PAGE 042 OF 150 · POWERING LEDS
Why Voltage Alone Is Not Enough
Small voltage changes cause big current changes in an LED.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
A steep curve
An LED's current rises very quickly once voltage passes Vf. A change of only a few tenths of a volt can double or triple the current.
Heat makes it worse
As the chip warms its forward voltage falls, so at a fixed voltage the current grows, which creates more heat. This feedback can end in thermal runaway and a burned-out LED.
The fix
A series resistor or constant current driver gives current a stable limit. The resistor makes the current change much less with small changes in supply voltage.
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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