PAGE 035 OF 150 · POWERING LEDS
Resistor Power Rating
Power equals current squared times resistance.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Power in a resistor
A resistor turns the energy it absorbs into heat. Power is P = I² × R, or P = V × I.
Ratings
Common resistors are rated 1/8 W, 1/4 W or 1/2 W. Choose a rating at least twice the calculated power so the part runs cool.
Example
A 12 V supply, 20 mA and a 470 ohm resistor: P = 0.02² × 470 = 0.19 W. A 1/4 W part is only just enough, so a 1/2 W resistor is safer.
If a resistor smells hot, discolours or is too hot to touch, use a higher resistance or a higher power rating.
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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