PAGE 010 OF 150 · LED BASICS
Why LEDs Are Polarised
Reverse connection blocks current and may stress the chip.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
One-way by design
An LED is a diode. Its junction passes current easily from anode to cathode but blocks current the other way, so the part only lights when connected correctly.
Reverse voltage limits
Blocking is not unlimited. Most small LEDs have a reverse voltage rating of only around 5 V. Beyond that the junction can break down and be damaged, which matters when driving LEDs from 9 V, 12 V or alternating current.
Protecting an LED
- Place an ordinary silicon diode in series, which blocks reverse voltage.
- Place a diode in anti-parallel across the LED to clamp reverse voltage to about 0.7 V.
- For AC, use a rectifier or two LEDs wired in opposite directions.
Practical habit
Identify anode and cathode before powering up, and mark polarity on breadboard layouts and boards.
REMEMBER
An LED is a diode, so current flows through it in one direction only.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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