HK1073964B - Circuit assembly for operating a luminous signal - Google Patents

Description

Circuit arrangement for driving a light emission signal

Technical Field

The invention relates to a circuit arrangement for driving a luminous signal, in particular an LED signal.

Background

Light emitting signals based on light emitting diodes instead of incandescent lamps are increasingly being used in many fields, especially in the field of signal technology. Relatively speaking, light emitting diodes are relatively inexpensive, durable and have high luminance. US-a 5939839 discloses a circuit for protecting light emitting diodes of optical signals from over-voltage damage. EP-a 0293921 relates to a protection circuit against overheating.

But LEDs are difficult to use in situations where the incandescent lamp should be replaced with an LED lighting signal without changing the control. This is particularly true for railway optical signaling circuits, in which the function that is in accordance with the regulations is generally monitored by means of signaling-technically reliable current measurements. In order to be able to continue to use the monitoring without modification, the current-voltage characteristic curve of the LED luminous signal must approximately correspond to the current-voltage characteristic curve of the incandescent lamp.

Another particular is to send signals outside of tunnels or environmental conditions with approximately constant lighting conditions. Here, in terms of circuitry, the optical power of the night drive is reduced compared to the day drive. The light sensitivity of the human eye in the day and in the night differs by a factor of about 1000. Therefore, if the light power is not reduced at night, the light that is hardly visible in the daytime is also very dazzling. However, especially in urban and railway traffic, dazzling light should be absolutely avoided, since there may be a risk that other signals are almost ignored due to excessive radiation. For light signals based on incandescent lamps in railway technology, the brightness between day and night is controlled by the supply voltage or supply current of the regulating mechanism. Since the light power of an incandescent lamp is exponentially related to the supply voltage or supply current, a small change in the supply current or supply voltage causes a large change in the light power. This means that to reduce the optical power to 20% of the output light intensity, the supply current or supply voltage has to be reduced to about 2/3% of the output value. In order to obtain a similarly advantageous characteristic curve for the light-emitting diodes, DE 19846753a1 proposes connecting a control circuit in parallel for each light-emitting diode. The disadvantage is that the obtained optical power difference between day and night is relatively small. Furthermore, component tolerances that lead to differences in the turn-on voltage (flusspannung) of the light-emitting diodes, transistors and other components, and the temperature influence of this turn-on voltage, cannot be compensated.

Disclosure of Invention

The object of the present invention is to overcome the above-mentioned disadvantages and to provide a circuit arrangement which improves the dynamic behavior of the optical power between day and night.

The object is achieved by a circuit arrangement for operating a luminous signal, in particular an LED signal. The light power of the light-emitting diode is controlled over a wide range by specifying the parallel current that does not flow through the light-emitting diode. In order to be able to achieve a constant led current even when the drive voltages in the day and night drive differ, a second controlled current source is provided according to the invention for predetermining the led current.

But it is also possible to realize a constant current for the light emitting diode without a second controlled current source. For this reason, the parallel current is not kept constant but controlled according to the drive voltage.

For the parallel current to be controlled and, if appropriate, the led current to be controlled, a setpoint-actual value comparison is carried out by means of a comparator, according to an embodiment of the invention. The predetermination of the respective current setpoint value is performed by a predefined switching threshold, which predetermines when the respective current is switched on or off. The switching threshold can be an approximately continuous predetermined parameter, but can also be purely digital information, preferably with hysteresis.

In a preferred embodiment of the invention, the switching threshold is temperature-compensated and/or the switching voltage is compensated. The element characteristics that change with temperature are balanced by voltage compensation. The conduction voltage compensates for the different conduction voltages of the light emitting diodes used. For example, it is conceivable to divide the light-emitting diodes used into groups of conduction voltages, from which the appropriate light-emitting diodes are selected during assembly.

An important characteristic of the entire device according to the invention is that the device can already drive current at small voltages. This function represents a protection of the circuit arrangement against coupled extraneous energy, which may occur over a longer distance when feeding the circuit arrangement. The voltage developed across the led must remain less than the turn-on voltage of the led even when the received extraneous energy is at a maximum. This prevents the light-emitting diode from starting to emit light impermissibly when external energy flows (i.e., when an interference voltage is present).

Drawings

The invention is explained in detail below with the aid of the figures. Wherein the content of the first and second substances,

figure 1 is a schematic circuit diagram of a circuit arrangement for driving a light emission signal,

figure 2 is a first embodiment of a control circuit,

figure 3 is a characteristic variation of the control circuit according to figure 2,

figure 4 is a second embodiment of a control circuit,

fig. 5 is a characteristic curve of the control circuit according to fig. 4.

Detailed Description

Fig. 1 shows the overall structure of an LED signal with n drivers T, each driver T controlling at least one LED D. Each LED D is connected in parallel with at least one LED controller St. The LED controller St is connected directly to the drive voltage U with one pole. And the other pole is connected to the drive voltage U in series with a resistor R. The resistor R is characterized by a defined interruption behavior, i.e. such that a specific fault, for example a complete short circuit, does not actually occur. The resistor R is also designed in such a way that a fault, for example a short circuit, of the LED d or the LED controller St only slightly influences the total current absorbed by the circuit arrangement. For example, in an LED signal with 60 drivers T, the total current rises only by approximately 5% when the LED controller St is short-circuited.

