. The switchable resistance 13 comprises a preload switch S2 here consisting of two transistors T3 and T4. Transistor T3 (here a field effect transistor) acts identically to the transistor T2, such that a "low" signal at signal output SO results in the potential of pole 4a (i.e. a positive potential) present at resistor R9, and a "high" signal at signal output SO results in the potential of lower pole 4b (i.e. mass) present at resistor R9. In turn, mass potential at the base of transistor T4 (here a bipolar transistor) makes the transistor T4 conductive, and through first preload resistor 11 some current is lead from pole 4a to pole 4b, in addition to some current flowing through second preload resistor 12. Positive potential at the base of transistor T4, in contrast, makes the transistor T4 non-conductive, and no current may flow through the first preload resistor 11. Then only some current may flow through the second preload resistor 12 from pole 4a to pole 4b.
Therefore, if the rectified voltage is above Vcrit (indicating daylight operation mode), then the signal at SO is "low", and the preload resistor 12 is effective, resulting in a high total resistance and a relatively high current flow from pole 4a to pole 4b, due to high current in the LEDs. In contrast, if the rectified voltage is below Vcrit (indicating night time operation mode), then the signal at SO is "high", and both preload resistors 12 and 11 are effective, resulting in a low total resistance and a relatively low current flow, due to low current in LEDs.
Thus the behaviour of a light bulb when its input voltage is changed is simulated. Note that by adequately choosing the ratio of preload resistors 11 and 12 the current contribution of the switchable constant current source 7 and, if necessary, of the comparator circuit 9 may be taken into account, too.

Claims

An electrical circuit (1) for an LED signal lamp,
comprising means for switching between a low level brightness, in particular a night time operation level brightness, and a high level brightness, in particular a daylight operation level brightness, of the LED signal lamp in dependence of an AC voltage at a voltage input (2) of the electrical circuit (1),
- a full wave rectifier (3) for rectifying the AC voltage supplied at the voltage input (2) of the electrical circuit (1), thus providing a rectified AC voltage at two poles (4a, 4b), and

- at least one LED block circuit (5) connected to the poles (4a, 4b),
and that the LED block circuit (5) comprises

- an LED arrangement (6) comprising at least one light emitting diode,

- a comparator circuit (9) switching a signal voltage at a signal output (SO) of the comparator circuit (9) if the rectified AC voltage crosses a critical level (Vcrit),
characterized in
that the electrical circuit (1) comprises

- a switchable constant current source (7), with its switching input (SI1) connected to the signal output (SO) of the comparator circuit (9), wherein the switchable constant current source (7) is connected in series with the LED arrangement (6),

- and a switchable resistance (13), with its switching input (SI2) connected to the signal output (SO) of the comparator circuit (9), wherein the comparator circuit (9), the switchable resistance (13) and the switchable constant current source (7) together with the LED arrangement (6) are connected in parallel at the poles (4a, 4b).
An electrical circuit (1) according to claim 1, characterized in that the electrical circuit (1) comprises several LED block circuits (5) connected in parallel at the two poles (4a, 4b).
An electrical circuit (1) according to claim 2, wherein only part of the LED block circuits (5), in particular only one of the LED block circuits (5), comprises a switchable resistance (13).
Electrical circuit (1) according to claim 1, characterized in that the comparator circuit (9) comprises a reference element (14), with its reference input (RI) connected to the poles (4a, 4b) via a potential divider, with one of its control contacts (CC2) connected to the signal output (SO) of the comparator circuit (9), and with one of its control contacts (CC1) connected to one of the poles (4b).
Electrical circuit (1) according to claim 1, characterized in that the signal output (SO) of the comparator circuit (9) is connected to one of the poles (4a) via a capacity (C2) and a resistance (R5), with said capacity (C2) and said resistance (R5) connected in parallel.
Electrical circuit (1) according to claim 1, characterized in that the switchable constant current source (7) comprises a constant current source (7a) connected to one of the poles (4a) via a resistance (R8) and a switch (S1), with said resistance (R8) and switch (S1) connected in parallel.
Electrical circuit (1) according to claim 6, characterized in that the switch (S1) comprises a transistor (T2), with its base or gate acting as the switching input (SI1) of the switchable constant current source (7).
Electrical circuit (1) according to claim 6, characterized in that the constant current source (7a) comprises a transistor (T1), with its base or gate connected to one of the poles (4a) via a voltage limiting component, in particular a diode such as a Zener diode (16).
Electrical circuit (1) according to claim 1, characterized in that the switchable resistance (13) comprises
- a series connection of a first preload resistance (11) and a preload switch (S2),

- and a second preload resistance (12),
wherein said second preload resistance (12) is connected in parallel to said series connection of the first preload resistance (11) and the preload switch (S2).
Electrical circuit according to claim 9, characterized in that the preload switch (S2) comprises a transistor (T3, T4), in particular with its base or gate acting as the switching input (SI2) of the switching resistance (13).
Electrical circuit (1) according to claim 1, characterized in that the electronic circuit (1) comprises a diode (8) connected in series with a pole (4a).
Electrical circuit (1) according to claim 1, characterized in that the AC voltage has a peak voltage of about 12 V, and that the critical level (Vcrit) is between 9.5 V and 11.5 V, in particular about 11 V.
Electrical circuit (1) according to claim 1, characterized in that the LED arrangement comprises several light emitting diodes, in particular two light emitting diodes connected in series.
Electrical circuit (1) according to claim 1, characterized in that the electrical circuit (1) is designed such that the current through the LED arrangement (6) changes by a factor of between 10 and 100, preferably between 20 and 60, most preferably about 50, when switching between low level brightness and high level brightness.
Use of an electrical circuit according to claim 1 in a railway signal or in a traffic light signal.