Emitting more than light

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LEDs are fast becoming a cost-effective alternative to incandescent light bulbs. And with good reason.

Light-emitting diodes actually are not light bulbs. A conventional incandescent light bulb consists of a piece of high-resistance wire called a filament, enclosed in glass in an inert atmosphere. When electric current is passed through the wire, resistance causes the filament to glow white-hot. The inert atmosphere keeps the filament from burning up immediately. But eventually, at somewhere between 200 and 15,000 hours, the filament burns out and the bulb must be replaced.

An LED is a solid-state semiconductor that emits light when electrical current is passed through it. Clusters of LEDs, hooked up in parallel, can meet the photometric requirements of truck and trailer lighting, thanks in part to advances in lens design.

There are several advantages to LEDs. First, with no filament to burn out, they live far longer than incandescent bulbs – by some estimates, more than 100,000 hours, or possibly longer than the vehicle they’re installed on.

They also draw less current and put less of a load on a vehicle’s alternator and batteries. Also, since they require less power, they leave more power available for other vehicle systems, helping ensure proper operation. This is especially important for double and triple trailers, where power is compromised significantly by the large number of lights and the length of wiring.

LEDs also offer two safety advantages: one, they light up about 200 milliseconds faster than incandescent bulbs. At 55 mph, a vehicle travels about 16 feet in that time, so there’s a safety margin for a following vehicle’s driver to notice a brake light and stop. Two, since they last so long, they reduce the number of instances where technicians need to climb ladders to replace marker lamps.

The Technology and Maintenance Council of the American Trucking Associations has addressed this issue in its Recommended Practices. TMC RP 137A, “Antilock Electrical Supply from Tractors Through the SAE J560 Seven-pin Connector,” and TMC RP 141, “Trailer ABS Power Supply Requirements,” constitute a performance standard for tractor and trailer ABS.

RP 141 specifies that a minimum of 9.5 volts should be available to the trailer ABS to ensure proper operation. Because of their high current demand, incandescent lamps contribute greatly to the problem of voltage drop. And by incorporating LED lighting, advises TMC, the amount of voltage drop will be significantly less than that of incandescent lamps.

On average, an LED marker lamp requires 0.05 amps at 14 volts, whereas a typical incandescent light requires 0.4 amps. An average LED stop lamp requires 0.5 amps at 12.8 volts, and a typical incandescent stop lamp requires 2.1 amps. Given these examples, it’s not hard to see that the cumulative effect of LED lighting all around can significantly reduce current draw and voltage drop.

But it should be pointed out that LEDs are diodes, and by definition, current can flow through them only in one direction. So a concern comes into play when making “custom” repairs. Mechanics who don’t know LEDs are polarized – and somehow manage to defeat the directional connectors and hook them up backward – may assume they’re defective, toss them and go back to the parts room for new ones.

Drawbacks aside, new and affordable LED products available from Grote, Truck-Lite, Peterson and others promise to provide cost-effective lighting for the future.