Voltage Divider and Power Prac

Transcription

1 Name: Teacher: Voltage Divider and Power Prac Part 1: Simple voltage divider incorporating a NTC Thermistor Figure 1 represents a simple voltage divider using a NTC thermistor. +5V 22k T h e r m i s t o r ( N T C ) VOUT 0V Figure 1 1. Wire up the above circuit on a breadboard. Connect the 5V supply to the circuit. Measure the supply voltage (VS) and the output voltage (VOUT) across the thermistor. VS = V VOUT = V 2. Using the general voltage divider formula given below calculate the resistance of the thermistor (RT). RT V OUT x V R 22k T S 3. Using the datasheet provided (see appendix A ) find out the temperature corresponding to the resistance of the thermistor. TR = C 4. What does this temperature indicate? 1

2 Part 2: Loaded voltage divider Figure 2 represents a simple voltage divider powered by a 5V supply. Figure 3 shows the same voltage divider being used to power a load resistor R k. +5V +5V 1.8k 1.8k 2.2k 2.2k Rk 0V 0V Figure 2 Figure 3 1. For Figure 3 show that the: a) total resistance in the circuit is: ( R) k R b) current through the circuit is: 5(2.2 + R) ma ( R) c) voltage across R is: 11R ( R) V d) operating power of R is: 121R ( R) 2 mw 2. Calculate the power at which R is operating for the values: R = 100, 220, 470, 820, 1k, 1.5k, 2.2k, 4.7k 2

3 3. Set up the circuit in Figure 3 above and record the current through and the voltage across R in each case. Hence, calculate the operating power for each value of R. R ( ) Current (ma) Voltage (V) Power (mw) k 1.5k 2.2k 4.7k 4. How do the theoretical and practical compare. 5. Estimate the value of R, which will have the maximum operating power. 6. Attempt to verify your estimate for R using a graphics calculator or otherwise. 7. For what values of R is the simple voltage divider formula a good approximation? VOUT = 2.2 x 5V ( ) 8. In terms of the physics principals involved, briefly explain your answer to question 7? 3

4 Problem Solving exercise The thermistor used in the above experiment is now used as a temperature sensor to control a fan motor for an amplifier power supply unit as shown in the circuit diagram below. +12V R1 47k CONTROL CIRCUITRY RT t VIN (RIN > 100M ) FAN MOTOR 0V The circuit is designed in such a way that when certain temperature is reached inside the power supply unit of an amplifier the fan motor switches ON to cool the unit and switches OFF when the unit cools down and reaches another set temperature. The Fan Motor switches ON and OFF automatically for these two temperature settings. This means that the control circuitry requires two different input voltages to switch ON and OFF the fan motor. These volta ges are: Fan Motor ON VIN = 3.87V Fan Motor OFF VIN = 6.93V Using the datasheet provided (see page 5 appendix A ) find out when: Fan Motor switches ON (i) the resistance of the thermistor (ii) the temperature inside the power supply unit Fan Motor switches OFF (iii) the resistance of the thermistor (iv) the temperature inside the power supply unit Note: The input resistance (R IN) of the control circuit is >100M. 4

5 APPENDIX 'A' - Characteristic Curve of NTC Thermistor Resistance (k-ohms) Temperature (Deg. C) 5

6 INTRODUCTION TO PICOSCOPE Three specialized pieces of apparatus used are: Signal Generator (Fig1), Picoscope (Fig2) and Variable AC Power Supply (Fig3) in addition to a digital multimeter. Figure 1 Figure 2 Figure 3 6

7 PART 1: Using the Signal Generator Figure 4 1. Set the signal generator frequency to about 1kHz (10 x100hz). Also set the WAVEFORM switch to sinewave (~). 2. Set the signal generator output level switch (ATTENUATOR) to 20dB (or x1). Adjust the output level rotary switch to half way. Note: If you are using a different signal generator to the one shown in the photo ask your teacher for help. Other signal generators may not have the word (ATTENUATOR) but will have a switch with settings labeled 20dB or x1. 3. Connect OUTPUT of the signal generator to the channel A input of the picoscope as shown in figure 4 above. The ground (black) terminal of the signal generator connects to the black lead of the picoscope. 7

