The Zener Diode: A Steady Output from a Shaky Input
Where the output stops as the input rises
The horizontal axis is the input voltage, the vertical axis is the output voltage, and the faint diagonal is the line where the output equals the input. As you raise the input, the output first climbs along that diagonal, but past Vz, marked by the dashed line, the Zener breaks down and the output goes flat. Slide the operating point to find where the output stops.
Reverse breakdown, on purpose
In B1, almost no current flowed through a reverse-biased diode. Yet raise the reverse voltage high enough and at some point a large current suddenly begins to flow. An ordinary diode can be destroyed here, but a Zener diode is built to operate safely in exactly that breakdown region, so it is a diode used backward on purpose. Low-voltage ones break down mainly by the Zener effect and high-voltage ones mainly by avalanche breakdown, but they are used the same way.
Why the voltage stays fixed
In the breakdown region the reverse curve stands almost vertical. Whether the current is small or large, the voltage across it barely moves from around the breakdown voltage Vz, so the Zener looks like a voltage source that delivers a constant voltage regardless of the current through it. The dynamic resistance rz, coming from the curve's very slight slope, sets the tiny variation that remains.
The regulator: a shunt circuit
Put a single series resistor between the input and the load and connect the Zener across the load. The series resistor takes up the difference between the input and Vz. Even if the input wobbles or the load changes, the current the Zener takes rises and falls by itself and the output stays held at Vz; this is the simplest constant-voltage circuit, used wherever a reference voltage is needed. But a minimum current must always flow to keep the Zener in breakdown, so if the load draws too much current, the output drops below Vz.
Back to the first screen
While the input was low, the output rose right along the faint diagonal. But the moment it passed the breakdown voltage the curve bent and went flat, and after that, however high you raised the input, the output stayed at Vz. That flat line is the constant voltage. Thanks to the nearly vertical breakdown curve, the voltage barely moves even when the current changes a lot, and the series resistor absorbs the variation of the input so that only a steady Vz reaches the load. This is how the reverse region of B1 becomes the simplest tool for a reference voltage.
What comes next
So far you have seen what a single diode can do. In the next unit you link four diodes like a bridge to see full-wave rectification, which flips even the negative half-cycles of AC to positive. Then a capacitor smooths that bumpy pulsation into something close to steady DC. Combine the rectification of B1 with the regulation of B3 and the front end of a power supply is complete.