Zener Diodes and Voltage Regulation

Past the breakdown voltage, reverse current rises almost vertically while the voltage barely changes. For an ordinary diode that is destruction; a Zener is built to survive it, and that near-vertical characteristic is what a voltage reference needs.

Breakdown itself is non-destructive. Heat is what kills the device, so the current must always be limited by an external resistor.

Around 5.6 V the two coefficients cancel, which is why precision voltage references cluster at that value.

A series resistor drops the surplus voltage and the Zener clamps what remains. The resistor's current splits between the load and the Zener, so the Zener absorbs whatever the load does not take — meaning it dissipates most at no load.

1. 1At minimum input and maximum load, R_s must still leave the Zener its minimum operating current.
2. 2At maximum input and no load, R_s must not let the Zener exceed its power rating.
3. 3Those two conditions pull in opposite directions.
4. 4If no resistor value satisfies both, the design needs a different topology — not a bigger Zener.

• The knee is steep but not vertical. Its slope is the Zener impedance r_z, a few ohms to tens of ohms, so load changes move the output.
• The surplus power is always burned as heat, so efficiency is poor and worst at light load.
• The breakdown voltage itself has a tolerance, typically ±5%, and drifts with temperature away from 5.6 V.

The fix is to stop asking the Zener to supply the load. Let it set a reference at a few milliamps, and put a pass transistor in the current path with its base following that reference. The output becomes V_z − V_be, and the Zener's own current barely varies, so r_z stops mattering.

That circuit is the ancestor of every three-terminal regulator IC. Wrapping an op-amp around the same idea removes almost all the remaining sag.

As an overvoltage clamp the Zener sits idle and dissipates nothing until a spike arrives — so efficiency and impedance are irrelevant. That is why Zeners and their faster relatives, TVS diodes, appear on almost every exposed input of a real product. It is the same device judged against a completely different requirement.