8 4. Connect the picoscope to the parallel port of the computer using the parallel port cable. Also connect the power to the picoscope using the AC adaptor supplied. Caution: Be very gentle with the DC plug, which connects to the picoscope. If it becomes loose, an error message will appear ADC200 unit not found on LPT1 and the picoscope won t work. 5. Run the picoscope program. Set channel A of the picoscope to measure AC voltage. Leave the voltage range to AUTO. 6. Switch ON the signal generator. 7. From the onscreen display determine and record the: PEAK Voltage Vpeak = V Period T = s Frequency f = Hz 8. Using the meter provided by the picoscope program, measure and record: Frequency f = Hz 9. State the theoretical relationships between: f and T 10. Record the AC signal to disk (save as a text file). At home import the data into an Excel spreadsheet and graph the signal. Label the graph showing Vpeak, T and f. 8

9 PART 2: Using the Variable AC Power Supply Figure 5 1. Disconnect the picoscope lead from the signal generator. 2. Get a Variable AC Power Supply unit. Select the OUTPUT switch to get AC voltage in the range of 0-6.3V and use the rotary switch to select 5.0V. 3. Now connect the channel A input of the picoscope to the OUTPUT of the Variable AC Power Supply unit as shown in figure From the onscreen display determine and record the: PEAK Voltage Vpeak = V Period T = s Frequency f = Hz 5. Record the RMS voltage using both the: picoscope VR MS = V & the multimeter VR MS = V 9

10 6. State the theoretical relationships between: Vpeak and VR MS 7. Use the data recorded above to verify the relationships between the two voltages in question 6. Show working. 8. Record the AC signal to disk (save as a text file). At home import the data into an Excel spreadsheet and graph the signal. Label the graph showing Vpeak, V RMS, T and f. 9. What is the significance of V RMS? 10

11 VOLTAGE AMPLIFIER AIM: In this experiment we aim to observe the behaviour of a transistor voltage amplifier. APPARATUS: Signal Generator Picoscope Amplifier board (pre-build), 9V Battery Alligator Clip Wires THE AMPLIFIER BOARD: Header Jumper Figure 1 (R) red wire (B) black wire (R) (B) (R) (B) (B) (R) PICOSCOPE CH B CH A SIGNAL GENERATOR Output Lead 11

12 PROCEDURE: 1. Obtain a Pre-build Amplifier board. Leave the 9V battery unconnected. Place the Header Jumper as shown in figure 1 above. 2. Connect the signal generator OUTPUT using the red and black alligator clip wires to metal pins a shown in figure 1 above. 3. Connect the red and black alligator clip wires of channel A and channel B of the picoscope to metal pins as shown in Figure 1 above. (PLEASE OBSERVE CORRECT POLARITY OF WIRES TO METAL PINS) 4. Connect the picoscope to the parallel port of the computer. Run the picoscope program. Set both channels of the picoscope to measure AC voltage. Leave the voltage range for both channels to AUTO. 5. Set the signal generator frequency to about 1kHz (10 x100hz). Also set the WAVEFORM switch to sinewave (~). 6. Set the signal generator output level switch (ATTENUATOR) to -20dB (or x1). Adjust the output level rotary switch to minimum. Switch ON the signal generator. 7. Now connect the 9V battery to the circuit. 8. Slowly increase the output level of the signal generator. 9. With the correct circuit setup, you should see on the computer screen sinewave of about 1kHz (or period, t=1ms) displayed for both input and output of the amplifier. 10 The amplitude of the output signal displayed is much greater than the input. The transistor amplifies the input signal. Also, notice that the output signal is 180 out-of-phase with the input. 11. Save your data to disk. Save as a text file. 12. Increase the output level of the signal generator until sufficient clipping occurs for both negative and positive cycles. 13. Again save your data to disk as text file. 14. Decrease the output level of the signal generator to zero. Remove the Header Jumper and place across the middle column (pins). 12

