EE 101 - Lab 4: PSPICE Evaluation of an RC Network
ALL GRAPHS MUST BE FULLY LABELED AND DRAWN NEATLY!
NO ANSWERS ARE COMPLETE WITHOUT THE UNITS! AS ALWAYS, SHOW
Circuit for prelab exercises
For parts 1, 2, and 3, assume the switch has been in position 1 for
a long time. At t = 0s, the switch moves to position 2 and the capacitor
begins to charge.
For part 3, assume the switch has been closed for a long time. At t
0s, the switch is opened and the capacitor begins to discharge.
Draw the charge curve for the capacitor using the 2/3 estimation method
given in class.
Answer the following:
What is the time constant for the circuit?
What will the capacitor voltage be at t = 150 ns?
What will the resistor voltage be at t = 150 ns?
What will the resistor current be at t = 150 ns?
How much time must pass for the capacitor voltage to reach 1 volt?
Draw the charge curve using the capacitor charge formula given in class.
Assume the capacitor is fully charged, and the switch moves to position
1 at t = 0s. Draw the discharge curve using the capacitor discharge
formula given in class.
(end of pre-lab)
Lab Exercise 4: PSPICE Evaluation of an RC Network
The purpose of this lab is to analyze an RC network using PSPICE.
Enter the following circuit (Figure 1) into the PSPICE schematic editor.
Unlike in last week's simulation, the voltage source you will
use today will be a 0 to 5v
pulse signal (Vpulse). The purpose of
the pulse is to model a switch opening and closing. When the pulse is low
(0 volts), the switch is modeled as open and when the pulse is high (5
volts), the switch is modeled as closed. You will need to define the parameters
of the pulse. For the purpose of this exercise, you will inject a waveform
that is 5 volts for the first 500 ns and 0 volts for the next 500 ns. To
accomplish this, double click on the Vpulse symbol and enter the following
attributes: V1 = 0V, V2 = 5V, TD = 0, TR = 0, TF = 0, PW = 0.5us, and PER
= 1us. (TR = risetime, TF = falltime, TD = delay time, PW = pulse width,
and PER = period of waveform)
Notice there are two voltage level markers on the schematic. These are
used in the probe
feature of PSPICE. Probe allows the user
to observe how a circuit affects a signal. Specifically, you will observe
how a 5 volt source charges the capacitor in an RC network (and how the
capacitor discharges in the same network). Place the voltage markers on
your schematic--one to measure the source voltage and one to measure the
capacitor voltage (under the Markers menu, choose Mark
Before you simulate this circuit, make sure your analysis setup is correct.
Under the Analysis menu, go to Setup and check
to see that the Bias Point Detail and Transient
options are enabled.
Perform an Electrical Rule Check and simulate your circuit.
After a bit, the MicroSim Probe window will appear. It should
look like this (Figure 2).
Notice that while the pulse reads 5 volts from 0 s to 500 ns, the capacitor
is shown to be charging, while during the low portion of the pulse wave
(500 ns to 1 us) the capacitor is discharging.
Add a set of cursors to the probe output (Tools - Cursor - Display).
Click the right mouse button to display one set of vertical crosshairs.
To make these crosshairs stick to the capacitor voltage curve, go down
to the curve label (V(C:2)) and use the same mouse button to click on the
small diamond to the left of the label. In the same way, but using the
left mouse button, add the second cursor. This time, click on the diamond
with the left mouse button to make the cursor stick to the charging curve.
A rule of thumb states that the time it takes for a capacitor to charge
from 10% to 90% of its maximum value is approximately equal to 2.2 times
the time constant for that curve. Using the left mouse button to control
one, and the right button to control the other, move one set of cursors
to 0.5 volts (10% of 5 volts) and the other to 4.5 volts (90% of 5 volts).
Keep track of the voltage value the cursor is at by watching the Probe
Cursor box at the bottom of the probe window. When you are close
to 0.5 and 4.5 volts, take note of the elapsed time difference between
them. Divide this time value by 2.2. Does this number basically agree with
the RC time constant value you calculated in the pre-lab?
Leaving one cursor at the origin, position the second cursor at the 150
ns mark. Read the capacitor voltage at this point. Does this value agree
with what you calculated in part 2b of the pre-lab?
Leaving the cursors as they are (in 1b), label your plot (Tools -
Label - Text) first with "RC = 50 ns" and then with your name.
Neatly place this information under the charging curve. Next, you will
print out the plot. To keep from using an entire piece of paper for the
print job, go to File-Print and then click on the Page
Setup button. Locate Plots Per Page and choose 4.
Then print out the plot and tape it into your lab book.
Using what you know about the series and parallel relationships of resistors
and capacitors, reduce the schematic in Figure 3 to a circuit with a single
equivalent resistor and capacitor. Fully label your schematic.
What is the RC time constant for the circuit?
Sketch the capacitor charging and discharging curves using the 2/3 approximation
method. Fully label your plots.
Enter the circuit in Figure 3 into the schematic editor in PSPICE.
For Vs, configure a pulse signal with a 40 volt peak amplitude that is
150 ns in length. Use a period of 300 ns, along with rise, fall, and delay
times of 0 s. Place voltage level markers at the appropriate nodes to measure
the input pulse and the voltage across the equivalent capacitance. Under
the Draw menu, use Text to place your name
in your circuit. Save your drawing, print out a copy (make it small enough
to fit nicely in your lab book!), and tape it into your lab book.
Perform an electrical rules check on your circuit, set up probe to run
a transient analysis, and simulate your circuit as before.
When your probe results are displayed, notice that the input pulse
is repeated until it gets to the 1.0 us mark. Your first simulation you
ran today used up the same amount of time, as well. However, that pulse
was of a lower frequency and thus took up the entire span. Over three of
the present pulses can fit in the 1.0 us time span because it is of shorter
Like before, determine the time constant for this circuit by using the
10% to 90% risetime method. Print out a copy of your plot that shows the
cursors in the 10% and 90% of total voltage amplitude positions. On this
plot, add a label with the time constant information and your name. Tape
the printout into your lab book.
Copyright 2001, New Mexico Tech