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ENGR 43
Lab Activity
Student Guide
Lab 4 – Speakers in Series and Parallel
Student Name: ___________________________________________________
Acknowledgements
Subject Matter Expert: Wayne Phillips, Chabot College, Hayward, CA
Purpose
In this lab activity you will build simple series and parallel circuits using audio speakers as the
loads. You will verify your connections by performing basic resistance checks with the
ohmmeter, and verify Ohm’s law and Watt’s law by calculating power delivered to the speakers
in series and parallel configurations. You will gain additional experience in using the function
generator, DC power supply, and oscilloscope; and you will see one of the fundamental
limitations of the oscilloscope in series circuits and learn one way to work around this limitation.
Systems Rationale
This lab activity demonstrates the function, configuration, and measurement techniques of the
core elements of an analog system: input (function generator), outputs (speakers), power source
(DC power supply), and the electronic controlling element (the audio amplifier).
Before Starting This Activity
Look at the Getting Started section of the Tektronix PS280 manual and the Getting Started
manual for the Fluke 271 Function Generator.
Links to these learning modules are on the GoogleDocs listing (http://tinyurl.com/engr43lablinks).
Student Learning Outcomes
Relevant knowledge (K), skill (S), or attitude (A) student learning outcomes include:
K1. Define peak-to-peak voltage, period, and frequency for AC waveforms
K2. Convert peak-to-peak measurements to RMS values
K3. Describe the function of the vertical, horizontal, and triggering sections of an oscilloscope
S1. Measure peak-to-peak voltages, period, and frequency of AC waveforms
S2. Measure resistance of series and parallel networks and AC voltages with the DMM
S3. Compile data into a test report.
A1. Recognize the significance of the oscilloscope as a primary tool for the technician.
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Process Overview
In this lab activity, you will measure the DC resistance of the speakers individually and in series
and parallel configurations to verify the set-up of the test circuit. The function generator will be
used initially as the signal source for the speakers, and you will characterize the loading effect of
the speakers on the output of the function generator. You will then connect the audio amplifier
module and power it with the DC power supply. You will measure the amplifier clipping level
and output voltage, and calculate output power.
Time Needed
Lab Performance:
It should take students approximately 3.0 hours to work through the entire lab.
Lab Deliverables:
It should take students approximately 1.0 hour of homework time to create the lab report
summary.
Equipment & Supplies
Item
24-ohm speakers
Mini-clip, alligator clip leads, and 22 ga. solid jumper wires
BNC test leads
LM380 audio amplifier module
Handheld DMM
Proto-board for circuit connections
Quantity
2
As req.
3
1
1
1
Special Safety Requirements
The sound produced by the audio speakers can be potentially damaging to your hearing if the
speakers are very close to your ears. Do not place your ears close to the speakers unless you
ensure that the audio levels are low.
Lab Preparation
Verify that your lab station has a function generator, an oscilloscope, and a DC power supply,
and that all are plugged in and powered up. Turn the two voltage knobs on the power supply
fully counter-clockwise (CCW). Turn the two current knobs to the mid position. Set the two
pushbutton switches between the knobs to the OUT position. Set the two slide switches to the
VOLTS position and verify the two displays both show 0 volts.
Introduction
All speakers produce sound by converting AC electrical power into pulsating waves of air
pressure. Most speakers use a linear electric motor to convert the electrical energy to magnetic
energy, which is then converted to mechanical energy to move the surrounding air. The loudness
you hear is controlled by the amount of electrical power delivered to the speakers. When one
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Student Guide
source is used to drive more than one speaker, the speakers may be connected in series, parallel,
or, in the case of several speakers, a combination of series and parallel.
Task
In this lab you will see the effects of connecting speakers in series and parallel, and the effect on
the power delivered to the speakers. You will calculate the power delivered to the speakers using
Ohm’s and Watt’s laws, and you will calculate RMS voltages from the peak-to-peak voltages
measured on the o-scope.
Performance
Part 1: Checking Voice Coil Resistance
1.
Place one of the speakers face down on the test bench with the metal connection lugs
facing you. Find the “+” and “–” markings near the lugs.
2.
Set the dial selector on the DVM to the “Ω” position to measure resistance. To
measure resistance, the DVM supplies a fixed current to the speaker, and measures
the resulting voltage across the speaker. Because of this, it is very important to
NEVER attempt to measure resistance of a component when it is installed or
connected to a circuit.
3.
Put the red lead of the meter (the Volt-Ohm terminal of the meter) on the + speaker
terminal and the black lead (the Com terminal) on the – speaker terminal. The speaker
resistance should be something less than 24Ω. The DC resistance of all dynamic
(voice coil) speakers will be less than the AC impedance that the speaker is rated at.