13 15. Slowly increase the output level of the signal generator until sufficient clipping occurs for the negative cycle only. Be sure that there is no clipping of positive cycles. 16. Again save your data to disk as text file. 17. Decrease the output level of the signal generator to zero. Remove the Header Jumper and place across the right column (pins). 18. Slowly increase the output level of the signal generator until sufficient clipping occurs for the positive cycle only. Be sure that there is no clipping of negative cycles. 19. Again save your data to disk as text file. 20. Make sure you remove the battery at the end of the experiment. ANALYSIS AND REPORT Import the data from your text files into an Excel spreadsheet and generate graphs (Input and Output on sam e graph) for: Normal amplification of sinewave (Graph 1) Clipping of both cycles (Graph 2) Clipping of negative cycles only (Graph 3) Clipping of positive cycles only (Graph 4) Using graph 1 calculate the voltage gain of this amplifier circuit. Using graph 2 and the DC voltages measured for V S and VC comment on the voltage level at which the clipping of both cycles occurs. Using graph 3 and the DC voltages measured for V S and VC-1.5k, comment on the voltage level at which the clipping of the negative cycles occurs. Using graph 4 and the DC voltages measured for V S and VC-220, comment on the voltage level at which the clipping of the positive cycles occurs. 13

14 PHOTONIC TRANSDUCERS AIM In this experiment we aim to observe the behaviour of vario us photonic transducers, example: LDR, LED, infra -red emitter diode and infra-red phototransistor receiver. APPARATUS Breadboard, 9V Battery and Holder Resistors - 220, 470, 4.7k. Photonic devices LED, LDR, IR Emitter Diode (Blue Lens), IR Phototransistor Receiver (Clear Lens) Digital Multimeter FLAT EDGE FLAT EDGE FLAT EDGE RED LED IR EMITTER DIODE IR PHOTO- TRANSISTOR RECEIVER (clear 14

15 PART 1 LDR AND LED. 1. Obtain a LDR and insert it, by itself onto a breadboard. 2. With normal light falling on the LDR use a multimeter to measure its resistance. Now fully cover the top of the LDR with your fingers and re-measure its resistance. R LI GHT = k R DARK = k LDR 5V RED LED + 3. Setup the above circuit on a breadboard. 4. Alternately, cover and uncover the LDR wi th your finger. 5. Explain your observations. 6. Replace the LED with a 4.7kΩ resistor. With normal light falling on the LDR, measure the voltage drop across the resistor and battery. V4k7 = V VBATT = V 7. Confirm your measurement for question 6 with the voltage divider formula (for LDR resistance use R L I GHT measured in question 1). 15

16 PART 2 INFRA-RED TRANSMITTER AND RECEIVER SWITCH V + RED LED 5V IR EMITTER DIODE + c IR PHOTO- TRANSISTOR e RECEIVER IR TRANSMITTER IR RECEIVER 1. Setup the above circuit on a breadboard. Use the left side of the breadboard for the transmitter and right side for receiver. 2. Bend the terminals of the emitter diode (blue lens) and phototransistor receiver 90. Insert them at least 6-8cm apart looking into each other. 3. For switch, simply use a jumper wire. Leave the switch open. Use 2 x 5V supply to power up the transmitter and receiver separately. 4. Does the red LED light up at the receiver side when the: switch is open? Switch is closed? 16

17 5. Open and close the switch couple of times to see the operation of the IR transmitter and receiver. Explain your observation? 6. With the switch closed, place a piece of paper in between the IR emitter diode and the IR phototransistor receiver path. What happens to the red LED? 7. Close the switch. Using the multimeter, measure the potential difference across the IR emitter diode and the current flowing through it. Hence calculate the power dissipated by the diode in mw. VD = V ID = ma PD = mw 8. Using the data sheet provided, what is the maximum power dissipated by the IR emitter diode. Is the IR emitter diode operating within its limit? 9. Measure the voltage drop across the collector -emitter junction of the phototransistor receiver, in turn when the switch at the transmitter is open and close. VC-E(open) = V VC-E(close) = V 17

