Nearly every consumer device makes use of the Light Emitting Diode (LED). This highly versitle device offers an easy way to add an indicator to any project, while drawing a relatively small amount of current. Once their operation is understood, adding them to any project is a simple task. This is an simplified explanation of how a LED works and how to select a current limiting resistor. The information here is enough to use LEDs in a project, but is not intended to be a through explanation.
A Diode is an electronic component that only conducts electricity in one direction. The Forward Voltage rating of a Diode will determine the minimum voltage difference between the Anode and Cathode in order to allow electrons to flow. For example, if you apply +1V to the Anode and 0V (GND) to the Cathode, and the Forward Voltage of the Diode is rated at 0.7V then current will flow. However, if you apply 0V (GND) to the Anode and +1V to the Cathode, current will not flow!
A Light Emitting Diode (LED) is a variant of the basic Diode with the same characteristics. The key difference is that when the LED conducts electricity it also generates Light. When looking at the specifications for a LED, there are two key ratings to note: the “Forward Voltage” and the “Forward Current.”
Forward and Reverse Voltage
The Forward Voltage defines the amount of voltage required in order to conduct electricity. Any voltages below this amount will cause the LED to remain “Open” and non-conductive. This also means any components in-series with the LED will not have current flowing through them either! Once the voltage dropped across the LED reaches the Forward Voltage, it will begin to conduct electricity. Not only that, but the LED will only drop its Forward Voltage at any given time.
For example, consider a LED with a Forward Voltage rated at 3.0V. Now what happens if you attach the Anode to the Positive (+) Terminal of a AA (LR-6) Battery and the Cathode to the Negative (-) Terminal? Will the LED do anything? No! The AA (LR-6) Battery only has a nominal voltage of 1.5V. Until you add a second battery, the LED will not light up.
So if you use two nominal AA (LR-6) batteries in series and connect them to this diode, it will light up and all is good, right? Well, No. What is really happening inside of the LED is that the Diode turns into a short-circuit once a Forward Voltage is applied. This means the LED will draw ALL the current it can from the Battery This isn’t good because you are basically short-circuiting the battery! Not only will this damage the battery, but will overheat or destroy the LED!
Forward Current (If)
As mentioned before, when the Forward Voltage is applied to a LED, it turns into a short-circuit and allows current to flow. As a short, the LED will draw all the current the supply allows AND will damage itself. So you must limit the amount of Forward Current flowing through the LED. There is where the name “current limiting resistor” comes in. By placing a resistor in series with the LED, the current that flows through it is effectively limited.
Diodes, and LEDs, drop a constant voltage regardless of the current that runs through them. So the Resistor and LED work together. The Resistor hold the amount of current constant and the LED holds the voltage dropped across each constant. The next question to address is, what value resistor is needed?
Yellow LED Example
To calculate the required current limiting resistor, two properties of the LED must be known: Its Forward Current (If) and Forward Voltage (Vf). Menitoned in the last section is that a LED will hold the voltage dropped across it constant. Regardless of the voltage applied, it will only drop the Forward Voltage (Vf) across itself. Using the datasheet for a Yellow LED (available at Sparkfun), we see these two values:
Notice that the Forward Voltage is defined with a certain amount of current as its Test Condition. Within a specified curve, the LED will drop its Forward Voltage. A property of Diodes (and therefore LEDs) is that they are not linear. Different amounts of current will cause different voltage drops. For the purpose of safely lighting an LED, this is not necessary to be understood at this point.
The goal is to set the Forward Current for the LED at 20mA which means the LED will drop 1.8-2.2V. In this case, make the assumption THIS LED is going to drop 2V. (Please note that many LEDs will have a forward Current around 20mA. However, their voltage drop will typically vary depending on color.)
Using Ohm’s law the value of the R_LIMIT can be calculated. The R_LIMIT and LED are in series. This means their voltages add and the amount of current going through them is the same. This means the LED is dropping 2V across it and that 3V will be dropped across R_LIMIT. Since these two components are in series, 20mA of current will flow through both.
Ohm’s Law says that Resistance = Voltage / Current. This means R_LIMIT = 3.0V / 20mA = 150Ω.
Again, depending on the exact LED being used, the value of this resistor will change. Generally it will be in the range 150-470Ω. When in doubt, select a slightly large resistance value.
Diodes are simple, yet versatile components. LEDs extend these properties to include lights. LEDs have plenty of cool Matrix-based projects as well as more practical uses like status indicators. The information shown here shows how to find the Forward Voltage and Forward Current of an LED from its datasheet. Then Ohm’s Law is shown to calculate the correct limiting resistor.