Wire Wilt: How Light-Emitting Diodes Fade As Temperature Increases
AbstractWhat's your favorite thing to do on the hottest day of the year? Dip your toes in an icy river? Hang out by the pool? Retreat to a cool basement? Lie motionless in the shade? You're probably not too eager to move around and put out a lot of energy, like mowing the lawn in the mid-afternoon sun. Well, you're not the only one. In this electronics science fair project, you'll find out that some semiconductor devices, like light-emitting diodes (or LEDs), act the same way. As their internal temperature goes up, their light output goes down.
Kristin Strong, Science Buddies
Edited by Steven Maranowski, PhD, Philips Lumileds Lighting Company.
To determine how the output of an LED flashlight changes over time as its temperature increases.
If you've ever gone camping deep in a forest, far from any town or city, you know well the comfort and security that a small bit of light can bring. Humans learned how to control fire, by some accounts, around 400,000 years ago, but the first portable electric lights didn't come about until the invention of the battery and the incandescent lightbulb, so were not available until 1898. These early portable lights (shown in Figure 1) were first tried out by the New York City police department. They ran on zinc-carbon batteries, which did not provide a good flow of current to the inefficient lightbulbs that were in use at the time. The combination of poor batteries and inefficient bulbs meant that the portable lights could only be turned on for brief periods of time, and then they had to "rest" before they could be used again. For this reason, they were named flashlights, because you could only use them to get a "flash of light" before having to turn them off again. Flashlight technology has improved considerably over the last 100 years, but the name still remains, at least in the United States. In much of the rest of the world, they are called torches.
Figure 1. This photo shows one of the first flashlights ever built, in 1899. (Wikipedia Commons, 2007.)
Traditional flashlights work by flipping the flashlight's on-off switch to the "on" position, which "closes the loop" and allows current to be drawn from the flashlight battery and pass through an extremely thin tungsten filament inside the flashlight's incandescent lightbulb. As free electrons pass through the filament, they bump into and vibrate atoms in the tungsten filament and heat them up. The heat raises bound electrons in the vibrating tungsten atoms to a higher energy state temporarily, and when they fall back down to their normal state, they give off that energy in the form of photons, the basic units of light. The light coming from the bulb spreads out in all directions, but a parabolic reflector collects and focuses the light into a narrow beam, so you can find your way to your tent in the dark.
Figure 2. This drawing shows the basic flashlight circuit. The loop is closed when the switch is closed (turned on) and current flows from the battery and through the incandescent lightbulb. (Modified from Energy Quest, 2007.)
In the past decade, the traditional flashlight has been modified to use solid-state electronics. The small incandescent lightbulb has been replaced with a semiconductor device, called a light-emitting diode or LED. Semiconductors are called "semi" conductors because they can conduct or carry electricity, but not as well or easily as a normal conductor, like copper wire, can. An LED does not have a filament inside it, like a lightbulb does. Instead, it has a diode containing one semiconductor material with extra electrons (called n-type) bonded together with another semiconductor material with extra "holes," or a deficit of electrons (called p-type). With this arrangement, current can only flow in one direction across the diode.
This drawing shows an LED made up of P (on the left) and N (on the right) material connected to a battery with the positive terminal going to the P-type material and the negative terminal going to the N-type material. The holes are shown moving out of the P-type material and into the N-type material. The electrons are shown moving out of the N-type material and into the P-type material. As electrons fall from the conduction band to the valence band, light is given off.
Figure 3. This drawing shows how LEDs produce light. (Wikimedia Commons, 2006.)
When a battery with enough voltage is connected across a diode in the proper direction—with the negative terminal connected to the n-type material, and the positive terminal connected to the p-type material—the free electrons in the n-type material are repelled by the negative charge and attracted to the positive electrode, while the holes go the other way, and current flows across the diode. The free electrons are at a higher energy state than the holes are, and when they "fall" into the holes, they release energy in the form of photons, the basic units of light. Whether that light is visible or not depends on how far they fall. LEDs are designed so that the electron falls produce light in the visible spectrum. The greater the fall, the more energy that is released, and the higher the frequency of the light will be.
