Electrical Circuit Simulator

This is a fun little project that was way more work than it's probably worth but a few of us have found it amusing.

If you don't want to read all this stuff, click this link Click the switches and see what happens. Then come back here.

I started this project to demonstrate an idea for editing electrical ladder diagrams in a web browser. Then someone wanted to click on the switches and have them change state. Of course the light bulbs had to light up. Then someone else wanted the "meter". I accidentally discovered the requirement for the wires to have finite resistance (it's a long story). By the time the LEDs only worked under forward bias, I had pretty much used up my enthusiasm. But it's a cool toy!

Brief Instructions

The circuit simulator works in one of two modes. In circuit editing mode, you can build and rewire your circuit. You can place circuit elements such as switches and lights and connect them up with wires. When you're ready to try your circuit, click on the battery switch to "close" it and turn on the juice.

In circuit testing mode, your circuit is operating. Lamps light, etc. If there are switches in your circuit, you can click on them to change them from open to closed and vice-versa. Your cursor arrow is also your meter probe. When you place it on a wire, the "meter" readout at the left shows the voltage at that point, with respect to the negative battery terminal, and the current through that segment. If you have your probe on a branching junction, the current may not display. When your probe is on a lamp or a resistor, the voltmeter shows the voltage drop across the element as well as the current through it. In this mode, the meter reads "dV" instead of "V".

When you click the link below, you'll start in the "circuit editing" mode of the circuit simulator. You know you're in this mode because the battery switch is open. Also, the palette is present on the left side of the screen. To build your circuit, click a circuit element from the palette. A large version of the selected element shows up at the top of the palette. As you move your cursor over the wiring area above the batteries, a ghostly image of the element is displayed. Position this where you want it and click to stick it down. The blank cell in the palette is the eraser. Use it to erase unwanted circuit elements.

It's best to start by sticking down lamps, switches, etc. Then select wire segments and connect everything together.

The simulator allows you to save your circuits for later recall. You are asked to use your email address as a key to store your circuits separately from everyone else's. If you are paranoid about giving me your email address, just make one up. But remember what you used! There is no other way to recall your saved circuits.

Things to Try

There are a small handful of example circuits available. You'll see these listed on the "Open" screen. Pull one of these up. Flip the switches. Play with the "meter". The circuit called "Series and Parallel" is the one I used for much of the debugging of the program.

Other experiments you might find entertaining:

Make a circuit where there are redundant paths from the battery to the lamp. How does the electricity know which way to go? Make one of the paths much longer than the other. Measure the current in each path.
Make the shortest possible dead short across the batteries. Measure the current. Doesn't that give you a feeling of power?
How many lamps at full brightness can you get on the page? If you try this on a notebook computer, how long will the battery last?
Take photographs of real-world light bulbs at various currents. Compare to the lamps in the simulator at the same currents. Let me know how it comes out.
Make a long wire by zig-zagging back and forth, filling the whole window with the longest possible single path that connects the battery terminals together. When you turn on the battery, of course the current is only limited by the resistance of the wire. Run the meter probe along the wire and watch the voltage change. Some people are easily amused, I guess.

Follow this link to the Circuit Simulator

Known Problems

Wierd things can happen if you put a three- or four- way junction right next to the battery. Try to use a vertical line or a corner here.

The batteries should have some internal resistance. Simulating resistance in the wires hasn't satisfied some critics.

The V/I curve of the LEDs is not very realistic. In fact, I'm tempted to take them out altogether.

The instructions are pretty pathetic. Feel free to write a better set and email them to me.

How does it work?

The circuit editor is done in Javascript because it has to work in the browser. The circuit simulation is on the server and was written in C.

The program on the server parses the diagram and writes a system of equations describing the voltage and current at each point in the diagram. This is really the hard part. The voltage equations are basically "Ohm's Law" and the current equations are "Kirchoff's Law". Once I have the equations, I solve the system of simultaneous equations using Gauss-Jordan reduction (with partial pivoting). This is a bit of an oversimplification. There are some heuristic transformations applied to the equations to handle loose ends, isolated loops, and the diode-action of the LEDs.

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