An AC circuit simulator with RMS meters and a scope
Mokxi is a free online AC circuit simulator that runs in your browser, with nothing to install and no account needed. A 60 Hz sine, 10 V peak, drives a resistor and a capacitor in series. Three AC meters read the current and each part’s voltage in RMS, and a two-channel scope draws the source and the capacitor, so you can see why AC voltages do not simply add.
- 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 RMS current and voltage on true RMS AC meters
- Change the frequency and watch a capacitor’s reactance fall
- See the phase shift between the source and the capacitor on the scope
- Check that the part voltages add like the sides of a right triangle
- Swap the capacitor for an inductor, or add one for an RLC circuit
- Rectify the AC with diodes and smooth it with a capacitor
What the running circuit shows
Two voltages that add up to more than the source.
The generator swings 10 V either way of ground, 60 times a second. An AC meter reads RMS, the DC voltage that would heat a resistor the same, which for a sine is the peak divided by √2: 7.07 V. At 60 Hz the 1 µF capacitor has a reactance of 1 / (2πfC), about 2.65 k, and with the 1 k the whole circuit is about 2.83 k. In the engine the current reads about 2.50 mA, the resistor about 2.50 V and the capacitor about 6.60 V.
2.50 V plus 6.60 V is 9.10 V, more than the 7.07 V going in, and nothing is wrong. The resistor’s voltage is in step with the current and the capacitor’s lags it by a quarter cycle, so they add like the two sides of a right triangle: √(2.50² + 6.60²) is 7.06 V. The scope shows the timing: the capacitor’s wave peaks at about 9.34 V, a little under a millisecond after the source’s. The current through the resistor peaks at about 3.5 mA, about 3.3 ms ahead of the source, where the arithmetic says 3.2 ms.
Turn the generator up to 600 Hz and the reactance falls tenfold, to 265 ohms: the current rises to about 6.84 mA, the resistor takes about 6.84 V and the capacitor keeps only about 1.81 V. At 50 Hz, the mains frequency in much of the world, the current is about 2.12 mA and the capacitor takes about 6.73 V. A capacitor passes AC more easily the faster it changes, which is the idea behind every filter and coupling capacitor.
How AC is modeled
A solve in time, and meters that average it.
The circuit is solved step by step in time, the way a SPICE transient run is, so the sine, the phase shift and any distortion come out of the parts, not out of a formula. The meters are true RMS on their AC ranges, with any DC offset left out, over a quarter-second window: 15 whole cycles at 60 Hz. The volts ranges have a 10 megohm input and the amps range a 1 ohm shunt, so the meters load the circuit as real ones do.
What it leaves out: the meters sample at 4 kHz, so above about a kilohertz an AC reading is under-sampled and reads low; use the scope there, as you would on a real bench. Below a few hertz a quarter second holds only part of a cycle and the reading is off. There is no mains outlet: the generator is a bench signal generator, and the circuit stays at a safe 10 V.
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.
Why don’t the voltages in an AC circuit add up?
They do, as phasors. Across a resistor the voltage is in step with the current; across a capacitor it lags by 90 degrees. RMS readings lose the timing, so 2.50 V and 6.60 V add to √(2.50² + 6.60²) = 7.06 V, not 9.10 V.
What is RMS?
Root mean square: the DC voltage that would heat a resistor the same as the AC does. For a sine it is the peak divided by √2, so a 10 V peak sine is 7.07 V RMS, which is what an AC meter shows.
Can I simulate an RLC circuit?
Yes. Add an inductor from the parts bin, or open the RLC circuit simulator, which runs a series RLC through resonance.
More simulators
Each one opens on a circuit that is already running.
Put AC through a capacitor
Open the circuit, change the frequency and watch the meters move. Nothing to install, and no account needed.