Join Barron Stone for an in-depth discussion in this video Light-emitting diodes, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] Light-emitting diodes, or LEDs, are the flashiest member of the diode family. Like normal diodes, they consist of a semiconductor junction that only allows current to pass through it in one direction. But, that junction is specially designed so that when current does pass through it, it'll convert some of the energy into light by giving off photons. The schematic symbol for an LED looks like the symbol for a regular diode, with the addition of two little arrows pointing away from it to represent the light that's given off by the diode.
Physically, LEDs come in a variety of shapes and sizes. Many consumer electronic devices use compact surface mount LEDs on printed circuit boards to indicate the operating status of a device. Those surface mount LEDs look like small rectangular prisms. For prototyping circuits on a breadboard, it's more common to use through hole LEDs like this, which consist of a plastic bulb with two long metal leads on one side. The plastic bulb will either be clear or tinted to indicate the color of the light it produces.
Since LEDs are a type of diode, they're polarized, which means the direction of the current passing through them matters. So you can tell which side is which, on most through hole LEDs, the metal lead connected to the positive anode terminal will be longer than the lead that's connected to the negative cathode. Additionally, there's usually a flat section on the cathode side of the bulb, and since I tend to get the terms anode and cathode mixed up in my head, I usually just remember that the flat section on the side of the LED bulb represents the flat R on the diode symbol.
If I ever build a circuit using an LED and it doesn't light up, the first step I always take to try troubleshooting the problem is turning the LED around, because there's a pretty good chance I put it in backwards. Just like all other types of diodes, LEDs require a certain amount of forward voltage across their junction before they allow current to flow through them to generate light. LEDs generally require a larger forward voltage than normal diodes, and it'll vary depending on the LED's color and intensity.
For example, this table shows how the forward voltage, maximum current, and luminous intensity ratings compare for two different colored LEDs. The red LED will require a forward voltage drop somewhere between two to 2.4 volts, whereas the blue LED requires a higher forward voltage drop between three to 3.4 volts. Generally, LEDs that have a higher forward voltage will also be able to handle more current.
And, on this table, I can see that the blue LED can handle a maximum current of 30 milliamps, whereas the red LED can only handle up to 20 milliamps. The higher voltage and current ratings of the blue LED mean it can dissipate more power than the red LED, which allows it to produce a brighter light. The blue LED has a luminous intensity of 400 millicandela, versus 200 millicandela for the red LED. Millicandela, or mcd, is a standard unit for measuring the intensity of a light source that's commonly used to describe the brightness of LEDs.
As a point of reference, an LED with 50 to 100 millicandela is good to use for a low-intensity indicator light. But, if you wanted to build an LED flashlight, you would need to use one of the ultra-bright LEDs which have intensities of over 10,000 millicandela. You can control the brightness of an LED by controlling the amount of current through it. Since the forward voltage remains constant when an LED is on, sending more current through an LED will increase the amount of power it dissipates and light it generates.
More current makes the LED shine brighter. However, there is such a thing as too much current. When an LED is turned on, it allows current to pass through it more or less unrestricted. So, the LED needs something else to limit the amount of current passing through it, or else it'll try to dissipate too much power and it'll quickly be burned up and destroyed. The simplest way to prevent that is to put a resistor in series with the LED to restrict the amount of current flowing through the circuit. And that raises an important question.
What size resistor should I use? As a simple rule of thumb, for most small-scale hobbyist projects which commonly use through hole LEDs and are powered by 3.3 or five volt sources, a 330 ohm resistor is generally a safe limiting resistor value to use to keep LEDs from burning up. However, if I want to make sure my LED is as bright as possible, I'll need to use Ohm's Law to calculate the smallest resistor value I can use that will give the LED the maximum amount of current within its limitations.
For example, if I plan to connect my red LED to a five volt source, which is a common voltage for microcontroller devices, I know that since the red LED has a forward voltage drop of two volts, there will be three volts left across the limiting resistor. My red LED is rated to handle a maximum current of 20 milliamps. So, to find the resistor value that will create the maximum amount of current, I'll divide three volts by 20 milliamps, which gives me 150 ohms.
Now, 150 ohms is not a common resistor value that I have in my parts kit, so I'll round up to the nearest value that I do have, which is 220 ohms. I always round this resistance value up, rather than down, because a higher resistance will restrict the current more, so that there's less current than the maximum of 20 milliamps. With a 220 ohm limiting resistor, my LED only gets about 13 1/2 milliamps of current, which it can safely handle.
If I had rounded my calculated resistor value down, instead of up, that would have allowed more current to flow through the LED and exceed its 20 milliamp rating, which would quickly burn it out. Once I've determined the smallest resistor value that I can safely use which will make my LED as bright as possible, I can still play around with using larger limiting resistor values to tweak the brightness and make my LED dimmer to meet the needs of a specific project.
For example, here are three identical red LEDs connected to the same five volt power source, but with different current-limiting resistors. The LED on the left is the brightest because it has the smallest resistor value that I can safely use with these LEDs, which is 220 ohms. The LED in the middle is slightly dimmer because it's connected to a much larger 10 kiloohm resistor. And the LED on the far right is the dimmest because it has a 100 kiloohm resistor, which limits the current through it down to about 44 microamps.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals