A Zener diode simulator on a two-channel scope
Mokxi is a free online Zener diode simulator that runs in your browser, with nothing to install and no account needed. A signal generator swings a 50 Hz sine 10 V either way of ground, through 1 k into a 5.1 V Zener diode. The scope draws the input and the output together, so you see the Zener break down on one half of the wave and conduct like a plain diode on the other.
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What you can do here
In the circuit above, or in the editor it opens.
- See the breakdown voltage and the forward drop on one trace
- Pick any Zener from 3.3 V to 12 V, the 1N4728A to the 1N4742A
- Change the series resistor and watch the clip level move
- Read the Zener’s current, forward and reverse
- Turn it into a regulator with a DC supply and a load
- Turn the diode round and see why the band matters
What the running circuit shows
Flat on top, a diode drop at the bottom.
On the way up, the Zener does nothing until the output reaches its breakdown voltage; then it conducts backwards and the 1 k takes the rest. In the engine the top of the wave is clipped flat at about 4.76 V while the input goes on to 10 V. On the way down the Zener is an ordinary diode the right way around, and clips at about -0.71 V with about 9.3 mA through it.
Why 4.76 V and not the 5.1 V on the part? A Zener’s voltage is specified at its test current, 49 mA for the 1N4733A, and through 1 k it gets about 5.2 mA at most, so it sits lower down its knee. Make the resistor 220 ohm and the current climbs to about 23 mA and the clip to about 4.91 V. That is the first thing to know about designing with one: give it enough current.
Turn the generator down to 4 V and the top passes through untouched, since the Zener never reaches breakdown. Change the model to the 12 V 1N4742A and the whole positive half gets through to 9.99 V; change it to the 3.3 V 1N4728A and the clip falls to about 2.58 V.
How the Zener is modeled
A SPICE diode with a breakdown, fitted to the datasheet.
Each of the fifteen 1N47xxA models is the standard SPICE diode with a reverse breakdown, written from the three numbers the datasheet leads with: the Zener voltage, the test current it is given at, and the impedance there. So a 1N4733A reads 5.100 V at 49 mA, less at a few milliamps, and about 7 mV more for every extra milliamp near its test current. The forward side is a plain silicon junction, solved with the rest of the circuit at every step.
What it leaves out: temperature, which moves a real Zener’s voltage by a few millivolts a degree, and power. A 1N4733A is rated for 1 W, and nothing here burns out if you give it more; work out the voltage times the current yourself before you build it. The scope’s probe has no capacitance.
Questions
Is it free?
Yes. The circuit on this page, every part in it and the editor it opens in are on the free plan, with no card.
Do I need an account?
No. The circuit runs the moment the page does, and the editor opens without one. A free account is only for saving a project to the cloud and sharing it by link.
Which way around does a Zener diode go?
Band toward the positive side, so it is reverse biased in normal use: it only conducts once the voltage across it reaches its breakdown. The other way around it is an ordinary forward diode and holds the node at about 0.7 V.
Why is my Zener voltage lower than its rating?
Its rated voltage is at its test current, tens of milliamps for the common 1 W parts. With a large series resistor it gets only a few milliamps and sits lower; here a 5.1 V part on 1 k clips at about 4.76 V.
Can I plot a Zener diode’s I-V curve?
Yes. Drive it from the generator with a triangle wave through a resistor, put one scope channel across the Zener and one across the resistor, and switch the scope to its XY view.
More simulators
Each one opens on a circuit that is already running.
Clip a sine wave
Open the circuit, watch the top of the wave go flat, then change the Zener. Nothing to install, and no account needed.