engg1203: introduction to electrical and electronic

ENGG1203: Introduction to Electrical and Electronic EngineeringSecond Semester, 2017–18

Lab 6Objective:

• To experiment with loading effect on a driving circuit• To learn to use an op-amp as a buffer

Equipment you’ll need in this lab:

(a) Multi-meter (b) Wire Kit (c) Breadboard

(d) Power Supply Ca-ble

(e) 10𝑘ΩPotentiome-ter

(f) Op-Amp (g) 680 Ω Resistor

(h) 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 partnerand the assigned table,

1. Log in to Moodle.

2. Select the assignment Lab 6 Partner

Please proceed to your assigned table.

ENGG1203: Introduction to Electrical and Electronic Engineering Lab 6

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2 The Power Need

So far you have been using the FPGA board to supply a 3.3V power as 𝑉cc. In real world circuits whenhigher power is required. In this lab, you will be using a power supply commonly found in labs.

The power supply you are using for in this class is shown in Figure 2(a). It has 3 output channels, labeledCH1, CH2 and CH3. In this lab, use the middle channel labeled CH1. A few notes about how thepower supply operates:

• Connect the ground of your circuit to the BLACK terminal, and Vcc to the RED terminal. Ignorethe 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.

• 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 bycurrent or voltage. In this class, always make sure the light is green, i.e. your output shouldbe controlled by voltage. If it turns to red, then turn up the dial for CURRENT until it turns backto green.

• The terminal outputs are active only if the OUTPUT button on the top left is depressed. Usethis button often. Whenever you are experimenting with the settings on the power supply, makesure 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 =

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2.2 Turn on OUTPUT Press the OUTPUT button to enable output on the power supply. What is thereading 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 immediatelyafter testing your circuit.In general, always turn off the output of the power supply when you are constructing ordebugging your circuit. Only enable output when you are testing the circuit. It helps to avoidcausing damage on the lab equipments.

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3 Motor Introduction

Now that you understand how a power supply works, let’s begin with driving a simple DC motor shownbelow:

(a) Photo

M+ M-

(b) Schematic Sym-bol

Figure 3: Simple Motor

As illustrated by its schematic symbol, a DC motor can be 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 = 𝑘m𝑉m

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 withthe following definition:

Pin Color Signal1 Black M-2 Red M+3 White Not Connected

Using a digital multi-meter, determine the resistance of the motor 𝑅m across the terminals M+ and M-.(Hint: you may want to use an external connection header to help position the DMM probes.)

𝑅m =

3.2 Now, connect the motor to a breadboard exactly like the following diagram. You will need togradually build the circuit later in this part so it is important that you make your breadboard connectionexactly 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.

As a convention, use the BROWN wire to connect to RED terminal, and YELLOW wire to connect tothe BLACK terminal.

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1 5

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1 5

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To Power Supply

0 - 6V

To Motor

Wire A(remove after Checkoff 1)

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 powersupply 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 inan 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 wayto eventually have such control using electronic circuits. For that, you will begin with driving a motorusing a potential divider similar to the one below. In this circuit, 𝑉cc = 6 V, 𝑅o = 680 Ω, 𝑅pot = 10 kΩ.

𝑉cc

𝑅o

𝑅pot𝑉p 𝑉mn+ −

Figure 5: Driving a motor using simple potential divider.

4.1 Based on your breadboard circuit from previous steps, construct an addition circuit that implementsthe potential divider in Figure 5. Leave one end of Wire B unconnected.

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1 5

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To Power Supply

CH1 (6V)

To Motor

Wire B

Untouchedfrom previous

parts

Figure 6: Breadboard connection for potential divider in Figure 5.

With the motor DISCONNECTED do the following:

• Set output voltage of power supply to 6 V.

• Turn ON power supply output.

• Adjust the POT such that the voltage 𝑉p is 3V. Measure 𝑉p using your DMM at Wire B.

• Turn OFF power supply output.

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4.2 Motor Driving Now, without touching the POT, complete the circuit by connecting Wire B to themotor circuit you have built before as shown in Figure 7

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1 5

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ab

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To Power Supply

CH1 (6V)

To Motor

Wire B

Figure 7: Breadboard connection for driving motor with potential divider as in Figure 5.

Test your circuit by performing the following steps:

• Turn ON power supply output.

4.3 Circuit Debug Your motor DOES NOT ROTATE even with the power supply turned on. Tosee what is happening to the motor, measure the value of 𝑉p with the motor connected. What is thevoltage? 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:

4.4 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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4.5 Clean up

REMOVE the 2 wires colored blue from Figure 7 before you proceed.

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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 bufferingcircuit. In particular, you will use a simple voltage follower as shown in the following diagram:

−

+

𝑉cc

𝑉p

𝑉cc

𝑅o

𝑅pot 𝑉mp 𝑉mn+ −

Figure 8: A simple motor driver.

Reminder: DISCONNECT any power supply from the breadboard before you pro-ceed 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 9(a)). The op-amp IChas 8 pins and contains two op-amps internally. Figure 9(b) shows how the two op-amps are connectedinternal to the IC. Only ONE op-amp is used in this lab.

(a) Photo

1

2

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8

7Inputs A

Inputs B

Output A

Vcc

Output B

VEE/GND

(b) Pinout of the op-amp IC

Figure 9: Op-Amp

The power of the two op-amps are both supplied from Pin 2 and Pin 4 of the IC. Connect VCC of theop-amp IC (pin 2) to the “+” rail at the bottom. Connect GND pin of the package (pin 4) to ground.

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Your final circuit should look like the connection in Figure 10. Note the special marking (a small halfcircle) on the top of the op-amp package. The location of this marking helps you to orient the IC andidentify the pins.

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To Motor

1 2 3 4

6 58 7

To Power Supply

CH1 (6V)

Figure 10: 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 shouldrun. Test your circuit by measuring the voltage 𝑉p and 𝑉mp on the breadboard using your DMM. Youcan 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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engg1203: introduction to electrical and electronic

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