ENGG1203: Introduction to Electrical and Electronic Engineering
Second Semester, 2013–14
Lab 5
Objective:
• To experiment with loading effect on a driving circuit
• To learn to use an op-amp as a buffer
• To begin constructing the light tracker stage for your project.
Equipment you’ll need in this lab:
(a) Multi-meter
(b) Wire Kit
(d) Power Supply Cable
(e) 10kΩ Potentiometer
(c) Breadboard
(f) Small Motor
Figure 1: Materials for this lab
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1 Finding your lab partner
You will be working with a randomly assigned partner for this lab. To find your assigned lab
partner and the assigned table,
1. Log in to Moodle.
2. Select the assignment Lab 5 Partner
Please proceed to your assigned table.
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2 The Power Need
Instead of relying on the FPGA board to supply 3.3V power as Vcc , you will use a power supply
to supply power for your project.
The power supply you are using for in this class is shown in Figure 2(a). It has 3 output channels,
labeled CH1, CH2 and CH3. In this lab, use the middle channel labeled CH1. A few notes
about how the power supply operates:
• Connect the ground of your circuit to the BLACK terminal, and Vcc to the RED terminal.
Ignore the green terminal marked GND in the power supply.
• Voltage and current output at the terminals of CH1 are controlled by the 2 knobs on the
RIGHT.
ENGG1203: Introduction to Electrical and Electronic Engineering
Lab 5
• The display shows the live voltage and current output at the terminals.
• The signal light marked C.V./C.C. (Figure 2(b)) indicates if the output terminal is controlled by current or voltage. In this class, always make sure the light is green, i.e.
your output should be controlled by voltage. If it turns to red, then turn up the dial for
CURRENT until it turns back to green.
• The terminal outputs are active only if the OUTPUT button on the top left is depressed.
Use this button often. Whenever you are experimenting with the settings on the power
supply, make sure OUTPUT is off. Then you can reenable it once the setting is done.
(a) Photo
(b) C.V./C.C. Indicator
Figure 2: Power Supply
2.1 Basic Power Tests Do the following:
1. Depress the OUTPUT button so it is OFF.
2. Turn the VOLTAGE dial so the output of the power supply is at 6 V.
3. Turn the CURRENT dial so the output of the power supply is at 1.5 A.
Use a digital multimeter (DMM) to measure the output voltage. What is the output voltage?
Output Voltage =
2.2 Turn on OUTPUT Press the OUTPUT button to enable output on the power supply.
What is the reading on the power supply? What is the value you are measuring using the
DMM?
DMM reading:
Power Supply reading:
WARNING: Turn off the power supply output immediately after testing your circuit.
In general, always turn off the output of the power supply when you are constructing or debugging your circuit. Only enable output when you are testing the circuit.
It helps to avoid causing damage on the lab equipments.
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ENGG1203: Introduction to Electrical and Electronic Engineering
Lab 5
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3 Motor Introduction
There are two types of motors you may use for your project. The operations of the two types of
motors are identical. The only difference between them is their power requirements.
Let’s begin with driving a simple motor shown below:
M+
(a) Photo
M-
(b) Schematic Symbol
Figure 3: Simple Motor
As illustrated by its schematic symbol, a motor is modeled as a simple two-terminal device.
Define Vm to be the voltage across the two terminals M+ and M-, then a motor has the following
properties:
• The rotational speed of a motor (ω) is proportional to Vm ;
• The direction of rotation depends on the polarity of Vm .
That is,
ω = km Vm
with the sign of ω representing the direction of rotation and km is a constant.
3.1 First, we will determine some basic properties of the motor. The motor has a 3-pin connector
with the following definition:
Pin
1
2
3
Color
Black
Red
White
Signal
MM+
Not Connected
Using a digital multi-meter, determine the resistance of the motor Rm across the terminals M+
and M-.
Rm =
3.2 Now, connect the motor to a breadboard exactly like the following diagram. You will need
to gradually build the circuit later in this part so it is important that you make your breadboard
connection exactly as shown below. Make sure you are connecting the power supply to the
BOTTOM power row. Note that we are NOT connecting the top and bottom power and
ground rows in this lab yet.
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ENGG1203: Introduction to Electrical and Electronic Engineering
Lab 5
20
15
10
5
d
c
20
10
5
1
15
b
Wire A
a
To
Power Supply
0 - 6V
e
f
g
h
i
j
1
To Motor
Figure 4: Connection of motor on the breadboard.
Now, start with ZERO volt output at the power supply, gradually turn UP the voltage from the
power supply to 6V. What is the voltage needed to turn on the motor? What is the corresponding
current? You can read these values directly from the power supply.