Diagram of the visible light region of the electromagnetic spectrum. The spectrum is broken up into three segments: infrared (left), visible light (middle), and ultraviolet (far right). The waves on the left have lower frequencies than those on the right.
Figure 4. This drawing shows the visible light portion of the electromagnetic spectrum, and the infrared and ultraviolet portions below and above the range of visible. Notice that red has a lower frequency (slower up-and-down waves) than the color violet. (NASA, 2007.)
The color red, for example, has a lower frequency than violet, so the electron "fall" required to produce the color red is shorter than the fall required to produce the color violet.
If you go to a frequency even lower than the visible color red, you'll enter the infrared radiation range. You cannot see infrared radiation, but you can feel it in the form of heat when you get close to a fire, an oven, or an incandescent lightbulb. In fact, most of the energy used in turning on a lightbulb goes toward generating (unwanted) infrared radiation—only 5 percent goes toward producing visible light. In contrast, LEDs feel relatively cool as you get close to them because, in general, they emit very little infrared radiation. They are more efficient than incandescent lightbulbs, meaning that their light output per unit power input (a ratio) is greater. LED efficiency varies widely and depends on things like an LED's color, how it was manufactured, and the amount of current passing through it. No matter what their efficiency, LEDs do radiate heat at their base though, at many other frequencies other than infrared, with the result being that some portion of the input energy goes toward producing visible light, and the rest is spent generating heat.
It turns out that it is important to remove this heat through thermal-management methods, like heat sinks, especially with high-power LEDs, because as the temperature increases, an LEDs efficiency and brightness decrease. In this electronics science fair project, you are going to investigate how the output of an LED flashlight changes over time, after you turn it on and it begins to heat up and approach its steady-state temperature, the point at which its internal temperature is no longer changing and the LED has reached its thermal equilibrium.
Terms and Concepts
- Incandescent lightbulb
- Zinc-carbon battery
- Tungsten filament
- Free electrons
- Bound electrons
- Energy state
- Parabolic reflector
- Light-emitting diode (LED)
- Infrared radiation
- Heat sink
- Steady state
- Thermal equilibrium
- Inverse square law
- How is light created in a traditional flashlight?
- How is a diode made?
- Can current flow in both directions in a diode?
- How is light created in an LED flashlight?
- Why do LEDs feel cooler when you get close to them than incandescent lightbulbs?
These sources describe how LEDs are made and how they work:
- Wikipedia Contributors. (2009, May 14). Light-emitting diode. Wikipedia: The Free Encyclopedia. Retrieved May 15, 2009.
- Harris, T. (2009). How Light Emitting Diodes Work. Retrieved May 15, 2009.
- Brain, M. (2009). How Semiconductors Work. Retrieved May 15, 2009.
This source describes the heat generated by LEDs:
- Whitaker, T. (2005, May). Fact or Fiction—LEDs don't produce heat. Retrieved May 14, 2009.
This source describes the electromagnetic spectrum:
- Netting, R. (2007, March 27). The Electromagnetic Spectrum. Retrieved May 15, 2009.
Materials and Equipment
- Shoebox or other small cardboard box
- Multi-color LED flashlight, ideally with an on-off switch on the end of the handle; available at some hardware, sporting goods, and electronics stores, or from online sellers, such as www.amazon.com
- Lux meter, also known as a light meter; such as the Light Meter LX1010B,50,000 Lux Luxmeter with lcd display, available at Amazon.com
- Tape, any sturdy packaging or masking tape
- Lab notebook
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- Before starting your experiment, read the instructions that come with your lux meter so that you know how to use it. Practice taking light measurements around your home from different sources. For example, see what kinds of light measurements you get from a lamp; a TV; beside a window, both with and without its blinds; and outside.
