Zener Diode
Diode
A diode designed to conduct in REVERSE at a precise breakdown voltage, holding that voltage steady. Used as a simple voltage reference or clamp.
Schematic symbol
Zener diode
How zener diode appears in a circuit diagram.
Types & variants
Key specs
Zener voltage (Vz)
VThe regulated reverse breakdown voltage (e.g. 3.3, 5.1, 12V).
Power rating
WMax dissipation: Pz = Vz x Iz. Common 0.5W and 1.3W.
Tolerance
%How close Vz is to the marked value (±5% typical).
Test current (Izt)
mACurrent at which Vz is specified.
Markings
Banded end = cathode (same as normal diodes). Vz printed or coded on body.
Standard values
2.4, 3.3, 5.1, 6.2, 9.1, 12, 15V... (1N4728-1N4764 series, BZX55/BZX79).
How to choose
Pick Vz for your reference/clamp voltage. Size the series resistor so Iz stays within rating across load range. Use TVS instead for surge protection.
Pinout & package
Axial — band marks the cathode (same as any diode)
Check first: In regulation a Zener is reverse-biased: cathode goes to the more-positive side.
2 leads, POLARISED. In a regulator the cathode goes to + via a series resistor. DO-35/DO-41, SOD SMD.
Example circuits
- Shunt voltage reference (R + zener)
- Over-voltage clamp on an input
- Setting a transistor's bias voltage
Common failures
Shorts or drifts under overpower; noisy as a reference. Series resistor often burns with it.
How to test
Diode mode shows a normal diode forward. To confirm Vz, bias it through a resistor and measure the voltage across it.
Substitutes
Same Vz and equal/higher wattage. For protection prefer a TVS of matching standoff voltage.