Patent attorney Rosie Hardy looks at the patent protecting a new innovation in LED signalling

Innovators across industry sectors often refer to their ‘light-bulb moments’ of inspiration and LED railway signals can aptly be described as one such moment for the rail industry.

Due to their long-life and reliability, LED signals are proving to be a more efficient and cost-effective alternative to traditional filament signals.

Unipart Dorman was the first company to introduce LED signals to the rail market in the UK and it remains the market leader. As might be expected, Unipart has obtained patents to protect its extensive range of LED signalling products. These patents provide a valuable monopoly which effectively blocks competitors from copying the patented innovation. The patents also enable Unipart to obtain Patent Box tax relief on worldwide profits earned from selling and licensing the patented innovation.

One such patent is GB 2480883 B which protects a new LED signal with a steady light output. Prior to the patent, it was conventional wisdom for LED signals to be powered by a constant current source to prevent large currents from damaging them. To prevent the light output from falling as the temperature increased, these LED signals also required additional complex controls.

After discovering that LEDs can be powered reliably by a constant voltage rather than a constant current, Unipart developed and protected a solution for stabilising LED light output as the temperature of the LED changes.

GB Patent 2480883 describes how this innovative solution uses control circuitry that selects a constant voltage and resistance to control the current through an LED.led-patent

As shown in the Figure on the right, the patented LED signal includes a resistance R coupled in series with an LED 110. A constant voltage VCONST is provided across the resistance R and LED.

Advantageously, this control circuitry allows the constant voltage VCONST and resistance R to be selected so that the current can be varied to maintain a desired light output as the temperature fluctuates.

In an example, a resistance R of 20 Ohms and constant voltage VCONST of 2.46 Volts are selected to provide a current of 14.3mA through the LED at -25°C and at current of 25.4mA at +60°C. The light output provided by the LED at 14.3mA and -25°C and at 25.4mA and +60°C is approximately the same as the nominal light output when the LED is operated at 20mA and +20°C. As a result, the protected LED signal is able to provide approximately constant light across a wide temperature range using a constant voltage, without the need for complicated controls.

Such patented innovation will further help to transform signalling technology and improve safety across the rail network.

Rosie Rosie HardyHardy is a patent attorney at leading European intellectual property firm Withers & Rogers