What Is a Zener Diode?
A Zener diode, unlike a normal diode, conducts in a controlled way in the reverse direction at a specific voltage (the Zener voltage, Vz). This property makes it ideal for voltage regulation and producing a reference voltage.
How It Works
When the reverse voltage reaches the Zener voltage, the diode conducts and holds the voltage across it almost constant. Even if the load current or input voltage varies within limits, the output voltage stays stable. That's why a Zener diode is also called a "voltage clamp."
Difference From a Normal Diode
| Property | Normal diode | Zener diode |
|---|---|---|
| Working direction | Forward bias | Reverse bias (breakdown) |
| Purpose | Pass current one way | Hold voltage steady |
| Reverse voltage | Causes damage (punch-through) | Normal operating region |
Voltage Regulator Circuit and the Series Resistor
A Zener is always used with a series resistor (R) that limits the current. For example, to produce a 5.1 V reference from a 12 V source with ~10 mA through the diode:
R = (Vin − Vz) / I = (12 − 5.1) / 0.01 = 690 Ω (nearest standard: 680 Ω).
This calculation rests directly on Ohm's law.
Uses
- Voltage regulation and reference voltage generation
- Protection against overvoltage (clamping)
- Waveform shaping and voltage shifting
Common Mistakes
- No series resistor: without a resistor, the Zener burns out from overcurrent.
- Exceeding the power rating: drawing too much current from a 0.5 W Zener overheats and destroys it; check the power with P = Vz × I.
- Ignoring load current: if the load draws too much current, the Zener stops regulating.
Frequently Asked Questions
Which way is a Zener diode connected?+
What's the difference between a Zener diode and a regulator IC?+
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