PAGE 031 OF 150 · POWERING LEDS
Ohm's Law for LED Circuits
Voltage equals current times resistance underpins every LED calculation.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
The basic law
Ohm's law links voltage (V), current (I) and resistance (R): V = I × R. Rearranged, I = V ÷ R and R = V ÷ I.
Applying it to an LED
The LED takes up its own forward voltage, and the resistor takes the rest. So the resistor voltage is the supply voltage minus the LED's forward voltage.
- Supply 5 V, red LED 2 V, so the resistor drops 3 V.
- For 15 mA (0.015 A), R = 3 ÷ 0.015 = 200 ohms.
- Use the next standard value up, 220 ohms, for a slightly lower current.
Units
Work in volts, amps and ohms. Convert milliamps by dividing by 1,000, so 15 mA is 0.015 A.
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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