LED (Light Emitting Diode)

    Designing an LED Circuit

    In this section, we will consider the scenario of powering a single LED at its nominal current rating from a dedicated voltage source, and picking the best resistor to match. To do this, we need to do the following:

    1. Find the forward voltage drop of the desired LED
    2. Find the operating current of the desired LED
    3. Find or set the voltage of the power supply being used
    4. Calculate the value of the series resistor used to limit the LED current
    5. Calculate the necessary power rating of the resistor

    For the case of a single LED, we will be analyzing the following circuit:

    LEDCircuit.png

    Process for a Single LED

    For a given LED, we will have a forward drop voltage , an operating current . We also have a power supply voltage .

    Calculate Resistor Value

    The first thing we calculate is the resistor value. To do this, we need to know how much of a voltage drop there is across the resistor. Since we are given the voltage source and the forward drop voltage , the voltage drop across the resistor can be found using Kirchhoff's Voltage Law (KVL) as

    Now that we know , we can calculate the resistor value that will yield the desired operating current. This is a simple application of Ohm's Law, , or, more useful to us now,

    Finally, the resistor value necessary is

    Calculate Resistor Minimum Power Rating

    Since we know the resistor value, , the current through the resistor, , and the voltage drop across the resistor, , we can calculate the power dissipated through the resistor as either

    or

    NOTE: is the minimum power rating necessary for the resistor. Any rating larger than this is acceptable. Any rating smaller than this could damage the circuit, and may start a fire from overheating!

    Example 1 - Red LED

    For this example, we will design a circuit around a common hobby LED, such as this one from SparkFun. This LED has two important parameters:

    • 1.8-2.2 VDC forward drop
    • Suggested current 16-18mA

    First, we will assume that , , and the power supply voltage . We can calculate the voltage drop across the resistor as

    Next we can calculate the resistor value necessary:

    You can either try to make a resistor by cleverly placing resistors in series, or you can use any resistor of a larger value, at the expense of some current. The closest standard 5% resistor is , and will result in a current draw of

    which is pretty close!

    Finally we can calculate the power rating necessary for the resistor as

    Most common hobby resistors are at least , so just about any resistor would be able to dissipate the heat generated with this circuit (but double check!).

    Example 2 - IR LED

    For this example, we will design a circuit around a more powerful infrared LED, such as this one from DigiKey. The parameters of this LED are:

    • Forward voltage drop of
    • Current usage of

    IMPORTANT: Do not try to use this LED directly from an Arduino, such as in the digital output tutorial. Arduino pins are capable of sourcing at most 40mA of current. Drawing more will damage your Arduino. Consider using a transistor instead!

    As in Example 1, we will assume that . The voltage drop across the resistor is

    The resistor value is

    The closest standard 5% resistor is , and will result in a current draw of

    Finally we can calculate the power rating necessary for the resistor as

    For this LED, we will need to step up to the next power level of resistors, which is

    Designing an LED Array

    [In Progress]

    Common LED/Resistor Combinations

    Description Supply Voltage LED Forward Voltage # LEDs in Series Desired Current Closest Resistor
    Red SMD 5V 1.8V 1 2mA 1.6kΩ 1/10W
    Red SMD 5V 1.8V 2 2mA 750Ω 1/10W
    Blue SMD 5V 2.85V 1 5mA 430Ω 1/10W
    Yellow SMD 5V 1.8V 1 2mA 1.6kΩ 1/10W
    Yellow SMD 5V 1.8V 2 2mA 750Ω 1/10W
    Green SMD 5V 1.7V 1 2mA 1.8kΩ 1/10W
    Green SMD 5V 1.7V 2 2mA 820Ω 1/10W
               
               

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