What is an LED?

An LED (light emitting diode) is  basically a solid-state semiconductor diode that emits visible light radiation or infrared radiation when a voltage is applied.

Solid-state diodes have been used in electronics for decades to pass current in one direction and limit current in the other direction much like a hydraulic or pneumatic check valve will pass fluid and compressed air in one direction but not the other.

A major application of diodes is in power supplies where alternating current is converted to direct current by using the diodes to limit current flow to one direction only.

The newer “water clear” light emitting diodes emit relatively narrow bands of colored light at specific wavelengths depending on the composition of the semiconductor material added during the manufacturing process. Wavelength is measured in nanometers and wavelength determines the color of the light emitted.  

LED Advantages in Aircraft Applications

• LEDs are very rugged, completely solid state devices that do not use gas to produce light and therefore have no delicate parts to break. 

• LEDs consume very little electrical energy – a fraction of what incandescent lamps use.  

• LEDs do not generate RF (radio frequency) emissions that interfere with aircraft avionics.  

• LEDs radiate a very pleasant “clean” light that substantially improves readability in the nighttime aircraft cabin environment.

• LEDs (when used correctly) have an approximate lifespan of 100,000 hours continuous use – about 11 years (give or take a few years, depending on conditions).  

• LEDs generate very little heat – nearly all the electrical energy used is converted to light. They are sensitive to heat though – more on that later. 

• LEDs are completely waterproof.

• LEDs are totally embedded in epoxy. There are no loose or moving parts within this solid enclosure. Aircraft vibration will not be a factor -- they are virtually indestructible.  Of course, we never say never.

 Possible Disadvantages

• LEDs are current dependent and sensitive devices. This means they emit light with an intensity that is linear and directly proportional to the forward current applied. This also means that LEDs can be irreversibly damaged with excessive current. Care must be taken not to overdrive them with excessive voltage and current beyond their rating.

• LEDs are polarity dependent. They will not work properly and can be damaged if polarity is reversed. The long lead is always positive.

• LED radiation beams are narrow when compared with an incandescent lamp. Forward radiation is approximately 15 to 30 degrees.  This is an advantage for cockpit and map lights when light is only needed and desired in one direction. For wider beams, LEDs are mounted in “clusters” that overcome the narrow beam limitation.

• As stated above, LEDs are sensitive to heat build-up and this can significantly shorten their lifespan. This is especially true when they are mounted in clusters and enclosed in containers where the heat cannot escape easily. There are however, several strategies for correcting this problem.

Some high-end automobiles use clustered LED headlamps. The LEDs are driven very aggressively and this does produce enough heat to damage the devices.  The heat build-up is reduced with built-in cooling fans that ventilate the enclosure and remove the heat energy.

With smaller clusters, other strategies are used. If LEDs are driven conservatively (less current), the heat build-up will be minimal and usually not a factor.  This strategy does reduce the light output but it is barely noticeable.

Also, when LEDs are enclosed in an aluminum housing, the aluminum, being a great conductor of heat will absorb and transfer heat build-up away from the LEDs.

Note: We use these latter strategies at Steelebrook.  

Voltage/Current Limiting Strategies 

LED forward voltage ratings range from 1.7 volts for dull red to 4.6 volts for bright blue -- far below the average 13.8 volt charging voltage of a 12-volt aircraft electrical bus and only a fraction of the charging voltage of 24/28 volt systems.

Solutions

LEDs can be connected in series to add the individual LED voltage requirements to approach the source bus voltage. This works very well in LED clusters. For good stability and predictable current consumption, the summed voltages should not exceed 80 percent of the supply voltage (11 volts for a 13.8 volt bus).

A resister should be placed in series with the LED or LEDs to dissipate the surplus voltage, limit the current and help stabilize the circuit. Only one resister is needed for the whole series string. Ohms Law for DC circuits can be used to determine the resister rating.

The resister in this application, is essentially an energy converter. It converts the surplus voltage (voltage not used by the LEDs) into heat and dissipates this heat into the ambient atmosphere. As they say in the physics realm, heat is the graveyard of energy.

LEDs in parallel are not recommended. LEDs become more conductive as they warm up and uneven current distribution among the resisters is a real possibility.  Series/parallel arraignments are ok if you use a resister in series with each LED or groups of LEDs in series.  The individual series circuits can then be connected in parallel.

Sample Solution:

Assume we want to use two “bright white” LEDs with a forward voltage rating of 3.6 volts each on a 12 volt electrical system (13.8 volt bus).

Since we will connect these LEDs in series, add the two voltages for a total of 7.2 volts. 

Subtract 7.2 from 13.8 for 6.6 volts. This means we need to drop 6.6 volts across the resister we plan to connect in series with the two LEDs.

A standard “bright white” LED is rated at about 20 milliamps maximum current. We don’t want to drive these devises at maximum so it’s prudent to de-rate them by 10 to 20% so let’s go for with 15% (20 x .15 = 3 and 20 – 3 = 17. That leaves us with 17 milliamps (.017 amps).

Using ohms law for DC circuits, divide the voltage drop (6.6 volts) by the above total adjusted ampere rating of the two LEDs – 34 milliamps or .034 amps. This will give us the value of the resister we need to use in ohms (6.6/.034 = 194.1).

The nearest standard resister value is 200 ohms so we’ll use it in the series string.

Be sure the resister wattage rating is adequate. It’s a good idea to double the resister rating for good reliability and heat dissipation.  Just multiply the voltage dropped across the resister by the total adjusted current (6.6 x .034 = .224). So a ½ watt resister will work fine in this application. Consider higher wattage resisters too as they are relatively cheap and dissipate the heat even better.  

And remember, polarity is very important. The longer lead is always positive.

A few words about the “bright white” LEDs and why they are more expensive than the pretty colored variety.

We know that when light from all parts of the visible spectrum overlap, the additive mixture of colors appears white. However, the eye doesn’t require a mixture of all the colors of the spectrum to perceive white light. Primary colors from the upper, middle and lower parts of the spectrum (red, green and blue), when combined, appear white. To achieve this combination with LEDs requires a sophisticated electro-optical design to control the blend and diffusion of colors. Variations in LED color and intensity further “complicate” this process. Bright white LEDs are still relatively cheap considering what they do.

Eye safety

Only a few LEDs produce sufficient intensity to require eye safety precautions. But for absolute eye safety, do not stare directly into the light beam of any LED at close range.  

We hope the few above paragraphs have helped to shed some "light" on the wonderful world of LEDs.  We know we've just scratched the surface for if you need more info, just give me a call or punch in "LED" on the Internet.  But beware pilgrim -- LED information overload on the "net' can be a bit overwhelming.

Bob at Steelebrook