Tony Fisher's Circuit Simulator



This is a circuit simulator, written in Java and C++, which can be accessed via the World Wide Web.

If you don't have a Java-capable browser, the best I can offer is a screen dump showing the simulator running.

Drawing and editing your circuit

You draw an analogue electronic circuit on the screen by ``pointing and clicking'' with the mouse in a way which is tedious to explain, but which a few minutes' experimentation will make clear. At the moment, you can include only passive components (resistors, capacitors and inductors), voltage sources and current sources, voltmeters, and op-amps. Other active devices, e.g. diodes and transistors, might follow one day.

Simulating your circuit

When you have drawn your circuit, press the ``simulate'' button. At this point, if one or more components lacks a value, you will be invited to supply as many values as are required. The component whose value you are being asked to enter turns red, so you know which one the program is talking about.

Once all components have been given values, the program analyses the circuit and computes the voltages at each node. This process is called ``simulation''. When the computations are complete, a number of graphs will appear. There will be as many graphs as there are voltmeters in your circuit, and each graph shows the voltmeter ``reading'' as a function of frequency. Voltmeters measure complex voltages, and the graphs show the magnitude in red and the phase in blue. The graphs are produced with the help of the excellent gd GIF manipulation library from Quest Protein Database Center.

A table of complex voltages at each node, for various values of frequency, is also given.


This is a teaching tool. It's not an ``industrial strength'' design tool; it's suitable only for teaching the elementary properties of simple circuits and circuit elements. It's not finished. There are bound to be bugs - especially since this is my very first Java program!

The error messages are sometimes not helpful. The message ``The nodal admittance matrix is singular'', for example, generally means that one or more nodes has an undefined voltage, or that your circuit involves infinite currents or voltages.


This simulator does not run under Microsoft Internet Explorer 4, which reports ``Class Cctsim not found''.

This simulator does not run properly under some versions of Netscape Navigator Gold for PCs. Symptoms of the bug are:

I am trying to track down this bug. Similar bugs have been reported by Netscape (``XOR drawing doesn't work'') and Sun (``fixed in JDK 1.1.2'', bug no.402 8864) and mentioned in some old news articles in I suspect that the setXORMode method is behaving in a way I did not expect.

Netscape claim that you need a 256-colour video driver in order to run Java applets under Windows.

If you are a Java expert and can guess what the problem is, please let me know.

Unix implementations (e.g. Netscape 3.01 and 4.05) appear to be fine.


Here are some example circuits for you to try.

To enter your own circuit

Click here:


I'd appreciate comments on this package by email to

Tony Fisher /     24 Jun 1997