4S6TMP.com ā Series, parallel & multi-LED configurations with circuit diagram
An LED is a current-driven device, not a voltage-driven one. Once the forward voltage is exceeded, the LED's internal resistance drops dramatically and it will draw as much current as the supply can provide ā destroying itself in milliseconds. The series resistor limits the current to a safe, predictable value.
The formula is derived from Ohm's Law: R = (Vsupply - Vf) / I
Where Vsupply is your power supply voltage, Vf is the LED's forward voltage drop, and I is the desired current in Amperes. The result is in Ohms.
Always check your specific LED's datasheet. "High-brightness" or "ultra-bright" LEDs often have higher Vf values than standard indicators.
Series: Multiple LEDs share one resistor. The supply voltage must exceed the sum of all Vf drops. Advantage: same current through all LEDs, consistent brightness. Disadvantage: if one LED fails open, the whole string goes dark.
Parallel with individual resistors: Each LED has its own resistor. Recommended because LEDs have manufacturing variation in Vf. Without individual resistors, the LED with the lowest Vf hogs current and burns out.
Parallel with shared resistor: Not recommended for LEDs due to current hogging. Only acceptable if LEDs are matched from the same production batch and thermally coupled.
The resistor dissipates power as heat: P = IĀ² Ć R or P = V Ć I where V is the voltage across the resistor. Always choose a resistor with a power rating at least 2Ć the calculated dissipation for safety margin. Standard sizes: 1/8W (0.125W), 1/4W (0.25W), 1/2W (0.5W), 1W, 2W, 5W.
Can I use a higher value resistor? Yes ā a higher value means less current, dimmer LED, and lower power dissipation. The LED will be safe but less bright.
What happens if I don't use a resistor? The LED will burn out almost instantly. A typical LED has a forward current of 20mA but will try to draw amps from a 5V supply without current limiting.
Can I drive an LED directly from a microcontroller pin? Most MCU pins can source/sink 20ā40mA max. Use a resistor to limit current to a safe value (typically 5ā15mA for indicator LEDs). Check your MCU datasheet.
What is the E12 series? A standard set of resistor values: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 ā multiplied by powers of 10. Use the nearest standard value.