Part IA Linear Circuits and Device

Spice used for solving DC circuits problems (Examples Sheet 1)

The paragraphs below show how Spice may be used to solve awkward numerical problems in DC circuit analysis.  Both examples are taken from Examples Sheet 1.

5. Determine the current in the 2W resistor in the circuit of Fig. 2 using nodal voltage analysis. [L2]

The image below shows how the labelled circuit diagram (or schematic) can be entered into the Spice package.  Note that there are some differences from the symbols to which you have become accustomed.  If required, Spice offers the flexibility to let you craft symbols to suit your own preferences.  Each component is uniquely labelled and a numbering system is used internally to identify connections or nodes.  The designer specifies values for each component (note the resistor values - units are ohms - and current sources).  To determine the current in the 2-ohm resistor as required, Spice lets the designer place a current monitor on the circuit.  This will report the current into the designated pin of the component R1 (which is Spice's notion of the 2-ohm resistor).

Pressing the 'Simulate' button gives the following result.  The program has been instructed to plot the current as a function of time over a period of one second.  As all sources are DC, there is of course no variation.  The graph shows quite clearly how the current in the 2-ohm resistor is 1 Ampere; the convention used by Spice ("current flowing into the pin") shows that the (conventional) current flows left to right in the circuit.

9. In the circuit of Fig. 5, find the current in X for V1 = 7V.

(HINT: Starting at the right hand side of the circuit, assume a current of 1A flows in resistor X, and work out the potential drops back to the voltage source. Then use scaling.)

Fig. 5

Again, the image below shows how the circuit schematic can be entered into the Spice package.  To determine the current in the resistor X as required, a current monitor is placed on the circuit to report the current into the designated pin of that component.

Pressing the 'Simulate' button gives the result shown below.  Again, for convenience, the program has been instructed to plot the current as a function of time.  All sources are DC, so no variation is seen.  The graph shows that the current in X is 125 mA, as expected

.

These simple problems vastly understate Spice's enormous range of applications.  It really comes into its own with complicated circuits using linear and non-linear devices, and signal waveforms of almost any arbitrary form.  Spice will determine for itself operating points and bias settings, and can be programmed to show the output from a circuit under almost any conceivable combination of inputs.  We shall see some of these capabilities in demonstrations later in the course.

Page last edited at 09:31 on 28 September, 2005