# (solution) ELEC153 Circuit Theory II M1A3 Lab: C and L Circuits Transient

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ELEC153 Circuit Theory II
M1A3 Lab: C and L Circuits Transient Analysis
Introduction
Before we get into the ?real? AC circuits, this very first lab is about transient analysis for RC and
RL circuits. We will investigate the transient properties of RC and RL circuits.
Procedure
1. Setup the following RC circuit in MultiSim. The voltage source is 12 V DC: Fig. 1. RC circuit transient analysis
2. Select ?simulate ? Analysis ? Transient Analysis? from the MultiSim Menu, the following
window appears: Fig. 2. Parameter settings
Select the ?Analysis parameters? as shown above and the ?Output? as shown below: Fig. 3. Output selection
3. Click ?Simulate? from the above window, you will see the following transient response: Fig. 4. Transient response of RC circuit
The ?Design1? title will automatically change once you change your file name. You can see that
since the initial capacitor voltage is 0 V and the voltage cannot be changed instantaneously, it
takes about 5 ms for it to reach 12 V (at which point the capacitor is like an open circuit and
there is no current flowing so the voltage drop on the resistor is 0 V). This is called the steady
state. The time constant, which is equal to RC, is a measure of how fast the RC circuit can reach
its steady state. An RC circuit with zero initial condition will need about 5 time constants (page
416 in textbook) to reach its steady state. In this setup, RC=1K*1uF=1000*0.001ms =1ms. So 5
ms is the time to reach the steady state, as confirm by the above figure.
Task 1: Change R1 in Fig. 1 to 2K? and keep C1 the same, perform the simulation, measure
the time constant and compare it to the calculated one. Do the same for R1=2K? and C1=0.5
uF.
4. Change the capacitor in Fig. 1 to an inductor, as shown in Fig. 5. Keep the simulation
parameters the same and change output to I(L1) (make sure you remove V(2)). Perform the
simulation and the RL transient response will appear (Fig. 6). Fig. 5. Circuit for RL transient analysis Fig. 6. Transient response of RL circuit
Again, you can see that since the initial inductor current is 0 A and the current cannot be changed
instantaneously, it takes about 5 ms for it to reach 12 mA (at which point the inductor is like a
short circuit and the circuit current is simply the maximum of 12V/1K? =12 mA). This is called
the steady state. The time constant, which is equal to L/R, is a measure of how fast the RL circuit
can reach its steady state. An RL circuit with zero initial condition will need about 5 time
constants (page 476 in textbook) to reach its steady state. In this setup, L/R = 1/1000 = 1ms. So
5ms is the time to reach the steady state, as confirmed by the above figure. Task 2: Change R1 in Fig. 5 to 2 K? and keep L1 the same, perform the simulation, measure the
time constant and compare it to the calculated one. Do the same for R1=2 K? and L1=2 H.
In general, for each lab you do, you will be asked to setup certain circuits, simulate them, record
the results, verify the results are correct by hand, and then discuss the solution. Your lab write-up
should contain a one page, single spaced discussion of the lab experiment, what went right for
you, what you had difficulty with, what you learned from the experiment, how it applies to our
coursework, and any other comment you can think of. In addition, you should include screen
shots from the MultiSim software and any other figures, tables, or diagrams as necessary. 