Physics 536 - Spring 2008

Electronic Techniques for Research

Another Example of Using Berkeley SPICE with nanoHUB

Example of an RLC circuit analysis:

Consider the circuit shown below:
In which the components have the following values:
V1 0 V for t<0, 10 V for t> 0
L1 470 μH
C1 5.4 nF
R1 to be determined
The goal is to determine the range of resistances that will produce oscillations. From the analysis of the resulting differential equation, the solutions will oscillate if R2/(4L2) < 1/(LC). Thus, we expect oscillations for cases when, for example, R = 100 Ω.

Oscillating circuit

The SPICE simulation input file for the circuit that should oscillate is as follows:

which simulates the transient response of the circuit over the time interval 0 < t < 100 μs in time steps of 1 μs. Clicking on Simulate produces the graph:

which indicates that the circuit oscillates with a frequency of 100kHz, as expected based on f = (1/2π)(1/LC)1/2.

Non-Oscillating circuit

The minimum value of R for which the circuit is not expected to oscillate is 590Ω. Simulating the same circuit but with R=600Ω gives the following graph:

Again, remember that the convention used in SPICE is that the current I(V1) is the current flowing into voltage source V1 and is therefore negative when V1 makes the transition to a positive voltage.