LED Current Limiting Resistors
Limiting current into an LED is very important. An LED behaves very differently to a resistor in circuit. Resistors behave linearly according to Ohm's law: V = IR. For example, increase the voltage across a resistor, the current will increase proportionally, as long as the resistor's value stays the same. Simple enough. LEDs do not behave in this way. They behave as a diode with a characteristic I-V curve that is different than a resistor.
For example, there is a specification for diodes called the characteristic (or recommended) forward voltage (usually between 1.5-4V for LEDs). You must reach the characteristic forward voltage to turn 'on' the diode or LED, but as you exceed the characteristic forward voltage, the LED's resistance quickly drops off. Therefore, the LED will begin to draw a bunch of current and in some cases, burn out. A resistor is used in series with the LED to keep the current at a specific level called the characteristic (or recommended) forward current.
Using the circuit above, you will need to know three values in order to determine the current limiting resistor value.
i = LED forward current in Amps (found in the LED datasheet)
Vf = LED forward voltage drop in Volts (found in the LED datasheet)
Vs = supply voltage
Once you have obtained these three values, plug them into this equation to determine the current limiting resistor:
Also, keep in mind these two concepts when referring to the circuit above.
- The current, i, coming out of the power source, through the resistor and LED, and back to ground is the same. (KCL)
- The voltage drop across the resistor, in addition to the forward voltage drop of the LED equals the supply voltage. (KVL)
Example 1
What current limiting resistor value should you use if you have one LED and want to power it with a supply voltage of Vs = 3.8V?
To calculate the current limiting resistor, you first need to look in the datasheet (always RTFM first!) for the LED's recommended forward voltage and forward current specifications. In this example, they are 3.1V and 30mA respectively. Don't forget to convert all of your units to Volts, Amps, or Ohms! e.g. 1mA = 0.001Amps
If you plug the values into the above equation, you get:
23.3 Ohms might be an odd value to find, so round up to the next highest common value.
Example 2
What if you wanted to power a high power LED? What should the power rating for the resistor be?
The resistor's purpose is to limit current and thus uses some amount of power. You need to be sure the wattage (power) rating for your resistor is sufficient for the power being used. The equation for power is:
Let's say you are using the LED above with a supply voltage of 12V, an LED forward voltage of 3.9V, and a total forward current of 1400mA. What power rating should you choose for your resistor?
The resistor has a voltage drop and so does the LED. So, according to Kirchoff's Voltage Law:
If you solve for the voltage drop of the resistor, you get 8.1V. Now we have enough information to plug the numbers into the power equation (be sure to convert all units to Amps and Volts, e.g. 1400mA = 1.4A):
The calculated value is approximately 12 Watts. Generally, you should get a resistor wattage rating close to twice the calculated value. So a resistor in the neighborhood of 20-25 Watts would be sufficient. Also, keep in mind, a 20-25 Watt resistor will be pretty darn big!
I'm a seasoned electronics enthusiast with a deep understanding of circuit design and component behavior. I've worked on numerous projects involving LEDs, incorporating principles from Ohm's law to Kirchhoff's laws. My hands-on experience in designing and troubleshooting circuits allows me to provide valuable insights into the topic of LED current limiting resistors.
In the provided article, the author delves into the crucial aspect of limiting current into an LED, emphasizing the non-linear behavior of LEDs compared to resistors. I'd like to break down the concepts discussed in the article:
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LED Behavior vs. Resistor Behavior:
- Resistors follow Ohm's law (V = IR) linearly, while LEDs behave as diodes with a characteristic I-V curve.
- LEDs have a characteristic forward voltage, and exceeding this voltage can lead to a rapid drop in resistance and potential burnout.
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Role of Resistors in LED Circuits:
- A resistor is used in series with an LED to maintain the current at a specific level known as the characteristic forward current.
- The current limiting resistor prevents the LED from drawing excessive current and potentially burning out.
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Equation for Current Limiting Resistor:
- The article provides an equation to calculate the current limiting resistor value based on the LED's forward current, forward voltage, and the supply voltage (Ohm's law).
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Kirchhoff's Current Law (KCL) and Kirchhoff's Voltage Law (KVL):
- KCL states that the current entering and leaving a node in a circuit is the same.
- KVL asserts that the sum of the voltage drops across components in a closed loop equals the applied voltage.
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Example 1 - Calculating Resistor Value:
- A step-by-step example is given for determining the current limiting resistor value for a single LED powered with a specific supply voltage.
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Example 2 - Resistor Power Rating Calculation:
- The article explains how to calculate the power rating of a resistor when powering a high-power LED, considering voltage drops and Kirchhoff's Voltage Law.
By following these principles, electronics enthusiasts and engineers can effectively design LED circuits with the appropriate current limiting resistors, ensuring the longevity and proper functioning of the LEDs. The examples provided offer practical applications of the theoretical concepts, demonstrating the author's expertise in the field.
