Zener diode
The 1 W 1N4728A to 1N4742A family. Reverse biased it blocks like any other diode until it reaches its own voltage, and then it holds it there.
That is the whole part, and it is why the first voltage regulator a class builds is a resistor from the supply and a Zener to ground.
Pins
| Pin | What it does |
|---|---|
A |
Anode. On a regulator this goes to ground. |
C |
Cathode, the banded end. On a regulator this goes to the positive side. |
A Zener goes in the opposite way around from a rectifier, and that is the single most common way to wire one in wrong. The band faces the supply.
Properties
model: one of fifteen, 1N4728A (3.3 V) through 1N4742A (12 V), with
1N4733A (5.1 V) the default because it is the one in every kit.
vz: a breakdown voltage of your own, in volts, which overrides the model card.
izt: the current in milliamps that vz is quoted at; 5 mA if you leave it.
A vz you type has no dynamic resistance behind it, because a datasheet's
Zzt is a third number and the panel does not ask for it. A vz Zener therefore
holds its voltage better than any real one does. The named models do not.
What the model gets right
The same Shockley junction every diode in Mokxi is, plus the reverse breakdown
term SPICE calls BV, IBV and NBV:
I = Is (exp(Vj / (N Vt)) - 1) - Ibv exp(-(Vj + BV) / (Nbv Vt)) Vd = Vj + I Rs
Every card is built from the three numbers a datasheet leads with (Vz at
Izt, with Zzt ohms of dynamic resistance there) and the two that fall out
of them:
Rs = Zzt - Vt / Izt the part of Zzt the sharp knee does not give
BV = Vz - Izt Rs so the terminal voltage at Izt is Vz exactly
which is how a manufacturer builds a Zener SPICE model. A few of the fifteen:
| model | Vz |
Izt |
Zzt |
Rs the model uses |
BV |
|---|---|---|---|---|---|
| 1N4728A | 3.3 V | 76 mA | 10 Ω | 9.660 Ω | 2.566 V |
| 1N4733A | 5.1 V | 49 mA | 7 Ω | 6.472 Ω | 4.783 V |
| 1N4735A | 6.2 V | 41 mA | 2 Ω | 1.369 Ω | 6.144 V |
| 1N4740A | 10 V | 25 mA | 7 Ω | 5.965 Ω | 9.851 V |
| 1N4742A | 12 V | 21 mA | 9 Ω | 7.768 Ω | 11.837 V |
So the two things a regulator lesson measures both come out right: the voltage
at the test current is the datasheet's exactly, and the voltage moves with the
load at the datasheet's own slope. A 1N4733A on 12 V through 220 ohms sits at
4.978 V with nothing on it and 4.943 V with a 1 k load, and the 35 mV between
them is Zzt times the 4.9 mA the load took. The operating point agrees with
ngspice 42 on the same model card to better than a percent.
What it does not model
Zzk, the knee impedance. A datasheet quotes hundreds of ohms at a
milliamp where Zzt is single figures. One exponential cannot be that soft down
there and still leak nanoamps below the knee, so this model keeps the leakage
honest and lets the knee come in sharper than a real part's: a 1N4733A here is
at about 4.75 V at a milliamp where a real one is near 4.9. The plateau, which is
where a Zener is used, is right.
The forward direction. The series resistance that carries Zzt sits in the
forward path too, so a Zener conducting forwards drops more than a real one at
high current: about 2.6 V at 200 mA on a 1N4728A against the datasheet's 1.2 V
maximum. If you want a rectifier, use the diode.
No power rating and no temperature. These are 1 W parts whose voltage drifts with heat, and whose temperature coefficient changes sign somewhere near 5 V. This model will pass an amp through a 1N4728A at 27 °C and tell you it is fine.
No junction capacitance, so no noise (which is what a Zener is famous for on a bench and is half the reason people use a band-gap reference instead), and no reverse recovery.
Common mistakes
Wiring it in like a rectifier, band away from the supply. Then it is just a forward-biased diode and the output is 0.7 V.
Forgetting the series resistor. A Zener straight across a supply is a short circuit with a voltage on it; here it will pass amps and report them, and on a bench it lasts about a second.
Choosing the resistor for no load. The resistor has to pass the load's current and enough to keep the Zener in breakdown (a few milliamps at least) at the lowest supply voltage you expect. Size it there and the Zener takes the surplus when the load lets go.
See it in action
Zener regulator is this part doing its one job, with the load line written out in the sketch: the resistor's straight line, the Zener's near-vertical one, and where the two cross. Open it at /templates, and read how faithfully a part behaves for the model card of every device in the simulator and where each number came from.