Note: This part of the lab is just to develop your intuition about
superposition. There are no responses that need to be checked.
Circuits with multiple sources can be hard to analyze as-is. For example, what is the voltage
between the two terminals on the right of Figure 1?
Figure 1. Example multi-source circuit
We can use superposition to make the analysis much easier.
The circuit in Figure 1 can be decomposed into two separate
subcircuits: one involving only the voltage source and one involving only the
current source. We'll analyze each circuit separately and combine the
results using superposition. Recall that to decompose a circuit for
analysis, we'll pick each source in turn and set all the other sources
to zero (i.e., voltage sources become short circuits and current
sources become open circuits). The circuit above has two sources, so
the decomposition produces two subcircuits, as shown in Figure 2.
(a) Subcircuit for analyzing contribution of voltage source
(b) Subcircuit for analyzing contribution of current source
Figure 2. Decomposition of Figure 1 into subcircuits
Let's use the DC analysis capability of the schematic tool to see superposition
in action. The sliders below control the resistances of R1, R2, R3 and R4 in all
the diagrams. As you move the sliders, the schematic tool will adjust the appropriate
resistance, perform a DC analysis and display the node voltages on the diagrams. Here's
what you want to observe as you play with the sliders:
The voltage for a node in Figure 1 is the sum of the voltages for
that node in Figures 2(a) and 2(b), just as predicted by
superposition. (Note that due to round-off in the display of the
voltages, the sum of the displayed voltages in Figure 2 may only be within
.01 of the voltages displayed in Figure 1.)