While standard diodes block reverse current, a Zener diode is intentionally heavily doped to break down and conduct in reverse at a precise, predetermined voltage called the Zener voltage. Once this threshold is hit, the voltage drop across the diode remains remarkably stable even if the reverse current changes. They are primarily used to generate stable reference voltages or to clamp voltage spikes to protect sensitive circuits.
Put a 5.1 volt Zener diode across the input to clamp any transients before they hit the microcontroller.
Always use a series current-limiting resistor with a Zener diode, or it will draw too much current and burn itself up when it reaches its breakdown voltage.
Related terms
Diode
A two-terminal semiconductor component that restricts electrical current to flow in only one direction.
Voltage divider
A simple passive circuit that scales down an input voltage to a lower, predictable output level.
Clamper
A circuit that shifts an entire waveform up or down to a specific DC voltage level.