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Prudent RVer warns:

Beware The LED Killers:
Over-Heating & Over-Voltage

Light Emitting Diodes, aka LEDs, are the greatest lighting source available, but they have two Achilles Heels -- Over-Heating and Over-Voltage. Both of these problems must be considered when choosing RV LEDs.

LEDs are tiny slivers of semi-conductor material (p-n junctions for the geeks who want to know), a piece of silicon doped with rare earths that is mixed to emit photons in the visible spectrum whenever a proper voltage is applied across the junction such that an electrical current runs through the semiconductor. Like a true diode, LEDs resist letting current run in the wrong direction. The chemical composition of the p-n junction determines the wavelength of the emitted photons, so you can have red, amber, green, and blue LEDs, among others.

White LEDs are most often made using a native blue LED that shines blue photons into a phosper that in-turn re-emits a spread spectrum of light across a range that looks like white light to the human eye (for the geek, that is Stokes radiation).

Comparing regular lights to LEDs, the standard incandescent light bulb is only 15% efficient in converting the wattage into useful light. Fluorescent lighting is at best 50% efficient. With today's semiconductor electronics, LEDs exhibit an efficiency of 85% or higher for converting the electrical energy into useable photons, making this light source the most efficient of all the choices we have to light our RVs, boats, and homes.

None of our light sources are 100% efficient -- there is always some residual heat. And it is the heat energy that destroys the light source. With incandescents, the 85% wasted energy is heating the filament to a temperature between 600 and 1200 degrees Fahrenheit. The filament begins to literally vaporize at those temperatures, and after a thousand hours or so is finally exhausted and the bulb burns out.

LEDs, too, are heated by the flow of current through the circuit. LEDs will immediately fail when the junction temperature exceeds 185 degrees Fahrenheit. Properly designed LED circuits require two very important features to be successful: protection from over-heating and protection from over-voltage. These two problems are closely related.

Even when an LED is operated at a carefully controlled voltage, it still generates some heat. This heat must be removed from the LED device at a rate that ensures the junction temperature does not exceed the operating limit. The best way to do this is to place the LED device on a "heat sink" that has the capacity to soak up the generated heat from the LED and move it to a place where it can be radiated into the air or some surrounding heat pit. Early generation LEDs did not bother with this problem since the printed circuit board they were used on was generally large enough to serve as a sufficient heat sink.

As LEDs became more advanced, and more LED emitters were placed upon a single semiconductor device, the problem of heat dissipation became more accute. The requirement for doing a good thermal analysis of the printed circuit board holding the LED chips became apparent to those who understood what they were doing. Others tried to use the old technology with the new chips, and their products consistently failed.

The heat generated by an LED device strongly depends upon the input voltage to the device. If an LED circuit is designed to handle the heat load and operate only in a 12.8-volt environment, it will not operate well in a 14.7-volt environment. It will deteriorate and die much sooner. It's lifetime will be only a few thousand hours, rather than 100,000 hours. And if the over-voltage ranges up to 18 or 24 volts, the LED device may die immediately.

A fully charged type-27 lead-acid battery typically provides a maximum of 12.8-volts. Some people assume that is the operating voltage of an RV. But if you connect to the Shore Power Pedestal and let your AC/DC converter charge your battery, the line voltage will rise to 13.8-volts. If you have a solar system, the controller may raise the line voltage to 14.7-volts. If you do a battery equalization run on your batteries, the line voltage may be 16.6-volts for two hours or more.

Many early LED designers simply assumed a 12.8-volt environment, and their products failed at a remarkable rate in today's RVs and boats. Some learned that if they did not provide some kind of voltage regulation or power regulation on their LED circuits, product failure rates went out of sight.

How the designer protects the LEDs becomes important when you are concerned with overall power efficiency. If the LED circuit has a simple voltage limiter, it "throws" away any excess voltage, reducing the efficiency of your LED in using the available power. On the other hand, if the LED circuit has a power regulator, it converts excess voltage into usable power by taking less current from the main line. Ask your LED supplier how they are protecting their LEDs and what is the cost for such protection. You might be surprised when you find out how much they know about the problem. Will they give you a decent warranty for all conditions, like the full 100,000 hours - about 11 years?

LEDs are a big investment. They are expensive, but if they are constructed properly and used properly, you will have many years of great service, and they will pay back their original cost many times over. But you must be sure the LEDs you use have sufficient protection against excess heat and against voltage vagaries that can destroy them sooner than they should die.

Go for quality, and ask for proof, and a warranty. The Prudent RVer has all three.


copyright Sam Penny, August 27, 2010