- Read the instructions that come with your flashlight and practice changing colors before you begin the experiment. Remember, never look into the LED flashlight or shine it on someone's face, as that can cause eye damage.
Preparing Your Test Box
Tape the sensor part of the lux meter (the part that receives light) to the inside middle of one wall of the box. You can place the base of the sensor on the bottom of the box, to make it more stable, if desired.
- Because the illuminance from a light source follows an inverse square law, falling off by the distance squared, choose a wall that has the shortest distance to its facing wall (choose a wall where the box is narrow).
- Let the display part of the lux meter hang outside one corner of the box, as shown in Figure 5.
Photo of an open box with a sensor, lux meter and LED flashlight. Taped on the inside middle left wall is the sensor from the lux meter. Poking about halfway through the right wall and pointed at the sensor is the LED flashlight. The lux meter display is resting outside one corner of the box.
Figure 5. This photo shows how to prepare your test box.
- Using a ruler, measure the location (in the horizontal and vertical direction) of the center of the lux meter sensor.
- Find the wall of the box that is facing the lux meter sensor, and, using your measurements from step 3, make a hole in the box with the tips of the scissors, directly across from the lux meter sensor, that is just big enough for the flashlight to fit through.
- Insert the flashlight through the hole so that the light source is inside the box and, if possible, the on-off switch is outside the box.
- Using a small piece of tape, mark on the flashlight how far through the box you pushed the flashlight, so that you will know how to position the flashlight in future trials. Because of the inverse square law, you want the distance from the flashlight to the sensor to remain the same for all trials.
Testing Your LED Flashlight
Make data tables, one for each LED color that you are going to test:
Record the time in the rows of the data table.
- Have time start at zero and increase in 15-second increments for at least 3 minutes.
- Record the illuminance for each trial in the columns.
- How long you will need to test depends upon your LED flashlight and how long it takes to come to thermal equilibrium. When you see the illuminance measurements changing very little, then you will know that the flashlight is at thermal equilibrium and has reached a steady state.
- Record the time in the rows of the data table.
- Check that the flashlight handle is at the tape mark and not sticking too far out or too far inside the box.
- Close the box. If the flaps do not stay closed, tape them lightly, or lay an object up on top of them to keep them closed.
- Turn on the lux meter. The display should read zero or close to zero.
- Reset the timer.
Have your helper start the timer while you simultaneously turn on the LED flashlight to the appropriate color.
- Keep your eyes fixed on the lux meter display and record the first value that you see show up. This will be your "time equals zero" illuminance value.
- Have your helper call out "time" every 15 seconds, while you record the value that you see on the lux meter display in the data table.
- Continue step 7 until the flashlight has reached thermal equilibrium (the illuminance measurement stops changing, or is changing very slowly).
- Turn off the flashlight and the lux meter.
- Wait 5 minutes to allow the LEDs to return to their unheated state.
- Repeat steps 4–10 two more times, so that you have a total of three trials for the current color being tested.
- Repeat steps 4–10 three more times for each remaining LED color you are testing.
Analyzing Your Data Tables
For each data table, plot the time on the x-axis and the illuminance on the y-axis.
- Are the shapes of the curves the same for each trial?
- Does each color take the same time to reach thermal equilibrium?
Make a bar chart that plots the color frequency on the x-axis and the peak illuminance for each trial on the y-axis.
- Which color has the highest peak illuminance?
- Which color has the lowest?
- Does this make sense in terms of what you know about electron falls, energy, and color?
Ask an Expert
- Can you devise a way to make the line-of-sight from the flashlight to the sensor more stable from trial to trial, and improve upon your results?
- Can you devise a way to measure the temperature of an LED as it comes to thermal equilibrium, and then plot the temperature on the x-axis and the illuminance on the y-axis?
- Investigate how the flashlight's external temperature affects how the illuminance changes over time for one color.
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