As is the case all over the world, countless signal lights regulate rail traffic in Germany. In the past, they used incandescent lamps, but the switch to LEDs is now in full swing. The small light-emitting diodes already serve as the light source in roughly 60 percent of all signal lights supplied by Siemens today. According to Zimmermann, who works for Siemens Mobility, this is nothing to be taken for granted. “Railroad experts are conservative for good reason; only mature and tested technology is suitable for railroad use,” he says in explanation of the long time it took to introduce LEDs here compared to other applications. LEDs are already widely used in traffic lights and automobile headlights, for example.

“That’s because the safety requirements in the railroad sector are extremely stringent,” he says. The signals expert at Siemens Industry Railway Automation applied for his first patent on a signal light with LEDs back in 1998. The first product was launched on the market in 2000.

The main advantages offered by LEDs in railroad signal lights are primarily related to their service life. “Light-emitting diodes continue to work reliably and without maintenance for years in harsh railroad environments. We have already had LEDs that lasted longer than 10 years,” says Zimmermann. Incandescent lamps have to be replaced frequently during this same period because their filaments burn through. In addition, the tiny light sources are true champions at saving energy compared to their predecessors. LEDs use up to 80 percent less electricity. Light-emitting diodes also feature outstanding color intensity, in particular when it comes to white signal aspects. Thanks to LEDs it is now much, much easier to differentiate white from yellow and thus more easily recognize the color difference from a distance. And that in itself is a good reason to replace the incandescent lamps with LEDs. “The challenge was to design LED signals so that they could be easily and inexpensively integrated into traffic control systems that had originally been conceived for incandescent lamps. Because their electrical and optical properties differ so vastly from those of incandescent lamps, LEDs pose an entirely different set of problems.” In particular, Zimmermann had to find a way to reliably ensure that the LEDs only light up when desired. What should be a trivial matter turns out to be a problem with an LED.

The cable, which often supplies electricity to signal lights that are spread out over several kilometers, can act as an antenna and “pick up” external energy out of the air.

"This external energy is not enough to illuminate an incandescent lamp, but it can certainly light up an LED – a dangerous thing if it happens at the wrong time,” says Zimmermann describing the first of two challenges that he had to solve concurrently. He was also working out how to switch over from daylight mode with high light intensity to night mode with low light intensity without having to replace all of the control technology in the signal boxes.

The solution to these problems is special control technology for the individual LEDs in a signal light. “We currently build signal lights with 20, 30, or 60 LEDs,” says the signals expert. These cannot be connected in series, since the failure of one light-emitting diode would mean that the entire series would no longer function and the complete signaling unit would have to be replaced. This would negate the major advantage of LED technology, the long service life. Zimmermann therefore invented a circuit that makes it possible to individually control each LED. Similar to a bypass, this control circuit can divert a portion of the electricity and thus ensure that the same electrical parameters prevail as with an incandescent lamp.

This method is used to both dissipate the external energy and regulate the brightness of the LEDs. With increasing operating voltage, the drop in voltage across an installed resistance increases. When this voltage drop exceeds the forward voltage of the diode, it begins to shine. In order for it to shine less brightly in night mode, a portion of the current simply has to flow across the installed resistance while the other portion flows through the light-emitting diode. If the operating voltage is increased further, an electronic switching element blocks this “bypass” and the entire current flows through the LED, which shines with maximum brightness for daytime operation.

Zimmermann and his colleagues in Berlin and Braunschweig, Germany, are already working on a new generation of signal lights that will be equipped with just a few LEDs.

With the advent of this new generation of LED signaling units, the control circuits familiar from the incandescent lamps will become a thing of the past.

“The new control circuits can be designed specifically for the needs of LEDs,” he says. Only then will it be possible to fully exploit the low energy consumption of LEDs.

It will be a few years before such a system completely replaces the old technology. After all, signal boxes are an extremely long-lived component of railroad technology. “Some signal boxes still in operation in Germany are over one hundred years old,” reports Zimmermann.

Zimmermann’s path to developing railroad signals was a long one. After graduating high school in the former East Germany, he initially learned to be an aircraft mechanic. He then studied information technology at the University of Transportation in Dresden, where he earned his degree in 1991. Because “the subject matter also included signals technology,” Zimmermann joined Siemens Rail Automation in Berlin immediately after graduation. He initially worked in the Development department, and most of his 26 invention applications, 93 granted individual patents and 25 IPR families date to this period. However, even after transferring from the Development department to product management in 2002, the husband, father, and hobby saxophonist continues to come up with improvements for signals technology.