Circuit simulation

Why it matters

Simulators let you build any circuit, at any voltage, and watch current flow without risk or cost. Engineers simulate designs before building them, technicians use simulators to understand a fault before opening a panel, and teachers use them to show what a meter would read. A free browser simulator is the safest place to try the things you must never try on real equipment, such as what a short circuit does to current.

Two free tools

• Falstad Circuit Simulator (https://www.falstad.com/circuit/) runs in your browser with no account. It animates current as moving dots and colours wires by voltage, which makes KVL and KCL visible. It includes resistors, capacitors, inductors, AC sources, diodes, transistors, transformers and logic gates, and has dozens of built-in example circuits under the Circuits menu.
• Tinkercad Circuits (https://www.tinkercad.com/circuits) from Autodesk needs a free account. It looks like a real workbench, with breadboards, batteries, LEDs, multimeters and even an Arduino, so it is ideal for rehearsing a physical build.

Engineers also use SPICE simulators such as LTspice (free from Analog Devices). SPICE is more accurate for detailed electronics design, but it has a steeper learning curve; the habits you learn here carry over.

Getting started in Falstad

1. Open the simulator. It loads an example (an LRC circuit). Choose File → New Blank Circuit to start clean.
2. Add parts from the Draw menu, or right-click the canvas. Common shortcuts: r resistor, v voltage source, w wire, c capacitor, l inductor.
3. Drag from one point to another to place a part. Ends that land on the same point are connected (a small dot shows a joint).
4. Double-click a part (or right-click → Edit) to change its value, for example 470 Ω or 9 V.
5. Hover over a part to see its voltage, current and power in the corner of the screen.
6. Use the Reset and Run/Stop buttons; the speed sliders on the right slow down or speed up the animation.

Worked example: checking the series-parallel lesson

Rebuild example 3 from the series and parallel lesson: a 10 V source, R1 = 40 Ω, and R2 = 60 Ω in parallel with R3 = 30 Ω.

• Hover over R1: it should show about 166.7 mA and 6.67 V.
• Hover over R2 and R3: 55.6 mA and 111.1 mA, both at 3.33 V.

If the simulator disagrees with your hand calculation, one of them is wrong. Usually it is a wire that does not connect where you think it does.

What simulators are good for

• Checking hand calculations for DC and AC circuits.
• Trying "what if?" changes quickly: double a resistor, add a branch, change the frequency.
• Seeing AC behaviour: add a scope trace (right-click a part → View in Scope) to see voltage and current waveforms and the phase shift in an inductor or capacitor.
• Safely exploring faults: an open wire, a shorted part, a failed component.

Limits of simulation

• Parts are ideal unless you model otherwise: real batteries have internal resistance, real wires have resistance, and real LEDs vary.
• A simulator does not model fire, arc flash or shock. A circuit that "works" at 480 V in a simulator says nothing about whether it is safe to build.
• Simulators do not check codes. Wiring for buildings must follow the electrical code adopted locally (often based on NFPA 70, the National Electrical Code: https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70) and is installed and inspected by licensed people. Checked on 2026-10-23.

Common mistakes

• Parts that look connected but are not. Look for the joint dot, or drag a part slightly to see if the wire follows.
• Forgetting a ground or return path. Current needs a complete loop.
• Wrong units. In Falstad, type 4.7k for 4.7 kΩ and 10u for 10 µF.
• Trusting the simulator blindly. Always estimate the answer first.

Recap

• Falstad: instant, no account, great for seeing current and voltage. Tinkercad: realistic workbench and breadboards.
• Hover to read values; double-click to edit; use the scope for AC.
• Simulators check your thinking; they do not make a real high-energy circuit safe.

Try it

Open the Do tab and simulate two circuits from earlier lessons, then break them on purpose.