Exploring Power Supplies with Multisim
P. C. Womble
Hooray! This is a Multisim only Lab!
Typical line voltages in the US are between 110 V to 120 V with a frequency of 60 Hz.
However, digital devices require DC power and thus the
supplies can be created.
From Physics 260, the ratio of the input voltage on a transformer to its output voltage
follows the relation of
is the number of turns on the coil on the primary side (input) and N
number of turns on the secondary (output) side of the transformer.
In Multisim, under the “Basic” family, a virtual transform can be found. Among its
properties is the primary to secondary turns ratio. Examining Equation 1 closely, a
secondary turns ratio of 100, will actually reduce the input voltage by a factor
of 100. This is called a “step down” transformer. If the primary to secondary ratio is
less than 1 (but greater than zero), then the transformer is a “step up” transformer.
Place connect a 60 Hz 120 V AC source on the
Multisim screen. Connect this component to the
virtual transformer described above
(shown to the
The AC source can be connected either to
the two wires on the left of the transformer. Th
transformer will then follow the relationships
Connect the multimeter between the topmost and
bottom most wires on the right side of the
transformer. Connect a ground to the “
“ of the
multimeter. Set the multimeter for AC voltage
surement. Simulate the circuit.
Fill in the worksheet for this section.
Diodes effectively conduct current in one direction. In the early days of electronics,
diodes were tubes with two elements: a heating element and a cathode. The heater
s called the “anode” and when current ran through the heating element, it would
emit electrons. These electrons were attracted to the “cathode” (the “+” side) and
thus current would flow. Reversing the current would be impossible because the
made of a different material.
Today semiconductor materials
are used to construct these device
. Some materials
can be “doped” or lightly c
ontaminated with other elements
. When this occurs, an
electron can be easily removed from the doped materials. T
hese are called “n
type” materials have been doped with elements in which there are
“holes” (absence of electrons). The n
type material is combined with the p
material. Imagine that you have two types of chocolate bars and you s
mash then ends
together to form a long chocolate bar. One end, for instance, may be a Milky Way
bar and other a Heath
. How the bar tastes, then, depends on which side of the bar
It is a similar in a p
n junction. If you attach a battery wi
th the “+” to the n
and the “
“ to the p
type side, no current flows. Why? The nearly free electron in
type material immediately flows there and the electrons from the “
flow into the holes. There is no current flow across the
However, if the + side is connected to the p
type side and the “
“ side is connected to
type side, current will flow across the diode.
We will use a “Wheatstone bridge” arrangement of diodes to
change or “rectify” the
voltages from the
Half wave rectifier
Simulate Circuit 2 using Multisim.
Fill out the section of the worksheet for Activity 2.
Set up Circuit 3 and simulate the output of the oscilloscope.
Fill out the section of the worksheet for Activity 3.
Circuit 3: Full Wave Rectifier
The previous exercises have assumed that you have a center
What if none is available? The next circuit, the rectifier bridge, does not use the
Set up Circuit 4 and simulate the oscilloscope
t the appropriate section of the worksheet.
Circuit 4: Bridge Rectifier. Note: Due to
a printing error, the top contact of T1 is connected to
the connection between D1 and D2.
A capacitor filter is used to make the waveforms more DC. The RC time constant is
changed to make a
smoother waveform as shown below.
Note: You may have to change the time step of the Multisi
m to avoid errors in the
calculation. This can be done under the “Simulation Parameters”.
Set up Circuit 5 in Multisim.
Change the value of the capacitor to create a waveform as smooth or smoother
than the one shown in the above picture.
Circuit 5. Bridge Rectifier with
Set up and simulate Circuit 6. The LM7815 and LM7915 can be found by
searching the components.
Fill out the appropriate section of the worksheet.
Circuit 6: Dual Power Supply