PAGE 050 OF 150 · WIRING LEDS
Calculating Series Resistors for Many LEDs
Add every forward voltage before subtracting from supply.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Formula
R = (Vs − n × Vf) ÷ If, where n is the number of LEDs in the string.
Example 1
12 V supply, three white LEDs at 3.2 V and 20 mA. Total Vf is 9.6 V, leaving 2.4 V, so R = 2.4 ÷ 0.02 = 120 ohms. Power is about 0.05 W.
Example 2
A 9 V supply cannot run three white LEDs, since 9.6 V is more than the supply. Use two: 9 − 6.4 = 2.6 V, so R = 130 ohms, or use 150 ohms for about 17 mA.
Always check that n × Vf is lower than the supply with room to spare.
REMEMBER
In series the same current flows through every LED.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.
Explore LED clocks, stopwatches and electronics at cosycom.com.