PAGE 040 OF 150 · POWERING LEDS
Powering an LED from 12 V
At 12 V you can often run several LEDs in series.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
A common supply
12 V comes from power adaptors, lead-acid batteries and vehicle systems. A running car may reach 13.8 to 14.4 V, so design for the higher figure.
Examples
- Red LED, 15 mA: (12 − 2) ÷ 0.015 = 667, so use 680 ohms, about 0.15 W.
- White LED, 20 mA: (12 − 3.2) ÷ 0.02 = 440, so use 470 ohms.
- Three reds in series: (12 − 6) ÷ 0.02 = 300 ohms, wasting much less power.
Advice
Use a half-watt resistor for margin, and string LEDs in series to improve efficiency.
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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