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Circuit simulation can be used to try out circuit configurations before (instead of) buying components and soldering.



It can be free (or you can pay for a simulator)

You can iterate rapidly

Waveforms can be observed at many nodes simultaneously without instrumentation inaccuracy or the test gear affecting the circuit

Most manufacturers make simulation models (usually Spice) available for their semiconductors.


It follows the "garbage in garbage out" principle, it is only as good as the models you feed in.

Leave important parasitics out and you will waste time investigating a topology that can never be realised.

List of free Spice type simulators and where to get them

  • Simetrix - Free cut-down version of the commercial product is limited to 150 nodes, scripting is disabled, but all else is fully working - Save, print, import 3rd party models - comes with extensive libraries of common parts, and circuit examples. Download from
  • SwitcherCAD/LtSpice - Spice version, dedicated to Linear Technologies' switch mode power supply chips. It includes hard-coded models for their switcher components which speeds up simulation. Can be used as a conventional Spice simulator, from
  • PSpice - watch this space

Other free simulators

  • PC-ECAP - Will calculate and plot: gain, phase, group delay, impedance, VSWR and return loss. This versatile program can also calculate your circuit's transient response, using one of eight waveforms. Shareware DOS program that requires DOSBox to run on Windows Vista. Download from
  • Falstad Java circuit simulator applet - Animates circuit behaviour graphically. Handy to visualise the basics, or just play with it and be mesmerised by the moving dots and changing colours.
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