askvity

How Do You Limit LED Current?

Published in Electronics 4 mins read

You limit LED current by placing a resistor in series with the LED. This resistor reduces the voltage available to the LED, preventing excessive current flow and protecting the LED from damage.

Why Limit LED Current?

LEDs are current-driven devices, meaning their brightness is directly related to the current flowing through them. Supplying an LED with more current than its rated maximum will lead to overheating and premature failure. A current limiting resistor ensures the current remains within safe operating limits.

How to Calculate the Resistor Value

The most common method involves using Ohm's Law. Here's how:

  1. Determine the LED's Forward Voltage (Vf): This is the voltage drop across the LED when it's conducting. You can find this value in the LED's datasheet. A typical red LED might have a Vf of around 1.8V.
  2. Determine the Desired Forward Current (If): This is the current you want to flow through the LED for optimal brightness. Again, consult the LED's datasheet. A common value is 20mA (0.02A).
  3. Determine the Supply Voltage (Vs): This is the voltage of your power source (e.g., a battery or power supply).
  4. Calculate the Voltage Drop Across the Resistor (Vr): This is the difference between the supply voltage and the LED's forward voltage: Vr = Vs - Vf
  5. Apply Ohm's Law (V = IR) to Calculate the Resistor Value (R): Rearrange the formula to solve for R: R = Vr / If

Example:

Let's say you have:

  • Vs = 5V (Supply Voltage)
  • Vf = 1.8V (LED Forward Voltage)
  • If = 0.02A (LED Forward Current - 20mA)
  1. Vr = 5V - 1.8V = 3.2V
  2. R = 3.2V / 0.02A = 160 ohms

Therefore, you would use a 160-ohm resistor in series with the LED. Since resistors come in standard values, a 160-ohm resistor might not be readily available. In this case, it is generally best to choose the next higher standard value resistor to ensure the current through the LED will be slightly lower than the target value. This will extend the life of the LED, at the expense of a small reduction in brightness.

Circuit Diagram

[Vs] ---[Resistor (R)]---[LED]---\
                                     ---[GND (Ground)]

Important Considerations

  • Datasheets are crucial: Always consult the LED's datasheet for accurate Vf and If values. These values can vary significantly depending on the LED type.
  • Power Rating of the Resistor: The resistor dissipates power, which is calculated as P = I²R. Choose a resistor with a power rating sufficient to handle the power dissipation. In the example above, P = (0.02A)² * 160 ohms = 0.064W. A 1/4 watt (0.25W) resistor would be more than adequate.
  • Tolerance of Components: Consider the tolerance of the resistor and the variation in the LED's forward voltage. These variations can affect the actual current flowing through the LED.

Alternatives to Resistors

While resistors are the most common and simplest way to limit LED current, other methods exist:

  • Constant Current Diodes (CCDs): These diodes maintain a constant current flow, regardless of voltage fluctuations.
  • LED Drivers (Constant Current Power Supplies): These are specialized power supplies designed to provide a constant current to LEDs. They are commonly used in high-power LED applications.
  • Current Limiting ICs: These integrated circuits provide precise current regulation and protection features.

Related Articles