Science Fair

My science fair project question is: How does the size of an LED affect the amount of current that it takes to burn out?

I decided to put my background paper on here for constructive criticism ;) Here goes!

[u]Science Fair Background Paper[/u]

Circuits are unsung heroes in our daily lives. They are in everything from dishwashers to remote control cars. Circuits can be so simple that they only power a light, or complex enough to power a supercomputer. A circuit consists of two main parts. A power source is the first, and the most essential part of a circuit. It provides the power for the rest of the circuit. A battery is one example of a power source. The second part of a circuit is made up of its components. The components of a circuit range from lights to motors to buttons and sensors. Drawing circuits is an easy way to show others what components make up your particular circuit. Sketching all of the components in a circuit is one way to record what it contains. Another way is to draw a circuit schematic. A circuit schematic uses symbols in succession to represent each item in a circuit. One other way to draw a circuit is to use the “PCB” technique. This uses squares, lines, and labels to represent the members of the circuit and their relationships. LEDs are one common component of a circuit. They are the lights used in almost all modern electronics. LEDs are cheap, accessible, and come in gobs of different shapes, sizes and colors. LED stands for Light-Emitting Diode. A diode is an electrical piece that can only be put in a circuit one way. This means that if you put an LED in a circuit backwards, it will not light up. In 1907, a British experimenter discovered electroluminescence, the essential principle in LEDs. No practical use was made of his discovery until a radio technician discovered the properties of semiconductors in 1955. Semiconductors are elements between conductors, like aluminum, and non-conductors, like carbon. They only partially conduct electricity. In 1961, two experimenters at Texas Instruments combined these innovations and created an infrared diode that, when electrically stimulated, produced light. Their design was improved, and the first light-emitting diode was patented the next year by a scientist at General Electric Company (GE). When they were first manufactured, LEDs cost around two hundred dollars per unit. Because of their small size and few practical applications, they were considered inefficient and therefore not widely used. However, as technological discoveries continued, the price of the lights decreased to a manageable amount. By then, light-emitting diodes were used in expensive equipment in laboratories and as replacements for neon lights. As the price of the diodes continued to fall, they took on a more common role in modern appliances. Today, you can find LEDs in lamps, calculators, scoreboards, televisions, traffic lights, and a myriad of other electronic items. LEDs have two prongs, or “leads”. The longer prong, called the anode, is connected to the power source. The second, and shorter, lead is known as the cathode. It connects back to the power source to complete the circuit. The lights work because the case of the LED contains a semiconductor, often a substance called silicon carbide. Different semiconductors produce different colors and shades of light. When electricity passes through a semiconductor in such a fashion, it glows, producing colored light. White light-emitting diodes are actually red, blue, and green LEDs placed together inside one casing. If the electricity that enters an LED is not controlled, it can short circuit, or “burn out”. When too much current flows through a light-emitting diode, the semiconductor inside stops being able to expel all incoming energy through light. Instead, the extra electricity is released through heat. This heat melts the semiconductor and surrounding materials. The case takes on a blackish hue and the diode ceases to conduct electricity. Resistors are an efficient way to reduce the amount of current on an LED. Resistors are another type of electrical component. They use insulators to reduce the flow of electricity. Resistors are very cheap and come in an enormous variety. They are used in almost every electrical appliance to control the flow of electricity. Resistors are measured in ohms. Resistors range from one to thousands of ohms. Ohms are represented with the symbol ?. They are named for German physicist George Ohm. He is famous for his equation that became the basis for all electrical theory, Ohm’s law. Ohm’s law is the foundation of measuring electrical current. The equation is “I = V/R”. “I” represents the flow of the current. This equation can also be shown as “V = IR” or “R = V/I”. Basically, the equation shows the relationship between ohms, amperes (or “amps”), and volts. Amperes measure the flow of current. They give electricity its power. Without amps, light-emitting diodes would never short circuit. Without amps, LEDs wouldn’t even turn on. Volts are “V” in the equation. Volts are a measurement of the actual amount of power. Batteries and other power sources are measured in volts. Most light-emitting diodes require one to four volts to run properly. Circuits are important parts of our daily lives. They are the essence of electronics. Without circuits, our day-to-day lives would be drastically different.

