DC Simulation
The reader is introduced to project creation, drawing schematics including component placement, wiring and labelling, component property editing, the meaning of the "ground" component, simulation boxes and how to issue a DC simulation. Also how to display and interprete simulation results. Using a voltage divider example circuit each necessary action is explained step by step.
Any step before an EE simulation is to know about the circuit and the domain to be analyzed. Analog simulators are usual used to compute unknown voltages and/or currents in circuit topologies. In the quiescent domain (quasi static, direct current, DC) this is a comparatively easy task.
This example will guide the reader through the setup and analysis of a resistive voltage divider in the DC domain.
After having successfully installed and started the application you are supposed to create a project if you do not have one already. This is accomplished e.g. by selecting "New Project" from the "Project" menu.
After having input the project name and pressed the "Create" button the project is opened and an untitled schematic is shown. Now it is about to place the circuit topology onto the schematic area. For this choose the vertical "Components" bar on the left hand side. In the above combobox you will find the resistor in the "lumped components" category.
Click on the resistor symbol once and move the mouse to the schematic area. Moving the mouse over the schematic area you see a scheme of the resistor. By right clicking you can rotate the component and left clicking places the component finally on the schematic. We place two resistors on the schematic for the voltage devider circuit as shown in the next figure.
In order to connect both resistors electrically we need wires. To enter the "wiring" mode press on the
icon or Ctrl+E on your keyboard. The first point of the wire to insert is defined by clicking on the schematic. The grid snapping helps you to hit the red circles (which are the connection ports of components) exactly. Click again to define the next point of a wire. If your last click hit a connection point again, then the "wiring" mode ended up. Otherwise not and you can continue to wire. In order to leave the "wiring" mode press ESC on the keyboard.
One more note about wiring: While moving the mouse with a wire at hand you will see that you can wire around rectangular corners. If you feel this is the wrong corner around press right mouse button to change between the corners. This makes life sometimes easier.
We have now two connected resistors of 50<math>\Omega</math> in the schematic area.
As long as the 
icon is pressed (enable/disabled by pressing ESC) you are in the "selection" mode allowing to edit a schematic. In "selection" mode we change now the values of the resistors. This is accomplished either by clicking directly onto the "50 Ohm" texts in the schematic and entering the new value (don't forget to apply the edition by pressing RETURN on the keyboard) or by double-clicking the resistor symbol. You will get a dialog where you can change the "R" property of the resistor.
Also we want to give names to the nodes which are currently unnamed. This is achieved by clicking on the 
icon. Move the mouse to the wire or node you want to name and click it. You will be asked to enter a node name. Finally we get the following schematic consisting of two connected resistors with the specified resistance values and with a named connection in between.
Our voltage divider is finally complete.
After our voltage divider has been drawn, an excitation for the circuit topology is required. Otherwise the circuit cannot perform electrical actions. Since we want to analyze the circuit in the DC domain a DC source is required. From the vertical "Components" tab on the left hand side we choose "source", grab the "dc voltage source" and place it in the schematic.
Since we want the voltage source to drive a current through both resistors, remember the rule "currents flow in circles" and connect the components appropriately. We leave the voltage value of 1V of the DC voltage source as is.
Finally we have a circuit topology including excitations on the schematic area. But is this all?
No. Do you remember the EE classes about nodel analysis? When calculating unknown node voltages and branch currents you have been up to determine a so called "reference node". By definition this reference node had a potential of zero volts. Sometimes called "ground". So for the simulator as well. It requires a reference node which you can insert by pressing the 
icon or Ctrl+G on the keyboard. Place it at the negative terminal of the DC voltage source for instance.
We are finally done with a complete schematic. Including
- circuit topology consisting of
- circuit components
- (named) wires
- excitations (sources)
- a reference node
Always remember these items when setting up schematics for circuit simulators.
After the schematic itself is complete we need to tell the simulator which domain we are intending to analyze. For this we select from the vertical "Components" tab on the left hand side the "simulations" category. Since we intend to do a quiescent (DC) domain analysis, grab the "DC simulation" icon and place it on the schematic.
Now save the schematic by pressing the 
icon or Ctrl+S on the keyboard. Thus the schematic will be available in your project which you previously created.
For DC simulations there are two ways to start a simulation. One way is running "Calculate DC bias" from the "Simulation" menu (pressing F8). This gives you the following result figure.
The simulation results have been annotated at each node. Apparently the voltage divider really worked fine.
The other way is to run "Simulate" from the "Simulation" menu or pressing the 
icon or F2 on the keyboard.
Usually after successful simulation the user is led to the schematics appropriate data display page. Here you can place diagrams, tables, etc. to visualize your results.
In the "Components" tab you will be automatically guided to different types of diagrams. For our DC simulation results which are single node voltages and branch currents we choose a tabular to show the results. Place the "Tabular" from the left hand side list in the data display page. You will get a diagram dialog as shown in the next figure.
On the left column you see the available data items. For the DC simulation you find "divide.V" referring to the DC node voltage at the node "divide" which we previously named; and "V1.I" which is the DC current through the source "V1". The large letters "node.V" and "source.I" after the dot refer to DC values. Double-clicking these items move them into the "Graph" list in the right hand side column.
Well, lets have a look at the results finally by pressing "OK" button in the dialog.
The 0.5V at the node "divide" we already know by the DC bias calculation. The current through the voltage source "V1" seems also correct. These voltage source currents are defined to flow from the positive to the negative terminal, hence the negative sign of the current. So we are done with this example. All unknowns in the single current loop with two nodes are now known.
Original author: Stefan Jahn (2009)