18 10. What can you say about the state of the phototransistor when the switch is closed? 11. Using the data sheet provided, what is the maximum collector -toemitter saturation voltage for the IR phototransistor. PART 3 TRANSMITTING AUDIO + 10 F Connect to ipad/iphone audio out Setup the above circuit on a breadboard. Take your transmitter up to the teacher s desk and connect your ipad/iphone audio out. The teacher will have a pre-built receiver circuit. 18

19 1. Send a speech signal from the ipad via IR transmitter to the IR receiver. Compare the clarity of the speech signal with that of music. Which is clearer and why? 2. Measure the maximum separation distance from the IR transmitter to the IR receiver to still receive an audible signal. PART 4 Problem Solving Task A similar infra-red emitter diode and phototransistor is used in a control circuit as shown below. +5V High Frequency Oscillator Driver Circuit c Control Circuit IR EMITTER DIODE e IR PHOTO- TRANSISTOR RECEIVER 19

20 The forward voltage of the emitter diode is 2.5V. With the phototransistor switched ON the collector -emitter voltage (V CE) is 0.2V. The rate at which the collector -emitter voltage changes from when phototransistor switched OFF (supply voltage) to when the phototransistor ON and vice-versa is 0.6V/ s. A high frequency oscillator generates pulses of square wave as shown in the timing diagram below. On the separate timing diagram given, show the voltage waveform as is appears across collector -emitter (V CE). V DIODE 3V 40 S 120 S V CE 5V 40 S 120 S 20

21 5mm Infrared Transmitting LED 5mm EMITTING DIODE looks like a 5mm LED & has a blue transparent lens. Specifications: Forward Current (If): 50mA max Peak forward current (Ip): 1.2A Forward Voltage (VF): 20mA Reverse Voltage (VR): 5V max Power Dissipation (Pd): 100mW max Viewing Angle: 30 Peak Spectral Wavelength (IR): 20mA Spectral Bandwidth (DI): 50nm@20mA Material: GaAs 21

22 PHOTOTRANSISTOR Features L-51P3C MECHANICALLY AND SPECTRALLY MATCHED TO THE L-53 SERIES INFRARED EMITTING LED LAMP. WATER CLEAR LENS. Package Dimensions Description Made with NPN silicon phototransistor chips. Notes: 1. All dimensions are in millimeters (inches). 2. Tolerance is ±0.25(0.01") unless otherwise noted. 3. Lead spacing is measured where the lead emerge package. 4. Specifications are subjected to change without notice. Absolute Maximum Ratings at T A =25 C P arameter r Maximum Ratin g Collector-to-Emitter Breakdown Voltage Emitter-to-Collector Breakdown Voltage P ower Dissipation at (or below) 25 C Free Air Temperatur e 30V 5V 100mW Operating Temperature Range - 40 C ~ +85 C Storage Temperature Range - 40 C ~ +85 C L-51P3C-1

23 Electrical / Optical Characteristics at T A=25 C S ymbol P aramete r M in. T yp. M ax. U ni t Test Condictio n V V BR CEO BR ECO Collector-to-Emitter Breakdown Voltag e V Emitter-to-Collector Breakdown Voltag e V I C =100uA, I B = 0 I E =100uA, I B = 0 V CE (SAT) Collector-to-Emitter Saturation Voltag e V I C =0.1mA, =2.5mW/cm H 2 IC EO Collector Dark Curren t na V C E =10V, 2 H=0mW/cm T R Rise Time (10% to 90% ) us T F Fall Time (90% to 10% ) us V C E =5V, I C =1mA, R L = 100Ω I ( ON) n State Collector Curren t O ma V C E =5V, Ee=1mW/cm 2, λ=940nm L-51P3C-2

24 L-51P3C L-51P3C-3