Fig. 2 shows a first embodiment of an exciter T as a detail of the LED signal according to fig. 1. The current I _ P in parallel with the LED D is controlled by the LED controller St. A controlled current source is used for this purpose. The comparator, which is an operational amplifier OPV, is connected at the input to a switching threshold Sch, which represents the setpoint value, and to an actual value setter Ist. At the output, the operational amplifier OPV is applied to a branch which is connected in parallel to the LED D. The switching threshold Sch specifies when the parallel current I _ P is switched on during daytime driving, during nighttime driving and during idling. The switching threshold Sch itself is loaded with a temperature compensation Tk and a conduction voltage compensation Fk. The temperature compensation Tk balances the element characteristics depending on the temperature, while the conduction voltage compensation Fk takes into account the LED-specific conduction voltage. The actual value setter Ist processes the drive voltage U or the drive current of the entire LED signal, wherein the actual value can also be predefined by other information, for example by other control lines of the control mechanism, or by information encoded in the supply current or supply voltage U. The current source for the parallel current I _ P is designed such that it can already drive a current at low voltages. Thus, the coupled external energy, the magnitude of which depends substantially entirely on the length of the supply line, is short-circuited in such a way that a high voltage cannot be built up and the LED D does not start to emit light as a result of the external energy.

Fig. 3 shows the current profile of the parallel current I _ P as a function of the supply voltage and the current I _ D flowing through the LED D, in comparison with a conventional incandescent lamp G. Fig. 3 shows a load resistance line W, which is determined by the resistance R and which is present only in the case of the resistance R without the driver T and the LED D. The load resistance line W describes the maximum possible current flowing through the circuit arrangement of fig. 2 as a function of the drive voltage U. In this circuit embodiment, the voltage drop at night is achieved by controlling the parallel current I _ P. As can be seen from fig. 3, the LED current I _ D is not constant over the voltage range both during the day and at night.

To obtain a constant current, a current embodiment with an additional LED current controller is employed. Fig. 4 shows such a current setting. Wherein the parallel current I _ P is controlled by means of a controlled current source, and furthermore the LED current I _ D is controlled. The associated current profile is shown in fig. 5. The set current and thus the signal brightness is ideally constant for both day and night driving.

When the signal 0 ═ U _ Nacht min is switched off, the signal does not emit light even in the presence of coupled extraneous energy. The LED current I _ D is equal to 0. However, the parallel current I _ P ideally corresponds to the maximum possible current. By this parallel current I _ P, the coupled extraneous energy is short-circuited, and thus a voltage causing the LED D to emit light cannot be formed.

When U _ Nacht min < ═ U _ Nacht max is driven at night, the LED current I _ D passing through the LED D is I _ D _ Nacht driven by the LED controller St. The LED D emits light with little optical power. The parallel current I _ P ideally corresponds to the difference between the current determined by the resistor R and the LED current I _ D.

When the daytime driving is performed with U _ Tag min ═ U _ Tag max, the LED current I _ D passing through the LED D is driven by the LED controller St. The LED D emits light with maximum optical power. The parallel current I _ P in turn corresponds to the difference between the current determined by the resistor R and the LED current I _ D.

In the overlapping region between day and night driving, the hysteresis of the parallel current I _ P and the LED current I _ D acts so that the LED signal is maintained in a stable state.

The circuit arrangement according to fig. 4 shows a very suitable embodiment. The LED signal is reliable in the sense of signaling technology, has a high stability, is almost independent of temperature fluctuations and component fluctuations, and provides a large dynamic between day and night driving. However, this adaptation requires correspondingly high circuit-technology costs. This higher cost can be solved with integrated technology. Tailored variants can also be used which achieve only partial functionality at a relatively low cost.

One embodiment takes into account the fact that the maximum possible light power should be achievable during the day and that the light power is limited to a determined extent during the night. For this reason, the LED current I _ D is kept constant only in the range between U _ Nacht min and U _ Nacht max, while in the range between U _ Tag min and U _ Tag max, the maximum possible LED current I _ D is driven under the condition that I _ P is 0.

Another embodiment relates to the setting of a direct voltage and an alternating voltage. For this purpose, the LED controller St is provided with a corresponding rectifier diode. For operation at ac voltages, it is expedient to configure the LED controller St such that the instantaneous value of the actual value is not used for comparison with the setpoint value, but rather an effective value is preferably used.

Furthermore, the LED controller St can be provided both for dc voltage and for ac voltage, since the LED controller St is also used for ac voltage when driven by dc voltage. But it should be noted that a different switching threshold Sch may be required. The required voltage type can be converted by a program inputted from the outside.

The invention is not limited to the embodiments presented. Many variations are possible which may also employ features of the present invention in essentially another type of embodiment.

Claims (4)

1. A circuit arrangement for driving a luminous signal has at least one light-emitting diode (D) which is connected in series with a resistor (R) and in parallel with a controller (St),

characterized in that the controller (St) has a first controlled current source for specifying a parallel current (I _ P) which is reduced by the current (I _ D) flowing through the light-emitting diode (D), and a second controlled current source for specifying the current (I _ D) flowing through the light-emitting diode (D), and the first controlled current source has a comparator, to the input of which a switching threshold (Sch) and an actual value specification unit (Ist) are applied.

2. The circuit arrangement according to claim 1, characterized in that the switching threshold (Sch) is loaded with a temperature compensation (Tk) and/or a conduction voltage compensation (Fk).

3. The circuit arrangement of claim 1, wherein the light emitting signal is an LED signal.

4. Circuit arrangement according to one of the preceding claims, characterized in that the first current source for a predetermined parallel current has means for driving the current such that the voltage developed across the light-emitting diode (D) at the expected maximum coupled extraneous energy is smaller than the turn-on voltage of the light-emitting diode (D).