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PROTECTED LED TECHNOLOGY: SEEING THE LIGHT

By Jim Renie, Square D Company

For years, the industrial control industry has been looking for a new light source that would provide longer life, better illumination and more color options for push buttons. Now there is a new technology that provides more visible light than conventional sources and can provide a 10-year service life without any loss of brilliance. The future is here today in the Light Emitting Diode (LED) source.

Over the last few decades, pilot lights and illuminated controls have used two types of light sources. The incandescent lamp, the oldest and most common, works on the principal of heating a tungsten filament within a vacuum to white heat, thus producing light. Incandescent lamps radiate light in all directions and are immune to the electromagnetic interference typically encountered in manufacturing. It is an inexpensive and readily available light source. The downside, however, is that the heating causes the filament to gradually vaporize, limiting the service life of the lamp to between 500 and 5,000 hours. Also, since the filament is suspended in a vacuum, there is little resistance to shock or vibration.

The second most common light source is the fluorescent (neon) type. Fluorescent lights comprise a pair of electrodes mounted within a sealed capsule filled with gas. Current is passed between the electrodes and the resulting electron emissions excite the gas, which in turn produces photons (light). Also low cost and readily available, fluorescent lights dissipate little heat and their service life typically exceeds 20,000 hours. Unfortunately, the light output is weak and the range of available colors is limited. In addition, these lights can only function at higher voltages (greater than 65V) and their performance degrades rapidly when used on DC current.

Neither of the traditional sources is ideal for use in an industrial environment. They tend to not be mechanically robust, nor are they suited for the full range of industrial lens colors.

By contrast, LED devices comprise a semiconductor "chip" encapsulated into a surface-mount (SMC) module, which is housed in a conventional lamp base. LEDs create direct photon emissions (light) when a current is passed through the "chip." Its low current consumption means that LEDs dissipate less heat than incandescent lamps. Also, these devices tend to be much smaller and with much stronger colors. While ultimate service life depends on factors such as operating current, temperature and color, 100,000 hours of continuous operation - the equivalent of 10 years of service - is easily attainable.

Though LEDs are efficient, they still have some constraints. LEDs require a supply voltage between 1.7 and 3.6V (depending on color) and are polarity-conscious. They're typically offered only in red, green, and yellow, and operating temperature must remain between -25°C and 85°C. LEDs also tend to be highly sensitive to electrostatic discharges.

However, there is new technology that improves on the LED. PROTECTED LED® is an integrated light module offering special control and protection circuits, which negate the effects of electrostatic discharge, capacitive coupling and electromagnetic radiation. In addition, these protective circuits inhibit illumination of the LED due to proximity switch or PLC I/O module leakage currents. Overvoltage protection is also incorporated to protect against the effects of voltage surges, inductive spikes generated by coil de-energization and reverse polarity. The protection device prevents spurious illumination of the LED by absorbing any interference, such as the energy resulting from coil de-energization. PROTECTED LEDs are also available in five colors - the standard red, green, and yellow, PLUS blue and white.