Reverse the connections, red on – and black on +. Is the resistance the same? Repeat
the measurement for the other speaker.
Speaker 1 = ______Ω
4.
Speaker 2 = ______Ω
Use a mini-clip or alligator clip to connect the – lead of one speaker to the + lead of
the other speaker. Measure the resistance between the open terminals of the speakers
to find the total resistance of the series connection.
Series Speakers = ______Ω
5.
With the other quick-connect lead, connect the open terminals of the two speakers
together. Measure the resistance between the + and – terminals of one of the speakers
to find the total resistance of the parallel connection.
Parallel Speakers = ______Ω
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Part 2: Make Some Noise
8.
Remove all connections from the speakers. Connect the BNC-clip lead to the Output
connector on the function generator.
9.
Set the following on the function generator: Amplitude: VhiZ=100 mVpp 50Ω,
Waveform: sinewave, Frequency 1.00 kHz.
10.
Connect the BNC-clip test lead to the function generator MAIN output. Connect the
red clip to the + terminal of one speaker, and the black clip to the – speaker terminal.
11.
Press the output enable button on the function generator. Set the DVM to AC volts.
Put the DVM leads on the + and – speaker leads. Adjust the amplitude until the DVM
measures a 50 mV output. (Remember, the DVM displays RMS voltage.)
12.
Calculate the power driving the speaker with the formula P=V2/R, using the 50 mV
and the 24Ω rated impedance of the speaker. Power = 0.0025/24 = 104 µW
13.
Set the frequency to 10.0 kHz and listen to the speaker. Set the frequency to 100 Hz
and listen to the speaker (you may have to put your ear closer to the speaker). Adjust
the frequency dial and listen to the frequencies. What are the lowest and highest
frequencies you can hear?
Low _________Hz
High _________Hz
14.
Set the function generator frequency to 100 Hz. Remove the function generator clips
from the speaker
15.
Connect two speakers in series as follows: use one of the mini-clip or alligator leads
to connect the – lead of one speaker to the + lead of the other speaker. Connect the
open + terminal to the red function generator clip and the open – terminal to the black
function generator clip. Measure the voltage from the + and – terminals that connect
to the source (the function generator) and adjust the func gen amplitude for 50 mV
RMS as measured by the DVM.
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16.
Measure the voltage at each speaker individually (+ terminal to – terminal). Calculate
the power delivered to each speaker (P=V2/R) and find total power by adding the
power of the two speakers.
Speaker 1 Power ______ + Speaker 2 Power ______ = Total Power ________
17.
Do not touch the amplitude adjustment of the function
generator! Remove all connections from the speakers. Use the mini-clip or
alligator leads to wire the speakers in parallel (+ to + and – to –). Hook the function
generator clips to one of the speakers, red to + and black to –
18.
Measure the voltage across one of the speakers. (In parallel wiring, the voltage will be
the same across both speakers). Is it still 50 mV? (This difference is called the loading
effect of the speakers).
Loaded voltage=________mV
19.
Reset the amplitude to restore the 50 mVrms across the speakers
20.
Calculate the total power delivered to each speaker (same formula and values for the
single speaker), and find total power by adding the power of the two speakers.
Speaker 1 Power ______ + Speaker 2 Power ______Total Power ________
Part 3: Connect the Amplifier, Make More Noise
21.
22.
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Remove all connections from the speakers. Insert the LM380 audio amplifier board
in the proto board. Pin 1 of the connector is on the left, and pin 9 is on the right.
Confirm that the power supply voltage displays read 0 volts. Insert a 22 ga jumper
wire in the proto board row to connect to pin 1 of the LM380 board. Double-check to
confirm that you are not one row to the right or left of pin 1. Connect a red banana5
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clip lead to the jumper wire, and plug the banana jack into the left red terminal on the
power supply.
23.
Connect a jumper wire to pin 3, and a black banana-clip lead to the jumper and to the
black power supply terminal nearest the left red terminal (not the green terminal next
to the red terminal).
24.
Connect jumper leads between pins 4 and 8
25.
Make connections as follows:
Func gen red to pin 9: + input
Func gen black to pin 4: ground
Amplifier + output (to speakers) to pin 6: output
Amplifier – output (to speakers) to pin 3: ground
Optional: as needed for amplifier stabilization, connect a 0.1µF capacitor between
pins 8 and 9.
26.
Ask the instructor to verify your connections. After your circuit is checked, adjust the
left voltage knob to set the power supply output to 10 volts.
27.
Repeat steps 13 through 20, except now you are driving the speakers with the LM380
audio amplifier, and the function generator is the input to the LM380. Make sure you
are setting the 100 mV level at the output of the amplifier, and not the function
generator voltage.
