A light-emitting diode or LED, is, in fact, a diode. An LED has all of the characteristics and properties of a normal diode with the addition of emitting light. Being a diode, the power flowing through an LED only experiences a voltage drop with very little resistance. Consequently, an LED always needs to have a resistor wire in series with it to limit the amount of current that will flow.
What are the Different Types of LEDs?
LEDs come in several different types. The main differences you will see are form factor, the wavelength of light produced (color, IR, UV, etc.), and power (which dictates the amount of light produced).
SMT Resistors with SMT LEDs
Indicator LEDs
Most LEDs produce a small amount of light in various colors and are used as indicators; meant to be viewed directly. These typical LEDs have a voltage drop of about 2V and usually handle from 5-30mA.
Lighting LEDs
LEDs used in lighting are much more powerful than those used for indication. Their voltage drops much higher than typical LEDs. They range from 2.7V and go as high as 120V. These incredibly high voltages are due to the LED being constructed of many LEDs in series, causing the voltage drop to multiply. Lighting LEDs are also designed with much larger current capabilities, ranging from 10mA all the way up to 10.5A!
Infrared LEDs
The last common type of LED we should discuss is the infrared (IR) LED. This LED is used in remote controls all around us. They produce infrared light, which is invisible to the human eye, but it is easily sensed by electronics with IR receivers. It is used in remote controls because it can not be seen by humans, so it is not a nuisance, but it also receives little interference from normal lights. IR LEDs have a low voltage drop of about 1.2V and a high current rating, usually between 50-100mA.
How Do I Use An LED?
As previously stated, LEDs are diodes and create a voltage drop, but they do not limit the about of current flowing. Therefore, an LED must always be wired in series with a resistor. What size resistors, you ask? The resistor size is dependent on the voltage drop of the LED, the voltage of the source, and the current rating of the LED. If you are using an LED with a maximum current rating of 30mA, you will probably want to run it a little lower, so that you are not abusing it.
Running an LED from a 5V supply at 20mA
Let’s say that you choose to run the LED at 20mA from a 5V supply and the LED has a 2V voltage drop. You would then begin to calculate the value of the resistor needed be determining how much voltage will be across the resistor. The voltage across the resistor is equal to the source voltage, minus the voltage drop of the LED. This is true whether the resistor is before or after the LED.
Now we know the voltage across the resistor and we can use ohms law (V = I x R) to calculate the current through the resistor. And we know that the current flowing through all components in series is equal, so this will be the same as the current flowing through the LED.
\(\large V = I \times R, \quad R = \frac{V}{I} = \frac{V_{resistor}}{I} = \frac{3V}{0.020A} = 150 \Omega\)
Now we have calculated that if we put a 150Ω resistor in series with the LED in our hypothetical circuit, we will have 20mA of current flowing.
Try it yourself!
In this example, you can see the voltage drop caused by the LED in the circuit and that it doesn’t matter whether the LED or resistor is first. If the resistor is first, the voltage drops from 5V to 2V across the resistor and 2V to 0V across the LED. If the LED is first, the voltage drops from 5V to 3V across the LED and then from 3V to 0V across the resistor. In either case, there is 3V across the resistor, making our calculation valid. The last part of the example illustrates that LEDs are in fact diodes by showing that current will only flow through an LED in one direction.
In the example, two LEDs are connected to an AC power source, in parallel, facing opposite directions. Only one lights up at a time, depending on which direction the power is flowing. This is how LED Christmas lights are wired so that they can run on AC wall power. Usually, they are blinking with the 60Hz wall power.
