PAGE 044 OF 150 · POWERING LEDS
Reverse-Polarity Protection
A series diode guards against a backwards battery.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Why protect
Connecting a supply the wrong way can damage LEDs, drivers and other parts.
Methods
- Series diode: simple, but drops about 0.7 V and wastes power.
- Parallel diode across the supply with a fuse: it shorts a reversed supply and blows the fuse.
- P-channel MOSFET: very low loss, used on modern boards.
- Keyed connectors, which make reversing physically impossible.
For LEDs
A small diode in anti-parallel with an LED clamps reverse voltage to about 0.7 V.
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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