Exploring Power Supplies with Multisim

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7 Οκτ 2013 (πριν από 3 χρόνια και 8 μήνες)

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Exploring Power Supplies with Multisim

Lab 3

P. C. Womble


Hooray! This is a Multisim only Lab!


AC Power:


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
power
supplies can be created.


From Physics 260, the ratio of the input voltage on a transformer to its output voltage
follows the relation of


Equation 1

where N
p

is the number of turns on the coil on the primary side (input) and N
s

is the
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
primary to
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.


Activity 1:

1.

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
right)
.


The AC source can be connected either to
the two wires on the left of the transformer. Th
e
transformer will then follow the relationships
described above.


2.

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
mea
surement. Simulate the circuit.


3.

Fill in the worksheet for this section.


Diodes:

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
wa
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
cathode was

made of a different material.


Today semiconductor materials
are used to construct these device
s
. 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. “P
-
type” materials have been doped with elements in which there are
“holes” (absence of electrons). The n
-
type material is combined with the p
-
type
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
TM

bar and other a Heath
TM
. How the bar tastes, then, depends on which side of the bar
you eat.


It is a similar in a p
-
n junction. If you attach a battery wi
th the “+” to the n
-
type side
and the “
-
“ to the p
-
type side, no current flows. Why? The nearly free electron in
the n
-
type material immediately flows there and the electrons from the “
-
“ battery
flow into the holes. There is no current flow across the

junction.


However, if the + side is connected to the p
-
type side and the “
-
“ side is connected to
the n
-
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

transformer.

Circuit 2

Half wave rectifier

Activity 2:

1)

Simulate Circuit 2 using Multisim.

2)

Fill out the section of the worksheet for Activity 2.


Activity 3:


1)

Set up Circuit 3 and simulate the output of the oscilloscope.

2)

Fill out the section of the worksheet for Activity 3.





Circuit 3: Full Wave Rectifier

A
ctivity 4:

The previous exercises have assumed that you have a center
-
tapped transformer.
What if none is available? The next circuit, the rectifier bridge, does not use the
center tap.


1)

Set up Circuit 4 and simulate the oscilloscope
readings.

2)

Fill ou
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.

Activity 5:


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”.


1)

Set up Circuit 5 in Multisim.

2)

Change the value of the capacitor to create a waveform as smooth or smoother
than the one shown in the above picture.

3)


Circuit 5. Bridge Rectifier with
filter capacitor.

Activity 6:

1)

Set up and simulate Circuit 6. The LM7815 and LM7915 can be found by
searching the components.

2)

Fill out the appropriate section of the worksheet.

Circuit 6: Dual Power Supply