Field of the Invention
The invention relates to a circuit configuration for signal transmitters with light-emitting diodes (LEDs) for emitting colored light. The LEDs are arranged in an electrical circuit supplied with an a.c. voltage. One group of the LEDs is connected antiparallel to a second group of LEDs. The LEDs are arranged in electrical sections which are connected antiparallel to one another, and only LEDs of one group are arranged in each section, and the two groups of LEDs are driven by the two a.c. phases. The invention relates, in particular, to such a circuit configuration for signal transmitters in road traffic signal systems.
To date, a customary incandescent bulb has been used in road traffic signal systems or traffic lights for the purpose of generating colored light. A colored filter plate is disposed in front of the bulb. It is disadvantageous in that case, however, that other light sources may reflect on the colored filter plate of an otherwise switched-off traffic light. The reflection may illuminate the plate and it may not be clear whether the corresponding signal is switched on or off. A further disadvantage in the use of incandescent bulbs is that the latter emit light in a broadband radiation spectrum and only a small percentage is filtered out in the desired color and, as a result, only a fraction of the energy used is utilized.
These disadvantages are eliminated by using LEDs (Light Emitting Diodes) as a radiation source. The LEDs produce virtually monochromatic light in the desired color and therefore do not require any filters which can cause the deceptive reflection described above. In addition, almost 100% of the energy generated is converted into light of the desired color when LEDs are used.
To date, the use of LEDs, also known as light-emitting diodes, in traffic light systems has been effected only in prototypes and is still subject to a number of problems. In this case, the LEDs are operated using a ballast comprising a power supply unit, possibly a regulator and also, under certain circumstances, a pulse generator for pulse control operation of the LEDs. It is thereby problematic that the components used in the ballasts, in accordance with DIN 0832, do not comprise fault exclusions. Therefore, the ballasts cannot be regarded as an intrinsically safe assembly. For this reason, and because the ballasts are so complex in their entirety that internal monitoring can no longer be carried out reliably, it is necessary to use sensors to measure or meter an inhibit or stop signal, in particular. A red light is usually used as the stop signal in traffic light systems. Complex ballasts of this type, which additionally require a failure recognition configuration, can only be produced with high costs.
U.S. Pat. No. 4,939,426 discloses an array with a plurality of rectified LEDs arranged in sections. Two such sections are connected antiparallel to one another. Each section is driven alternately by the two a.c. phases when an a.c. voltage is applied. This has the disadvantage that when a single LED fails, the entire associated section becomes nonfunctional, with the result that flickering light is produced.
French patent application FR A 2 350 034 describes a system in which a plurality of sections of rectified LEDs are disposed next to one another. Within that arrangement of sections containing rectified LEDs, junction points between two LEDs of one section are, in each case, connected to a junction point between LEDs of a neighboring section in order to form a matrix.