PAGE 033 OF 150 · POWERING LEDS
Resistor Value Worked Example
A 5 V supply, 2 V LED and 10 mA needs about 300 ohms.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Example 1: red LED on 5 V
Target 10 mA, Vf 2.0 V. R = 3.0 ÷ 0.010 = 300 ohms. Use 330 ohms, giving 3.0 ÷ 330 = 9.1 mA. Resistor power is 0.0091² × 330, about 0.027 W, far below a quarter watt.
Example 2: white LED on 12 V
Target 20 mA, Vf 3.2 V. R = 8.8 ÷ 0.020 = 440 ohms. Use 470 ohms, giving 8.8 ÷ 470 = 18.7 mA. Power is 0.0187² × 470, about 0.16 W, which a quarter-watt resistor can handle.
Always check
Confirm the actual current stays under the LED's maximum, and that the resistor power is under its rating.
REMEMBER
The resistor sets the current, so the resistor sets the safety.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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