PAGE 032 OF 150 · POWERING LEDS
Choosing a Current-Limiting Resistor
Subtract the forward voltage from supply, then divide by current.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
The formula
R = (Vs − Vf) ÷ If, where Vs is the supply voltage, Vf is the LED forward voltage and If is the current you want.
Steps
- Find Vf and a safe If in the datasheet.
- Subtract Vf from the supply to get the resistor voltage.
- Divide by the current in amps.
- Choose the nearest standard value at or above the result.
- Check the resistor power rating.
Example
A 9 V supply and a blue LED of 3.2 V at 15 mA gives (9 − 3.2) ÷ 0.015 = 387 ohms. The nearest standard value is 390 ohms, and the real current is 5.8 ÷ 390, about 14.9 mA.
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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