Master the Basics: What is a Zener Diodes

Zener diodes are special-purpose diodes designed to conduct reverse current once a precise reverse voltage—called the Zener breakdown voltage—is reached. Unlike regular diodes, which block reverse current, Zener diodes are used to stabilize voltage and create precise reference voltages in circuits.

What Makes Zener Diodes Unique?

When reverse voltage across a Zener diode reaches its Zener voltage (e.g., 5.1V), the diode enters breakdown mode and allows current to flow in reverse. The voltage across the diode remains nearly constant at this Zener voltage, regardless of changes in the input (as long as current is within limits).

Symbol Tip: The Zener symbol looks like a diode symbol, but with bent edges on the cathode line. Don’t confuse it with the Schottky diode, which has curled ends.

Simple Zener Voltage Regulator Circuit

A basic voltage regulator can be built using:

• A Zener diode connected in reverse

• A series resistor to limit current

When the input voltage exceeds the Zener voltage, the diode maintains a steady output voltage. For example, using a 5.1V Zener:

• If input is below 5.1V, output equals input (Zener is off)

• If input exceeds 5.1V, output clamps to 5.1V

The resistor drops any extra voltage, protecting the Zener from overcurrent.

Limitations of Zener Regulation

This setup has practical limitations:

• High input voltage leads to large voltage drop across the resistor

• That drop results in power dissipation as heat

• The resistor must be rated to handle this power

• Zener diodes are not suitable for high-current loads

Example:

• Input = 6V, Zener = 5.1V, Resistor = 100Ω

• Drop across resistor = 0.9V → Current = 9mA

• The load can draw up to 9mA; excess current goes through the Zener

If the load tries to draw more current:

• Voltage across the resistor increases

• Voltage across the Zener drops

• Zener exits breakdown mode and output becomes unstable

When (and Why) to Use a Zener Diode

Zener diodes are not ideal for supplying power to devices. They are best used as a reference voltage source for:

• Microcontrollers

• Analog sensors

• Comparator circuits

For instance, to generate a 3.3V reference from a 5V supply, use a 3.3V Zener and a resistor.

TVS Diodes: Zener's Protective Cousin

TVS (Transient Voltage Suppression) diodes are related to Zeners but designed to protect circuits from voltage spikes or electrostatic discharge.

TVS diodes don’t conduct in normal operation. But if voltage spikes past a set threshold, they divert the surge to ground—like a fast-reacting electrical shield.

Symbol Tip: TVS symbols often resemble two Zener diodes back-to-back (cathode-to-cathode).

Example Use Case:

• A 5V TVS diode on a Raspberry Pi protects the board from voltage spikes on the USB power line. It doesn’t regulate—it only clamps unexpected surges.

Key Takeways

Zener diodes are essential tools for generating precise voltage references, not for powering high-current loads. Use them wisely for tasks like reference generation and spike suppression in low-current, low-power sections of your circuit. For surge protection, consider a TVS diode—a powerful, responsive cousin of the Zener.