ENGR43Lab5

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ENGR 43 Lab Activity Student Guide Lab 5 – Speakers in Series and Parallel Student Name: ___________________________________________________ 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. 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 documents are on the GoogleDocs listing ( http://tinyurl.com/engr43-lablinks ) . 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. 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 2.0 hours to work through the entire lab. Lab Deliverables: It should take students approximately 1.0 Lab 5 – Speakers in Series and Parallel © 2012 ENGR 43 1

ENGR 43 Lab Activity Student Guide Equipment & Supplies Item Quantit y 24-ohm speakers 2 Banana plug leads As req. BNC test lead 1 1x/10x scope probes 2 Audio amplifier module 1 Handheld DMM 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 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 oscilloscope. Performance Part 1: Checking Voice Coil Resistance 1. Place one of the speakers face down on the test bench with the banana plug connecters facing you. On the speaker the red connector is “+” and the black connector is “–”. 2. Set the dial selector on the DMM to the “Ω” position to measure resistance. To measure resistance, the DMM 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. Connect the red lead of the meter (the Volt-Ohm terminal of the meter) to the + speaker terminal and the black lead (the Com terminal) to 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 rated AC impedance of the speaker. Reverse the Lab 5 – Speakers in Series and Parallel © 2012 ENGR 43 3

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ENGR 43 Lab Activity Student Guide 4. Repeat the measurement for the other speaker. Speaker 1 = ____Ω, Speaker 2 = ____Ω 5. Use a banana plug leads 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 6. Co nnect the speakers in parallel as shown. Measure the resistance between the + and – terminals of one of the speakers to find the total resistance of the parallel connection. Notice that the banana plugs can be stacked. Parallel Speakers = ______ Ω Part 2: Make Some Noise 1. Remove all connections from the speakers. Connect the DMM to one speaker using banana plug leads. Set the DMM to AC volts. 2. Connect the BNC-clip test lead to the function generator MAIN output. Connect the red clip to the + terminal of the speaker, and the black clip to the – speaker terminal. The clips can connect into one of the holes of the banana plug. 3. Set the following on the function generator: Amplitude: VhiZ=100 mVpp 50Ω, Waveform: sinewave, Frequency 1.00 kHz. 4. Press the output enable button on the function generator. Adjust the amplitude until the DMM measures a 50 mV output. (Remember, the DMM displays RMS voltage.) 5. Calculate the power driving the speaker with the formula P=V 2 /Z, using the 50 mV and the 24Ω rated impedance of the speaker. (Impedance is the opposition to AC current flow, similar to the resistance to DC current flow). Verify that your calculation matches this value: Power = (0.05) 2 /24 = 104 µW 6. 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 ________kHz 7. Set the function generator frequency to 100 Hz. Remove the function generator clips from the speaker Lab 5 – Speakers in Series and Parallel © 2012 ENGR 43 5

ENGR 43 Lab Activity Student Guide Part 3: Connect the Amplifier, Make More Noise The amplifier module is an example of a simple electronic system: it has an input (from the function generator), an output (to the speaker), a power source (the bench DC supply), and a process (amplification) We are going to characterize the amplifier as it operates in a typical configuration. 1. Remove all connections from the speakers. Verify that the bench power supply voltage displays show 0 volts. 2. Connect the amplifier to one speaker as follows: a. Function generator output (red lead) to amplifier input, terminal 6. b. Function generator ground (black lead) to amplifier input common, terminal 5 c. Amplifier output, terminal 1, to speaker + terminal. Use banana to alligator clip leads. d. Speaker – terminal to amplifier ground, terminals 3&4. e. Bench power supply output 1 + red terminal to amplifier DC power input terminal 2. f. Bench power supply output 1 – black terminal to amplifier ground, terminals 3&4. 3. Set the function generator for a 10 mVpp amplitude, 400 Hz sinewave output. 4. Ask the instructor to verify your connections. After your circuit is checked, adjust the left voltage knob to set the power supply output to 6 volts. 5. Connect scope probes to the oscilloscope channel 1 and 2 inputs. Switch the probes to 1x and the scope input probe multiplier setting to 1x. (Always verify that the scope input multiplier matches the probe multiplier setting.) Connect channel 1 to the amplifier output, channel 2 to the amplifier input, and the probe ground clips to the amplifier ground terminal. Lab 5 – Speakers in Series and Parallel © 2012 ENGR 43 6

