PAGE 037 OF 150 · POWERING LEDS
Powering an LED from a 3 V Coin Cell
A low supply leaves little headroom for a resistor.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
What a coin cell gives
A CR2032 cell is about 3 V when fresh and falls toward 2 V as it is used. It can supply only a few milliamps without its voltage sagging.
What this means
- Red, yellow and orange LEDs usually light, since their forward voltage is about 2 V.
- Blue, white and pure green LEDs often will not, or will be dim.
- A small resistor of 100 to 150 ohms is a safe choice for red.
Note
Do not rely on the cell's internal resistance to protect the LED, as it varies between cells and brands. Use a resistor and keep currents low for longer life.
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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