A diode simulator: silicon, Schottky and an LED on one supply
Mokxi is a free online diode simulator that runs in your browser, with nothing to install and no account needed. A 5 V supply, two 330 ohm resistors, a 1N4148 silicon diode in one branch and a red LED in the other, and a meter across the diode. Change the diode’s model or turn it around, and the drop and the current change the way its datasheet curve says.
- 455parts on the bench
- 43boards running now
- 1.00xreal time, on every board
What you can do here
In the circuit above, or in the editor it opens.
- Read a diode’s forward drop on a meter, at the current the circuit sets
- Swap between the 1N4148, the 1N4007 and three Schottky diodes
- Turn the diode around and watch it block
- Rectify an AC signal, half wave or with a bridge
- Clamp a motor’s or a relay’s coil with a flyback diode
- Hold a voltage steady with a Zener
What the running circuit shows
The same resistor, three different drops.
Each branch is 330 ohm into a diode, across the same 5 V. In the engine the 1N4148 drops about 0.74 V and passes about 12.9 mA. Change its model to the 1N4007 rectifier and it drops about 0.68 V; to the 1N5819 Schottky, about 0.36 V, so about 14.1 mA flows. The red LED in the other branch drops about 1.65 V and passes about 10.2 mA, which is why an LED’s resistor is worked out differently from a plain diode’s.
Turn the 1N4148 around and it blocks: the current falls to nothing and the meter reads the full 5.00 V across it, because with no current the resistor drops nothing. That is the one-way valve, and it is how a diode protects a board from a battery put in backwards.
How the diode is modeled
The Shockley equation, not a fixed 0.7 V.
Each diode is the Shockley equation with a series resistance, solved with the rest of the circuit, so no forward drop is written down anywhere: the drop is what the curve gives at the current the circuit passes. The cards are fitted to datasheet curves, and a Schottky drops a third of a volt where a 1N4148 drops two thirds because its junction is different, not because a table says so. The operating point agrees with ngspice on the same card to better than a tenth of a percent.
What it leaves out: junction capacitance and reverse recovery, so every diode switches instantly and a 1N4148 and a 1N4007 differ only in their drop. Every junction sits at 27 C, there is no reverse breakdown on these cards (the Zener is a part of its own), and nothing stops a small diode being asked for more current than it would survive.
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.
What is the forward voltage of a diode?
It depends on the current and the kind of diode. In the circuit above, at about 13 mA, a 1N4148 drops 0.74 V, a 1N4007 0.68 V and a 1N5819 Schottky 0.36 V, and a red LED at 10 mA drops 1.65 V. Mokxi works it out from each diode’s curve rather than using a fixed figure.
Can I simulate a Zener diode?
Yes. The Zener is its own part, conducting backwards at the voltage you choose, and the Zener regulator page holds a load at that voltage as the supply changes.
Can I plot a diode’s I-V curve?
Yes. Open Analysis in the editor’s more menu and use the DC sweep to step the supply and plot the current. The LED I-V curve page does it for an LED, and the same sweep works with a diode in its place.
More simulators
Each one opens on a circuit that is already running.
Turn a diode around
Open the circuit, change the model and read the meter. Nothing to install, and no account needed.