Circuit Simulator Online: Analog & Digital, DC & Transient

Build a schematic and watch live current flow, voltages, and waveforms via an in-browser MNA solver.

Draw a schematic, drop in resistors, sources, diodes, transistors, an op-amp or logic gates, and the simulator assembles a Modified Nodal Analysis matrix and solves it — the same numerical method real SPICE-class tools use, run in your browser with mathjs's linear-equation solver. It supports a DC operating point and a full time-domain transient; it deliberately does not do frequency-domain (AC small-signal or Bode) analysis, and says so on screen rather than quietly faking one.

How to build and simulate a circuit online

Pick a part from the palette (passives, sources, semiconductors, op-amp or logic gates) and click the canvas to place it; select a placed part to rotate it, delete it or type its value.
Switch to the Wire tool, click one pin and then another to connect them, and add at least one Ground symbol so the solver has a reference node.
Choose DC Operating Point or Transient (time-domain) in the Analysis mode menu.
For a transient run, pick the element to probe and whether to read voltage across it or current through it, then press Run and watch the oscilloscope trace.
If a run misbehaves, press Load example to get back to the known-good voltage divider and rebuild from there.

What Modified Nodal Analysis is actually solving

Every node in the schematic except ground gets an unknown voltage; every element that can't be expressed as a simple conductance between two nodes — voltage sources, inductors, the op-amp's output, logic-gate outputs — adds an extra branch-current unknown instead. The solver stamps each component's contribution into a conductance matrix and a source vector, then hands the whole system to mathjs's LU-decomposition solver in one shot for a linear circuit.

Nonlinear elements make that one shot insufficient. Diodes, transistors and MOSFETs are solved with Newton-Raphson iteration — up to 100 iterations, converging to a tolerance of 1e-9 — where each pass linearizes the nonlinear device around its current guessed voltage (a 'companion model' with an equivalent conductance and current source) and re-solves until the guess stops moving.

The component library

Passives: resistor, capacitor, inductor, and a ground reference.
Sources: DC and AC voltage sources, DC and AC current sources.
Semiconductors: a diode using the Shockley exponential model, NPN and PNP bipolar transistors, and NMOS/PMOS MOSFETs using a level-1 square-law model.
Everything else: an op-amp modeled as a voltage-controlled voltage source with an editable open-loop gain (100,000 by default), and seven logic gates — NOT, buffer, AND, OR, NAND, NOR and XOR — each with an editable supply voltage (5 V by default).

Starter circuits to wire from the parts palette

RC charging: a DC source, a resistor and a capacitor in series to ground. Run a transient and probe the capacitor to watch the exponential rise; the default 1 kΩ and 100 µF parts give a 0.1 s time constant.
Half-wave rectifier: an AC voltage source (60 Hz by default) driving a diode into a load resistor. Probing the resistor shows only one polarity of each cycle getting through.
Inverting amplifier: an input resistor into the op-amp's inverting input, a feedback resistor from the output back to that input, and the non-inverting input tied to ground; the output should follow −Rf/Rin times the input.
Transistor switch: an NPN with a base resistor from a DC source and a load resistor from the supply to the collector. Raising the base drive should pull the collector voltage close to zero.
Logic check: two DC sources set to 0 V or 5 V feeding a NAND or XOR gate, with a resistor from the gate output to ground as the probe point, lets you step through the truth table one DC run at a time.
None of these ship as presets; the only built-in schematic is the voltage divider, so each one is a wiring exercise.

DC operating point and transient analysis

The DC operating point treats every capacitor as an open circuit and every inductor as a dead short, exactly like a real SPICE .OP analysis, and gives you the steady-state node voltages and branch currents before anything starts moving in time. Selecting transient analysis seeds capacitor voltages and inductor currents from that DC solve, then advances the circuit with backward-Euler time steps, re-running the same Newton-Raphson process at every step for any nonlinear elements in the loop. The oscilloscope panel plots whichever node or branch you probe against simulated time.

What it will not do is a frequency sweep. The menu lists 'Frequency Sweep (AC / Bode)' as an option, and choosing it shows a warning rather than a chart: this tool solves the DC operating point and time-domain transients only, and small-signal AC analysis is out of scope. If you need a Bode plot or an impedance-vs-frequency curve, this is the wrong tool for that specific question even though it can draw the same schematic.

Building and reading a schematic

Components sit on a grid and connect through wires drawn pin to pin. To measure something, choose an element in the Probe menu and set it to read the voltage across that element or the current through it; DC mode also lists every node voltage.
The page opens on a voltage divider — a 9 V source across a 2 kΩ resistor over a 1 kΩ resistor — and Load example restores it at any time, so the layout conventions are visible before you place your first part; the 1 kΩ resistor should read 3 V.
Solve errors are reported with the specific failure rather than a blank chart: a missing Ground symbol, a node with no path back to ground, a voltage source shorted across one node, or a Newton-Raphson loop that does not converge.

Circuit size limits and shareable links

A schematic can hold 40 components and 30 distinct electrical nodes. The panel header counts components (for example 12/40), and placement is refused with an explanation when a new part would break either limit.
These limits are identical for every visitor: the simulator has no account, paid tier or daily quota attached to it.
After each edit the schematic, analysis mode and probe choice are written into the page URL, so copying the address bar shares the whole circuit.
When that saved state would run past roughly 1,800 characters the link stops updating, and a notice says the schematic is too large to share via link.
Transient analysis advances in 100-microsecond backward-Euler steps, computing 40 of them per animation frame.

Questions people ask about Analog & Digital Circuit Simulator

Why can't I run an AC frequency sweep or get a Bode plot?: The solver only implements DC operating-point analysis and time-domain transient stepping; it has no frequency-domain linearization or complex-impedance machinery at all. Choosing the Frequency Sweep option in the mode menu shows an explicit warning instead of a chart, because faking a Bode plot from transient data would be misleading rather than useful.
How does the simulator handle a diode or transistor, which aren't linear?: It runs Newton-Raphson iteration on the whole circuit: each nonlinear device is temporarily replaced with a linear equivalent circuit (a conductance plus a current source) based on the previous guess for its voltage, the linear system is solved, and the guess is updated and re-solved up to 100 times until the answer settles within a tolerance of one billionth of a volt.
What happens to capacitors and inductors before I run a transient?: The DC operating point that seeds a transient run treats every capacitor as an open circuit and every inductor as an ideal short, which is the standard steady-state assumption for a DC source that has been connected long enough for transients to settle. The transient analysis itself then integrates their actual charge and current behavior forward from that starting point.
Can I simulate digital logic alongside analog components in the same schematic?: Yes. The gate library covers NOT, buffer, AND, OR, NAND, NOR and XOR, each stamped into the same MNA matrix as an ideal source branch referenced to a configurable supply voltage, so a logic gate driving a resistor-capacitor network or feeding into an op-amp solves in the same DC or transient run as any analog circuit.
Is this circuit simulator free, and how many components can I use?: It is free, with no sign-up. Every schematic is capped at 40 components and 30 electrical nodes, a limit set by the solver's design rather than by a paid plan, so it applies the same way to everyone who opens the page.
Do I need to download or install anything?: No. The editor, the matrix solver and the oscilloscope all run as JavaScript in an ordinary browser tab, with no plugin, desktop program or SPICE installation involved, and the circuit is never sent to a server for solving.
Can I share a circuit with a link?: Yes, for circuits small enough to fit. The page address updates as you edit, so sending it reproduces the schematic, analysis mode and probe; once the encoded circuit passes about 1,800 characters a warning replaces the link and you would need to simplify the design.

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Content reviewed 2026-09-27.