FVCC Engineering 116!
Monday, March 1, 2010
Using Multisim to simulate electronic circuits
Each week we try to introduce a new component used in electronics or a new tool. We
have worked with function generators, oscilloscopes and multimeters. We have
introduce capacitors, inductors and resistors. Today we introduce a ubiquitous tool
which has all of these tools and components and much more. That tool is the circuit
simulator. In fact, as an engineer you will use simulators extensively.
For more information on Multisim, use the software manuals which also contain tutorials
for the use of this simulation.
Exercise 1.
Wire a voltage divider. Divide 12V DC source into 4 volts and 8 Volts. Use a
multimeter to display your result. Note that figure 1 shows an example circuit ( not
exactly the voltage divider you are building) with a multimeter wired in. Pick virtual
resistors for the circuit.
Questions for Exercise 1:
a) What values for resistors did you use?
b) What voltage values where measured by the multimeter?
Exercise 2.
Remove the DC voltage source and replace it with a function generator. Use the
function generator to input a sine wave. Replace the multimeter the an oscilloscope.
Measure the voltage drop across each resistor with the oscilloscope.
Questions for Exercise 2:
a) What was the frequency of the function generator?
b) What was the time base on the oscilloscope?
c) Is it possible to display the voltage drop across the first resistor on channel A and
the voltage drop across resistor 2 on channel B,...simultaneously?
5!
1
FVCC Engineering 116!
Monday, March 1, 2010
Exercise 3
Wire an RC circuit with a square wave input. Use Oscilloscope to display the signal on
the capacitor. Use a 1 microfarad virtual capacitor and two additional resistors of your
choice. Call these resistors #1 and #2. Figure 1 is an example of the circuit you need.
Adjust the frequency on the function generator so the the capacitor very nearly charges
and nearly discharges. Measure the voltage signal on the capacitor with the
oscilloscope. Figure 2 displays
exactly what you should get for a
pattern on the oscilloscope. Note that
the settings of the scope will change
based on your circuit values.
!
!
!
!
!
!
Figure 1. Basic RC circuit in Multisim
Figure 2. RC circuit in simulation
5!
2
FVCC Engineering 116!
Monday, March 1, 2010
Questions for Exercise 3:
a) what were your chosen resistor values?
b) what was the input frequency of the square wave?
c) what voltage value is the function generator set at?
d) what is the oscilloscope voltage reading for the signal?
! if these don"t match, why not?
Measure the voltage signal across one resistor only. Choosing the resistor closest to
the capacitor, you should get the signal in Figure 3.
Figure 3. Signal across resistor in RC circuit
Careful examination will reveal that the patterns shown in Figs. 2 & 3 are not the same.
Questions for Exercise 3 (part 2):
a) What is different about them?
b) What is causing the difference?
5!
3
FVCC Engineering 116!
Monday, March 1, 2010
Exercise 4
Change the input on the RC circuit such that an AC sine wave is the source. Measure
the voltage signal across the capacitor on Channel B and put the initial wave from the
function generator on Channel A. Measure the phase difference in the signals. Figure
4 shows this. Also measure the peak to peak voltage. Finally, measure the voltage
across the capacitor using the multimeter.
Figure 4. Measure phase on an AC circuit.
Figure 5. Multimeter from Multisim.
Questions for Exercise 4:
a) What is the measured phase?
b) What is the calculated phase?
c) What is the Peak-to-Peak voltage across the capacitor?
d) What is the voltage measured by the multimeter?
e) If there is a difference in the voltage measurements, what is it an why is it there?
5!
4
FVCC Engineering 116!
Monday, March 1, 2010
Equations
The equation for the charging capacitor is:
t
!
"
%
V ( t ) = V0 $ 1 ! e RC '
#
&
The equation for the discharging capacitor is:
V ( t ) = V0 e
!
t
RC
The equation for the phase angle between voltage from the generator and the voltage
across the capacitor is:
" R%
! = arctan $
# XC '&
The equation for the capacitive reactance is:
XC =
5!
1
2! fC
5