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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. Lab 4 ENGR 43 1 © 2012 ENGR 43 Lab Activity Student Guide 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 Lab 4 © 2012 2 ENGR 43 ENGR 43 Lab Activity 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 Lab 4 ENGR 43 3 © 2012 ENGR 43 Lab Activity Student Guide 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. Lab 4 ENGR 43 4 © 2012 ENGR 43 Lab Activity Student Guide 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. Lab 4 ENGR 43 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 © 2012 ENGR 43 Lab Activity Student Guide 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. Lab 4 ENGR 43 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. 6 © 2012 ENGR 43 Lab Activity Student Guide 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 = _____________ Lab 4 ENGR 43 7 © 2012 ENGR 43 Lab Activity Student Guide 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? ____________________________________________________________ ____________________________________________________________ Lab 4 ENGR 43 8 © 2012 ENGR 43 Lab Activity Student Guide 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? Lab 4 ENGR 43 9 © 2012 ENGR 43 Lab Activity Student Guide 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? Lab 4 ENGR 43 10 © 2012