Vm, on =
Im, on =
3.3 Now, try reversing the connection of the motor (NOT the power supply). Is the motor
turning in an opposite direction? Does it have a similar turn-on voltage as above?
3.4 Checkoff 1
Show the working motor to your TA and answer the following questions:
• What is the internal resistance, turn on voltage, and current of the motor?
• How do you reverse the rotational direction of the motor?
3.5 Disconnect the Motor
• Disconnect the motor from the breadboard.
• Remove Wire A in Figure 4.
• Keep the remaining wires intact.
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4 Motor Driving
Instead of controlling the speed of the motor manually using knob on the power supply, you
need a way to eventually have such control using electronic circuits. For that, you will begin
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ENGG1203: Introduction to Electrical and Electronic Engineering
Lab 5
with driving a motor using a potential divider similar to the one below. In this circuit, Ro =
680Ω, Rpot = 10kΩ.
Vcc
Ro
Vp
Rpot
+ −
Vmn
Figure 5: Driving a motor using simple potential divider.
4.1 Construct the breadboard circuit below that implements the potential divider in Figure 5.
You should build on top of the circuit in previous section. DO NOT CONNECT Wire B on
the breadboard yet.
25
30
25
30
20
15
10
5
g
h
i
j
1
To Motor
d
c
b
20
15
10
5
1
a
To
Power Supply
CH1 (6V)
e
f
Wire B
Figure 6: Breadboard connection for Figure 5.
With the motor DISCONNECTED (no Wire B), do the following:
• Turn ON power supply output.
• Adjust the POT such that the voltage Vp is 3V. Measure Vp using your DMM.
• Turn OFF power supply output.
4.2 Motor Driving Now connect the motor to your potentiometer and test your circuit by
performing the following steps:
• Turn OFF power supply output.
• Connect the output of the potentiometer (Vp ) to the input of the motor by Wire B.
• Turn ON power supply output.
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ENGG1203: Introduction to Electrical and Electronic Engineering
Lab 5
At this point, even if your circuit was built correctly, your motor SHOULD NOT BE RUNNING.
To see what happened to the motor, measure the value of Vp with the motor connected. What
is the voltage? What is the current and voltage reading from the power supply?
Vp with motor connected:
Voltage output from power supply:
Current output from power supply:
4.3 Checkoff 2
• Explain the behavior of the motor. Why is the motor not running?
• Explain the reading of Vp .
• Explain the voltage and current readings from the power supply.
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5 Buffering with an Op-Amp
To drive the motor efficiently, you will use an operational amplifier (op-amp) to construct a
buffering circuit. In particular, you will use a simple voltage follower as shown in the following
diagram:
Vcc
Ro
Vcc
Rpot
Vp
+
Vmp
+ −
Vmn
−
Figure 7: A simple motor driver.
Reminder: DISCONNECT any power supply from the breadboard before
you proceed with the construction of the op-amp circuit.
5.1 Op-Amp as Buffer Construct the above buffering circuit on your breadboard.
You will be using an op-amp in a standard integrated circuit package (Figure 8(a)). The op-amp
IC has 8 pins and contains two op-amps internally. Figure 8(b) shows how the two op-amps are
connected internal to the IC. Only ONE op-amp is used in this lab. You will use both in later
labs.
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ENGG1203: Introduction to Electrical and Electronic Engineering
Output A
1
8
Vcc
2
7
Output B
3
6
VEE/GND
4
5
(a) Photo
Lab 5
Inputs A
Inputs B
(b) Pinout of the op-amp IC
Figure 8: Op-Amp
The power of the two op-amps are both supplied from Pin 2 and Pin 4 of the IC. Connect VCC
of the op-amp IC (pin 2) to the “+” rail at the bottom. Connect GND pin of the package (pin
4) to ground.
Your final circuit should look like the connection in Figure 9. Note the special marking (a small
half circle) on the top of the op-amp package. The location of this marking helps you to orient
the IC and identify the pins.
20
25
30
20
25
30
15
10
5
8
7
6
5
1
2
3
4
d
c
b
15
10
5
1
a
To
Power Supply
CH1 (6V)
e
f
g
h
i
j
1
To Motor
Figure 9: Implementation of the op-amp buffer on breadboard.
5.2 Test your breadboard circuit. Now, turn on the output of your power supply. The motor
should run. Test your circuit by measuring the voltage Vp and Vmp on the breadboard using
your DMM. You can adjust the voltage Vp by adjusting the POT. Fill in the following table:
Vp
0
1
2
3
4
5
6
Vmp
5.3 Checkoff 3
Show to your TA the completed motor driving circuit and answer the following questions:
• Why is the voltage Vp adjustable in this case but not when there was no buffer?
• Where does the motor get its running current from?
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