PAGE 039 OF 150 · POWERING LEDS
Powering an LED from 9 V
Higher supplies need larger resistor values.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
About 9 V batteries
A PP3 battery gives about 9 V but has a small capacity, roughly 500 mAh for a good alkaline type, and it sags under load.
Example
Red LED at 15 mA: (9 − 2) ÷ 0.015 = 467 ohms, so use 470 ohms. Resistor power is about 0.1 W.
Running time
At 15 mA a 500 mAh battery gives around 33 hours in theory, and less in practice. Lower currents such as 5 to 8 mA make a noticeable difference.
Higher voltages mean more energy lost in the resistor. Wiring several LEDs in series wastes less.
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.
Explore LED clocks, stopwatches and electronics at cosycom.com.