3-Way Light Switches

Two SPDT switches let either location change the lamp from on to off or from off to on. “3-way” describes this two-location switching arrangement; each switch has one common contact and two throws.

Follow the path

V1 supplies 12 V DC on the left. SW1 sends it along one of the two straight traveler wires to SW2. SW2's common contact feeds the lamp on the right. The lower wire returns current to the source. The upper traveler joins NC to NC; the lower joins NO to NO. The second switch is mirrored to keep these paths clear.

Each switch's State property selects its contact: 0 = COM–NC, 1 = COM–NO. Both switches start at 0. The lamp uses a 120 Ω resistance, with 0.1 Ω for each closed contact and 100 MΩ for each open contact.

Read the results

The operating-point table shows Vlamp in green and Ilamp in blue. Current is positive toward the lamp. The lit symbol is a power-driven indication. This is a steady-state result, so the chart is hidden.

$$I_{\mathrm{on}}\approx\frac{12}{120+2(0.1)}=99.834\,\mathrm{mA}$$ $$V_{\mathrm{lamp}}\approx11.980\,\mathrm{V}$$ $$P\approx1.196\,\mathrm{W}$$

SW1 State	SW2 State	Lamp	Approximate current
0	0	On	99.834 mA
1	0	Off	0.240 µA
0	1	Off	0.240 µA
1	1	On	99.834 mA

The tiny off-state current comes from the two finite open-contact resistances. It is about 28.8 µV across the lamp, far below visible illumination.

Try the switches

1. Change SW1 → State to 1, leaving SW2 at 0, then click Run simulation. The lamp goes out. Set SW1 back to 0, then change only SW2 to 1 and run again: it also goes out.
2. Set both State values to 1 and run again. Current now uses the lower traveler, and the lamp lights. Toggling either one switch always changes the result.

You can also click a switch blade to toggle its State, then run the simulation. Restore SW1 = 0 and SW2 = 0 for the saved starting arrangement.

Model limits

This is a low-voltage conceptual example. The lamp is a fixed resistor with a visual glow, so filament heating, cold-start current and contact bounce are omitted. The table reports electrical voltage/current, not calibrated brightness. All ground symbols represent the same electrical reference.