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ENGR 43 Lab Activity Student Guide Part 4: Amplifier Gain and Clipping 1. The voltage gain of the amplifier, A V , is the ratio of the AC voltages v out / v in . Measure the voltages with the oscilloscope and calculate the gain. _______ v out / _______ v in = ________A V 2. Amplifier clipping occurs when the output voltage increases to the level where the amplifier can no longer accurately reproduce the input signal. This is primarily a function of the input DC supply voltage. To make the next test easier on the ears, set the function generator frequency to 200 Hz. 3. Increase the amplitude of the function generator while observing the amplifier output waveform on the oscilloscope and listening to the output of the speaker. Continue to increase the amplitude until you see and hear the distortion of the output waveform. Reduce the amplitude so that you have the maximum output without distortion. Vpp =______________ V 4. Calculate the maximum output power with the formula P=V 2 /Z, using the measured RMS output voltage of the amplifier and the 24Ω rated impedance of the speaker. To convert peak-to-peak sinewave voltage to RMS, V PP / (2*  2) = __________V RMS max power = ____________ W 5. Increase the DC supply voltage to 10 V. Repeat the clipping voltage test, measure the maximum output voltage, and calculate the maximum output power max power = ____________ W Part 5: Speakers in Parallel 1. Use the mini-clip or alligator leads to wire the speakers in parallel (+ to + and – to –). Connect the amplifier output to the speakers, output to + and ground to – Leave the oscilloscope connections on the amplifier input and output. 2. Set the DC bench power supply voltage to 6 V. 3. Repeat the clipping voltage test, measure the maximum output voltage, and calculate the maximum output power. Use 12Ω for the total parallel impedance. Vpp =_____ V V RMS = ___________V RMS max power = ____________ W . 4. Calculate the total power delivered to each speaker (same voltage as step 3, 24Ω impedance), and find total power by adding the power of the two speakers. Speaker 1 Power ______ W + Speaker 2 Power ______ W = Total Power ________ W Lab 5 – Speakers in Series and Parallel © 2012 ENGR 43 7

ENGR 43 Lab Activity Student Guide Part 6: Speakers in Series 1. Connect two speakers in series as shown: 2. Repeat the clipping voltage test, measure the maximum output voltage, and calculate the maximum output power. Use 48Ω for the total parallel impedance. Vpp =______________ V max power = ____________ W . 3. With the DMM, measure the RMS voltage at each speaker individually (+ terminal to – terminal). Calculate the power delivered to each speaker (P=V 2 /24) and find total power by adding the power of the two speakers. Speaker 1 Power ______ + Speaker 2 Power ______ = Total Power ________ 4. Move the channel 2 scope probe from the amplifier input to the + terminal of speaker 2. What is the peak-to-peak voltage, and what is the calculated RMS voltage? V 2p-p = ________ / (2*  2) = V 2RMS ________ This value should be very close to your DMM measurement. 5. 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 oscilloscope. Choose the “Ch1-Ch2” function. What voltage are you seeing on the math channel? _______________________________ _______________________________ _______________________________ Deliverable(s) Complete Lab 4 Activity Guide and Performance Report (which starts on the next page). Lab 5 – Speakers in Series and Parallel © 2012 ENGR 43 8

ENGR 43 Lab Activity Student Guide Lab 3 – Speakers in Series and Parallel Student Name: ___________________________________________________ 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? Lab 5 – Speakers in Series and Parallel © 2012 ENGR 43 9

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ENGR 43 Lab Activity Student Guide How did the measured series and parallel resistances compare with the series and parallel power measurements? Hint:P=V 2 /R In Part 2, how did the constant 100 mV driving the speaker (the input) compare to the loudness (the output) at different frequencies? What do you think can account for this? Lab 5 – Speakers in Series and Parallel © 2012 ENGR 43 10