Zener Shunt Regulator
Series resistor window and worst-case zener dissipation for a simple shunt regulator.
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The engineering
The design squeeze in one line: R must be small enough that at the lowest input and full load the zener still gets its minimum keep-alive current — but every ohm you remove pours more current into the zener when the input is high and the load unplugs. This card sizes R at the largest legal value and then reports the price at the opposite corner.
The worst case assumed here is brutal and real: input at maximum, load disconnected, all the resistor current shunted into the diode. If that milliwatt number embarrasses your 500 mW zener, either the input range is too wide for a shunt regulator or the load belongs behind a series regulator using this zener only as its reference.
Where this math comes from
Clarence Zener never built a diode — his 1934 paper explained dielectric breakdown as quantum tunneling of electrons through the band gap. When Bell Labs' silicon junction diodes of the early 1950s showed a sharp, stable, non-destructive reverse breakdown, the effect (and below ~5 V, the mechanism) carried his name.
For two decades the zener-plus-resistor was *the* regulated supply of small electronics, and it still owns the corners: bias rails, clamps, crowbar references — anywhere a three-terminal regulator is too much part for too little job. Bandgap references (Widlar, 1971) took over precision; the shunt zener kept simplicity.
- 1934Clarence ZenerQuantum-tunneling theory of electrical breakdown in solids.
- 1954Bell Labs / early silicon eraSilicon junction diodes show stable reverse breakdown — 'zener' diodes, circa.
- 1971Robert WidlarBandgap reference — precision moves on, zeners keep the simple jobs.
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