B3 · Diodes

The Zener Diode: A Steady Output from a Shaky Input

Pushed deep into reverse, a diode breaks down backward and holds a voltage. Raise the input voltage yourself and watch the output stop following past a point and settle at one value.

Find where the output stops even as the input rises

The horizontal axis is the input voltage, the vertical axis the output. Raise the input and at first the output rises right along with it. But past a certain voltage the Zener breaks down and the output goes flat.

Input voltage VinVin = 2.0 V

Output voltage Vout

Vout = Vin

Still below the breakdown voltage. The Zener is nearly blocked, so the output just follows the input.

Output follows the input

Reverse breakdown, on purpose

In B1 the reverse diode barely conducted. But raise the reverse voltage high enough and at some point a large current suddenly begins to flow. An ordinary diode might be destroyed, but a Zener diode is built to operate safely in exactly that breakdown region. It is, in effect, a diode used backward on purpose.

Why the voltage stays fixed

In the breakdown region the reverse curve is almost vertical. Whether the current is small or large, the voltage across stays nearly unchanged around the breakdown voltage Vz. In other words, the Zener looks like a voltage source that delivers a constant voltage regardless of the current through it. Only the very small dynamic resistance rz, set by the slope of the curve, decides the tiny remaining variation.

The regulator: a shunt circuit

Put a single series resistor between the input and the load, and place the Zener in parallel with the load. The series resistor absorbs the difference between the input and Vz, dropping exactly that much voltage. When the input wavers or the load changes, the Zener raises and lowers the current it takes on its own to hold the output at Vz. This is the simplest voltage-regulator circuit, used wherever a reference voltage is made.

ObserveVout = Vz (V > Vz)

In breakdown the output is fixed at Vz.

ChooseIR = (Vin - Vz) / ?

The series-resistor current is the input-minus-Vz difference over R.

Fill inIR = Iz + ?

The resistor current splits between the Zener and the load.

On your ownΔVout ≈ rz ?

The tiny remaining output variation is set by the small dynamic resistance.

Back to the first screen

When the input was low, the output rose right along the diagonal. But the moment it passed the breakdown voltage the curve bent flat, and after that, however high the input climbed, the output stayed at Vz. That flat line is the regulated voltage. Thanks to the nearly vertical breakdown curve the voltage barely moves even as the current changes a lot, and the series resistor absorbs the input swings so the load sees only a steady Vz. The reverse region of B1 thus becomes the simplest tool for a voltage reference.

A Zener diode is a diode operated on purpose in reverse breakdown. Past the breakdown voltage Vz, the voltage across it stays nearly constant at Vz even as the current changes greatly. Place it in parallel with the load behind a series resistor (a shunt regulator) and the output is held at Vz despite input swings: a regulated voltage. The small dynamic resistance rz sets the tiny remaining variation.

What comes next

So far you have seen what a single diode does. The next unit weaves four diodes into a bridge that flips even the negative half-cycles of AC into positive: full-wave rectification. Then a capacitor smooths that bumpy pulsation into steady DC. Combine the rectification of B1 with the steadiness of B3 and the front end of a power supply is complete.