I see you have put some thought into this... but you will probably have to do less direct quoting from Wikipedia and more real world explanation about WHY things work the way they do. You CAN do this by running experiments... but you should be showing that this OVERLOADING=FAILURE result is not just an LED thing... it's a semiconductor and passive component thing. (Yes, I am belng vague... I'm leaving you room to experiment and draw conclusions)

NOTE: There are people that make their career out of semiconductor testing and analysis to document information for datasheets or for quality control. You could actually have a job testing things till you break them... if that interests you. ;-)

In your project will need to explain WHY a semiconductor device fails. Example: Lets say you have LED with a 3.3V Forward Voltage characteristic. Other characteristics from it's data sheet might be 30mA nominal forward current with MAX current = 100mA.

In your experimentation or documentation, I would expect you to first show how a Semiconductor P/N works. (AKA A DIODE) Blocks DC polarity one way and allows it another way. Remember, an LED is a light emitting "DIODE". Connecting a DIODE across a 5V power supply and Ground in one direction allows NO CURRENT TO FLOW but change direction of the diode and anything more than about 1.2Volts looks like a dead short to the circuit. The same rule applies to the LED.

What am I Saying? If you allow more than 100Ma to flow through the LED I mentioned above and the voltage is at 5Volts, you have exceeded the limits of the device and it will begin to try to dissipate the "dead short" created. If you kept your supply voltage close to 3.3V on the other hand... you have a slight chance of not burning the LED since it is very close to the non-conducting mode of the LED. In that area, the behavior of the diode provides some minute amount of conductivity control. But, increase that voltage, and your current flow changes and then you are in the "land of smoke". To keep the LED within it's documented limitations, you need to keep the MAX current flowing through it "under control" using a resistor and also to prevent the dead short condition.

You talk about the history of "circuits", but you would probably benefit from explaining this from a Physics perspective; n-type material: semiconductor containing more electrons than holes and p-type material: semiconductor containing more holes than electrons and how these are used to create the PN Junction of a diode and other more complicated devices.

White light-emitting diodes are actually red, blue, and green LEDs placed together inside one casing.

This is not true, while you can get white this way most white LEDs are UV LEDs with a phosphor that glows white on excitation.

Also note that over current of an LED can result in shortening of life as well as sudden death. I had a situation at work where someone had not taken temperature variations into consideration and at the top end of the specified temperature range there was slightly too much current through the LEDs. This resulted in an average life of about 9 months with the first failures not showing up until six months.

most white LEDs are UV LEDs with a phosphor that glows white on excitation.

Actually, most white LEDs are BLUE LEDs with a phosphor that glows yellowish on excitation. The combination of blue and yellow combine into approximately white. (this shows up nicely on emission spectra:

The three-emitter white LED and UV/Phosphor LED are alternate techniques, but so far it is the blue-led based version that is setting all the performance records.

(one doesn't normally think of blue light as causing fluorescence, since you're taught that that's what happens with UV light. The UV case is particularly interesting because the excitation is invisible and the resulting fluorescence happens in the visible region, but the physics works with incoming excitation of any frequency.

By the way, carbon is not a "non-conductor", in fact it conducts electricity pretty well. It is better considered as a semi-conductor. When in highschool We used to draw some heavy lines with pencils on paper, and used a lighter piezoelectric stuff to give high voltage to the line. We could see the sparks on the line as a little lightning.

Carbon is pretty much EXACTLY a semiconductor. Conductivity depends on exact molecular arrangement (for example, graphite conducts pretty well, diamond is an insulator.) Carbon seems to come in a rather large variety of forms (buckyballs, nanotubes, graphene...) with different properties AND the possibility of being doped with impurities. Making electronic devices out of carbon is a hot field these days...