Ohmforge Transient Simulation

Simulate a circuit over time: waveforms, Vpp/RMS/frequency, CSV and a chart.

SPICE netlist

Subset: R C L V I D Q M E G K, .model, SIN/PULSE/PWL sources, engineering suffixes (k, m, u, n, p, meg). Node 0 is ground. Leave empty when you pass an Ohmforge project instead.

Ohmforge project JSON (optional)

Alternative to the netlist: a .ohmforge.json project as saved by the app (format "ohmforge", version 1).

Stop time s

Simulated duration. Empty = use the .tran line of the netlist (or the project's transient settings).

Output step s

Spacing of the returned samples (also the largest internal step). Empty = .tran value or stop time / 1000.

Integration

Start from initial conditions (skip the operating point)

Capacitors start at their IC (default 0 V). Needed for oscillators.

Nodes to return

Node names; empty = all nodes (max 8 in the chart).

Try an example: RC step response (τ = 1 ms) · Half-wave rectifier with smoothing

How it works

Runs a transient (time-domain) simulation of a SPICE netlist with trapezoidal or backward-Euler integration, adaptive time steps and breakpoints at source corners. Returns resampled waveforms of the chosen nodes, oscilloscope-style measurements (min, max, Vpp, mean, RMS, frequency, duty cycle, rise time), rating warnings, a chart and CSV.

Starts from the DC operating point (or from initial conditions with "uic"), then integrates capacitors and inductors with companion models: trapezoidal (2C/h) or backward Euler (C/h); inductors and coupled inductors use branch currents.
Each time step is solved with Newton–Raphson. The step size adapts: the difference between the corrector and a linear predictor estimates the local truncation error; failed steps are retried with h/8.
Source corners (PULSE edges, PWL points) are breakpoints: the integrator lands on them and restarts with backward Euler to avoid trapezoidal ringing. Behavioural events (555 latch, relay pull-in, fuse, current limit) are localised by step bisection.
Returned samples are linearly interpolated onto a uniform grid (output step). Measurements: time-weighted mean and RMS, Vpp, frequency from mid-level crossings with 10 % hysteresis, 10–90 % rise time.

Teaching-grade simulation: device models are simplified (Ebers–Moll BJT, level-1 MOSFET, behavioural op-amps and 555) and part ratings are typical values. Verify real designs on the bench and against manufacturer data before building anything connected to mains voltage.

Sources

SPICE: A Computer Program to Simulate Semiconductor Circuits (L. W. Nagel, UC Berkeley ERL-M520, 1975) (University of California, Berkeley)
The Modified Nodal Approach to Network Analysis (Ho, Ruehli, Brennan, IEEE Trans. Circuits and Systems, 1975) (IEEE)
ngspice User Manual (device models, convergence options, source syntax) (ngspice project)

For scripts and AI agents

JSON API: POST https://ohmforgetransient.usefulweb.site/api/v1/calculate (same calculation as this page)
Interface description: OpenAPI 3.1 · AI guide: llms.txt
MCP server (Streamable HTTP): https://ohmforgetransient.usefulweb.site/mcp · tool name ohmforge_transient

Example request

curl -s -X POST https://ohmforgetransient.usefulweb.site/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{"netlist":"* RC step response, tau = 1 ms\nV1 in 0 PULSE(0 1 0 1n 1n 10m 20m)\nR1 in out 1k\nC1 out 0 1u\n.tran 10u 5m\n.end","probes":["in","out"]}'

Related tools

Ohmforge DC Operating Point – SPICE netlist in, node voltages, branch currents, power and overload warnings out.
Ohmforge AC Sweep (Bode plot) – Frequency response of a circuit: Bode magnitude/phase, −3 dB corner and unity gain.