Stage 2 · Lesson 2 of 7
LEDs and current-limiting resistors
An LED (light-emitting diode) only conducts one way and, once it does, has almost no of its own. Without a resistor in series it draws as much current as the source allows and damages itself or the pin driving it. Choose the resistor with R = (supply voltage minus LED ) / current.
The forward voltage depends on the colour, about 2 V for red and around 3 V for blue or white, and the datasheet gives the exact value. A few milliamps is plenty for an indicator LED. pins have current limits per pin and for the whole chip, listed in the datasheet.
Common traprunning pins at their absolute maximum current, which the datasheet treats as a limit to avoid, not a target.
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Resources for this stage
Start with
- BookFreeAll About Circuits Textbook
All About Circuits · Beginner
Free: read the DC volume chapters on Ohm's law, series and parallel circuits and dividers
Practise
Keep handy
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