Circuit simulator online: SPICE-style DC, AC (Bode plot) and transient analysis of RLC circuits in your browser

Type or paste a SPICE-style netlist of resistors, capacitors, inductors and voltage or current sources, then get the DC operating point, an AC sweep with the Bode magnitude and phase at any node, or the voltage against time with pulse, sine or piecewise sources. Nothing is uploaded.

Keep it one tap away for everyday use.

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Circuit

Example Netlist, one part per line

Each line is name node node value. Names start with R (resistor), C (capacitor), L (inductor), V (voltage source) or I (current source). Node 0 (or gnd) is ground. Values take SPICE letters: f p n u m k meg g (so 4.7k, 100n, 1meg). Sources: DC 5, AC 1 (for the AC sweep), and for the transient PULSE(v1 v2 delay rise fall width period), SIN(offset amplitude frequency) or PWL(t1 v1 t2 v2 …). Lines starting with * are comments. The examples are made up.

Analysis

Run Show the voltage at node Start frequency (Hz) Stop frequency (Hz) Points per decade Time step Stop time Method Also show node

Result

A teaching and sanity-check simulator for linear circuits, for study and planning, not a substitute for professional judgement. Real parts have tolerances, resistance in wires and capacitors, and limits that this model leaves out.

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What it does

This is a small circuit simulator in the style of SPICE, for linear circuits: resistors (R), capacitors (C), inductors (L), and independent voltage (V) and current (I) sources. It builds the circuit equations by modified nodal analysis (one equation per node, plus one per voltage source and inductor) and solves them three ways:

DC operating point: every node voltage and every part’s current and power, with capacitors open and inductors shorted.
AC sweep: the gain and phase at a node over a range of frequencies, drawn as a Bode plot, with the −3 dB points, bandwidth and roll-off.
Transient: the voltage against time when the sources switch on, pulse or oscillate, by the trapezoidal rule or backward Euler.

It is for study and planning, not a substitute for professional judgement. Diodes, transistors and op-amp models are not included.

How to use it

Pick an example or type a netlist, one part per line: R1 in out 1k puts a 1 kΩ resistor between nodes in and out. Node 0 (or gnd) is ground, and every circuit needs it.
Values use SPICE letters: f, p, n, u (micro), m (milli), k, meg, g. So 100n is 100 nF and 1meg is 1 MΩ. Note that m is milli, not mega.
Sources: V1 in 0 DC 5 AC 1 PULSE(0 5 0 1u 1u 1m 2m). DC is used for the operating point, AC (amplitude and optional phase in degrees) for the sweep, and PULSE (low, high, delay, rise, fall, width, period), SIN (offset, amplitude, frequency, delay, damping, phase) or PWL (time-value pairs) for the transient. A current source I1 a b 1m pushes current from a through the source to b.
Choose the analysis and the node to show. For the transient, choose a time step and stop time (such as 10u and 10m) and optionally a second node to compare.
Download the data as CSV, or the report as an image, PDF or Word file.

Worked example

The first example is an RC low-pass filter: a 1 kΩ resistor feeding a 1 µF capacitor. Its corner frequency is 1/(2πRC) = 159.2 Hz. The AC sweep from 1 Hz to 100 kHz shows −3 dB at 159.2 Hz, a phase of −45° there, and a slope of −20 dB per decade at the top end. Switch to Transient and the 1 V pulse charges the capacitor with a time constant of 1 ms: about 63 % after 1 ms and 99.3 % by 5 ms.

The series RLC example (10 mH, 1 µF, 10 Ω) rings at about 1.59 kHz when the step arrives. Its AC sweep shows −3 dB points near 1.514 kHz and 1.673 kHz, a bandwidth of 159 Hz and Q = 10.

Tips

Use a time step at least ten times shorter than the fastest edge or period you care about. Edges shorter than one step are spread over that step.
The trapezoidal rule is accurate but can ring around sudden steps in circuits with an inductor or a capacitor directly across a source. Backward Euler damps that ringing but also damps real oscillations a little.
A node joined only through capacitors has no DC level; the simulator ties it to ground through a tiny leak, as SPICE does, and says so.
Two voltage sources in parallel, or a voltage source shorted by an inductor at DC, cannot be solved: add a small resistor.

FAQ

Is this real SPICE? No. It follows the same netlist style and methods for linear parts, written from scratch in TypeScript. Files from other simulators work if they only use R, C, L, V and I lines; lines starting with a dot (.tran, .ac) are not read.

How big a circuit can it handle? Up to 80 nodes and 200,000 time steps, which runs in a moment on a phone.

Does the AC sweep include the DC level? No. AC analysis is small-signal: each source gives only its AC amplitude, and the result is the steady sine response at each frequency.

Where the numbers come from

Modified nodal analysis, the trapezoidal rule and backward Euler companion models, and the PULSE, SIN and PWL source conventions as described in circuit simulation textbooks (for example Vlach and Singhal, Computer Methods for Circuit Analysis and Design) and the public Berkeley SPICE documentation (method only; no code copied).
Checked against an independent NumPy nodal solve, the analytic transfer functions of RC and series RLC circuits, and SciPy solve_ivp on the circuits' differential equations with the same sources.
The example circuits are made up for this tool.

Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.

Last updated 2026-11-09.