Series:
Func gen voltage ______ (Amplifier output voltage = 100 mV)
Speaker 1 Power ______ + Speaker 2 Power ______ = Total Power ________
Parallel:
Loaded voltage=________mV
Speaker 1 Power ______ + Speaker 2 Power ______ = Total Power ________
Part 4: Measure the clipping voltage with the O-scope
28.
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Re-connect the two speakers connected in series with the amplifier output. Set the
function generator for a 500 Hz frequency and output amplitude at minimum.
Connect the o-scope channel 1 red lead to the + speaker terminal connected to the
amplifier output, and the black scope lead to the – speaker terminal connected to the
amplifier ground.
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29.
Turn on Chan 1 display, if not already on. Set the channel 1 volts/div to 100 mV and
the horizontal scale to 1 ms/div. Slowly increase the amplitude while watching the oscope. Set the function generator output to produce an o-scope waveform 2 divisions
high. What is the peak-to-peak voltage?
2 div X 100 mV/div = _____________
30.
Change the horizontal scale to 500 µs/div. Note where one of the cycles of the
waveform crosses the horizontal axis in the upward direction. Count the number of
divisions (and partial divisions, each tick mark is 0.2 divisions). What is the period of
the waveform?
_____ div. X 500 µs/div = _____________
What is the frequency of the waveform? 1/period = ____________
(Remember, kHz = 103 and milliseconds = 10-3. Did the exponent come out right?)
31.
Amplifier clipping is the waveform distortion produced when the desired output
waveform is greater than the positive and/or negative voltage limits of the amplifier
output. Increase the function generator amplitude until the amplifier is clipping the
output. Notice the difference in the sound of the speaker with and without clipping.
The clipping is adding harmonics to the sinewave.
32.
Reduce the frequency to 200 Hz (to make it a little easier on the ears). What changed
on the o-scope display? Set the horiz time base to 2 ms/div.
33.
Adjust the function generator amplitude to produce the maximum output without
clipping. (Tip: switching the waveform on the function generator to triangle makes is
easier to see the clipping.) Switch the waveform back to sinewave.
What is the peak-to-peak output voltage?______________
34.
Measure the output voltage with the DVM.
What is the voltage measured by the meter?________________The DVM measures
RMS voltage. For sinewaves, to convert pk-pk to RMS, divide by 2.828.
Calculated RMS voltage=_________
35.
Calculate the power driving the speakers with the formula P=V2/R, using the RMS
voltage and the 48Ω for the series speaker impedance.
Power = _____________
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36.
Measure the voltage at each speaker individually with the DMM (+ terminal to –
terminal). Calculate the power delivered to each speaker (P=V2/R) and find total
power by adding the power of the two speakers. (What value do you use for R in this
formula?)
Speaker 1 Voltage ______ + Speaker 2 Voltage ______=Total Voltage ________
Speaker 1 Power ______ + Speaker 2 Power ______Total Power ________
37.
With the DVM measure the voltage at the input to the amplifier. Calculate the gain of
the amplifier with the following formula: VOUT / VIN = AV
__________ / __________ = ________
38.
Connect a BNC test cable to the channel 2 input of the o-scope. Measure the peak-to
peak voltage at the amplifier input. Compare this with the amplifier output on channel
1 and calculate the gain, this time with the peak-to peak voltages, but with the same
formula: : VOUT / VIN = AV
__________ / __________ = ________
39.
Move the channel 1 probe to measure the voltage across Speaker 2, the one with the –
terminal connected to the amplifier ground. What is the peak-to-peak voltage, and
what is the calculated RMS voltage?
V2p-p = ________ ÷ 2.828 = V2RMS ________
40.
Can you move the channel 2 probe to measure the voltage across Speaker 1? Why
not?
____________________________________________________________
____________________________________________________________
41.
With the channel 2 probe connected across Speaker 2, and the channel 1 probe
connected across the amplifier output (both black leads connected to ground), press
the “Math” channel button on the vertical selection of the o-scope. Choose the “Ch1Ch2” function. What voltage are you seeing on the math channel?
____________________________________________________________
____________________________________________________________
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Deliverable(s)
Save your completed Lab 4 Activity Guide and Performance Report (which starts on this page)
in your Lab Activity Binder.
Scope Measurements
How did the maximum output power with the speakers in series compare with the output power
of the two speakers in parallel?
Considering the difference in power delivered to the speakers, when would you ever want to
connect two speakers in series?
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How did the measured series and parallel resistances compare with the series and parallel power
measurements? Hint:P=V2/R
How did the constant 50 mV driving the speaker (the input) compare to the loudness (the output)
at different frequencies? What do you think can account for this?
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