To limit the current to a safe level for an LED powered by a 5V source, you should use a resistor of 150 ohms or higher. The precise value depends on the LED's forward voltage.
Understanding the Calculation
The resistor value is calculated using Ohm's Law and the following factors:
- Voltage Source (Vs): In this case, 5V.
- LED Forward Voltage (Vf): This varies depending on the LED's color. Common values are:
- Red: 1.8V - 2.2V
- Yellow: 2.0V - 2.2V
- Green: 2.0V - 3.5V
- Blue: 2.5V - 3.7V
- White: 2.5V - 3.7V
- Desired LED Current (I): Typically, LEDs are run at around 20mA (0.02A) for good brightness without damage.
The formula to calculate the required resistance (R) is:
R = (Vs - Vf) / I
Example Calculation
Let's assume you're using a red LED with a forward voltage of 2V and you want to run it at 20mA.
R = (5V - 2V) / 0.02A
R = 3V / 0.02A
R = 150 ohms
Therefore, a 150-ohm resistor would be a suitable starting point. Using a slightly higher value (e.g., 220 ohms, 330 ohms) will reduce the current slightly, making the LED less bright but potentially extending its lifespan.
Resistor Values to Consider
Given the reference recommending 150 ohms or higher, here's a breakdown of common resistor values and their effect:
| Resistor Value (Ohms) | Approximate LED Current (mA) (assuming Vf = 2V) | Effect on Brightness |
|---|---|---|
| 150 | 20 | Relatively Bright |
| 220 | 13.6 | Moderately Bright |
| 330 | 9.1 | Dimmer |
| 1K | 3 | Very Dim |
| 2K | 1.5 | Extremely Dim |
| 10K | 0.3 | Almost Off |
| 100K | 0.03 | Off |
Important Considerations:
- LED Datasheet: Always consult the LED's datasheet to determine its specific forward voltage and maximum current rating.
- Experimentation: It's often best to start with a higher resistor value and then decrease it until you achieve the desired brightness. This prevents you from accidentally burning out the LED.
- Power Rating: Ensure the resistor's power rating is sufficient. The power dissipated by the resistor is calculated as P = I2R. In our 150-ohm example with 20mA, P = (0.02A)2 * 150 ohms = 0.06W. A 1/4 watt resistor (0.25W) would be sufficient.
