Breakdown — on purpose
Back in guide 1, a diode in reverse bias was the closed valve: push voltage the wrong way and almost no current flows. But "almost" is the key word. Crank the reverse voltage high enough and every diode eventually gives way — at some reverse voltage the I-V curve bends sharply downward and current floods through. For an ordinary rectifier that breakdown is a catastrophe you design to avoid. The Zener diode flips the script: it is a diode built to break down at a precise, low, repeatable reverse voltage — its Zener voltage Vz — and to do it gently, without self-destructing, as long as you limit the current.
In forward bias a Zener behaves like any silicon diode, with the familiar ~0.7 V forward drop you met earlier — nobody uses it that way. The interesting half is the reverse curve. Below Vz it blocks like a good diode (just a tiny leakage). Right at Vz the curve goes nearly vertical: the voltage across the part barely moves even as the current swings over a wide range. That near-vertical wall is the whole trick — a component that fights to hold a fixed voltage across itself, almost regardless of how much current you send through it. (Two different physics produce it — quantum tunnelling below about 5 V, called true Zener breakdown, and avalanche multiplication above about 6 V — but the part is called a Zener either way, and both are non-destructive when current-limited.)
The shunt regulator
Put that near-vertical wall to work. Feed a messy, higher voltage through a series resistor Rs into a Zener tied to ground, and take your output from the junction between them. The resistor drops whatever is left over; the Zener pins the output at Vz. Picture a pressure-relief weir on a reservoir: the resistor is the inlet pipe, and the Zener is an overflow that dumps any excess to ground the instant the level tries to climb past Vz. The output sits at a steady Vz even while the input sloshes around above it. This is a shunt regulator — "shunt" because the regulating part sits in parallel with the load and bleeds off the surplus.
Rs Vout = Vz (held steady)
Vin >--[====]--+----------------+----> Load (draws Iload)
| |
(Zener) [Load]
Vz, reverse; |
sinks Iz |
GND >----------+----------------+---->
KCL : I_Rs = Iz + Iload
R : Rs = (Vin - Vz) / (Iz + Iload)The arithmetic is just Ohm's law plus the current law. The resistor carries the sum of two currents — the load's and the Zener's own — so I_Rs = Iz + Iload, and the voltage across it is the leftover Vin - Vz, giving Rs = (Vin - Vz) / (Iz + Iload). Here is the heart of how it regulates: when the load suddenly draws more, less current is left for the Zener, but as long as some still flows, the Zener stays on its vertical wall and Vz barely budges; when the load draws less, the surplus simply diverts into the Zener instead. The Zener silently soaks up the difference, holding the output steady — that is the whole job.
Sizing the resistor, respecting the limits
Picking Rs is a worst-case game with two opposite corners to survive. If Rs is too big, then on the worst day — lowest input and hungriest load — so little current reaches the Zener that it falls off its wall and stops regulating. If Rs is too small, then on the other worst day — highest input and no load at all — the entire current piles into the Zener and its power dissipation cooks it. A good design clears both corners with margin. Let us walk a real one.
- Set the target and pick the part. You want roughly 5 V, so choose a 5.1 V Zener (Vz = 5.1 V). Say the input is a smoothed-but-rippling supply that wanders between 11 V and 13 V, and the load draws anywhere from 0 to 20 mA.
- Decide a minimum Zener current to stay in regulation — read it from the datasheet, often a few mA; take Iz_min = 5 mA.
- Size Rs for the hungriest corner (Vin lowest, load highest): Rs must still pass Iload_max + Iz_min. So Rs <= (11 - 5.1) / (0.020 + 0.005) = 5.9 / 0.025 ≈ 236 ohm. Pick a standard 220 ohm, comfortably under the limit.
- Check the cruellest power corner (Vin highest, load zero): all the current goes into the Zener. I = (13 - 5.1) / 220 ≈ 36 mA, so Pz = Vz times Iz = 5.1 times 0.036 ≈ 0.18 W. A common 0.5 W Zener handles that with room to spare.
- Don't forget the resistor itself: it drops (13 - 5.1) = 7.9 V at 36 mA, burning 7.9 times 0.036 ≈ 0.28 W — so specify a 0.5 W resistor, not a tiny 0.25 W one.
Only a crude reference
That near-vertical wall is not perfectly vertical. The Zener has a small slope — its dynamic or Zener resistance rz, typically tens of ohms — which acts like a built-in output resistance right at the output node. So the output is not bolted to Vz; it drifts a little. When the load current changes by ΔI, the Zener current changes by the same amount and the output moves by ΔV = rz times ΔI — that is the regulator's loading showing through. And the input's leftover ripple from the reservoir cap is not erased, only attenuated: it divides down by roughly rz / (Rs + rz). With Rs = 220 ohm and rz = 20 ohm, a 1 V input ripple shrinks to about 1 times 20 / 240 ≈ 83 mV — much better, but far from clean.
Worse, Vz drifts with temperature. Low-voltage Zeners (below ~5 V) have a negative temperature coefficient, high-voltage ones a positive one, and the two happen to cancel near 5-6 V — which is why a 5.6 V Zener is the most temperature-stable choice. Even so, a plain Zener is a crude voltage reference: a tempco of hundreds of parts-per-million per degree, audible-band noise from the breakdown process, and a Vz that varies part-to-part. When you need a reference you can trust to a fraction of a percent over temperature, you reach for a bandgap reference, which cleverly sums two voltages with opposite tempcos to land near zero drift. The Zener's virtue is not precision; it is being one resistor and one cheap part away from a usable, roughly-right voltage.
From regulating to clamping
Run the same part in a different spirit and you get protection instead of regulation. In the shunt regulator the Zener conducts all the time to set a steady level. As a clamp it does nothing at all in normal operation and only wakes up when the voltage tries to exceed Vz — a guardrail, not a road. Wire a Zener from a signal or supply node to ground with Vz chosen just above the normal operating voltage: as long as things behave, it sees less than Vz and draws only leakage; the moment a spike pushes the node past Vz, the Zener snaps onto its wall and shunts the surge to ground, pinning the node near Vz and saving whatever fragile chip sits downstream.
This is the everyday job of overvoltage and ESD protection: a dedicated clamp — a Zener, or its faster cousin the transient-voltage-suppressor diode — standing across an input, ready to catch the static zap or the inductive kick. It is the same family of idea as the freewheeling diode you will meet in the next guide, which clamps the spike when current in a coil is suddenly cut. And it shades into the AC clamps and clippers of guide 5, where ordinary diodes — not breakdown — reshape a waveform by pinning it to a chosen level. The thread through all of them: a diode that stays out of the way until the voltage crosses a line, then conducts hard to hold that line.
So the Zener earns its keep twice over from the very same physics. Pointed downstream of a resistor and conducting steadily, it is a humble voltage reference and shunt regulator — crude, wasteful, but instantly useful. Standing quietly across a node and conducting only in emergencies, it is a clamp that absorbs the spikes that would otherwise kill the rest of your circuit. Hold onto both faces: in the next guide the diode family widens again into LEDs, freewheeling diodes, and signal clippers, and the same one-way, threshold-crossing behaviour will explain all of them.