ECE 2010 Laboratory # 3 J.P.O Rourke

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Nathan Gilmore
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1 ECE 21 Laboratory # 3 J.P.O Rourke Prelab: Simulate all the circuits in this Laboratory. Record the simulated results for each part of the lab. Your Prelab is due at the beginning of lab and will be checked off by the TA's after the lab starts. Purpose: The objective of this laboratory is to investigate the properties of a Potentiometer(Pot), Thermistor, and Photocell.. The last two components are Sensors and will handed out in lab along with a switch and must be returned at the end of the lab period. Part 1: The use of a potentiometer is quite common when it is desired to vary a voltage to another circuit. However it should be noted that the effectiveness of this is dependent on the load the circuit presents to the Pot output. If it desired to have a linear variation then the choice of the Pot's ohmic value and wattage is critical. The wattage value will not be evaluated here but in general can be calculated from the Pot and load resistance and the applied voltage. Set up the following circuit using the 1K potentiometer and 1 ohm resistor in your kit and a switch that will be handed out with the Sensors. This circuit will be used to investigate the loading effects on a 1k pot for a load of 1 ohms and 1K ohms. The results should show that linear operation is a function of the load resistor. 1 V1 12 V R1 1.kΩ 5% J1 2 3 Key = Space XMM1 R2 1Ω Load Resistor Start off with a 1 ohm load resistor and use a DVM to measure the Pot output voltage. With switch J1 open, use the DVM to adjust the Pot for 12 volts output. This condition is when the Pot is unloaded since the switch is open. Now close the switch J1 and measure the output again with the DVM. This new voltage value shows the effects of Pot loading if it is a factor. For the 12 volt setting, there should be little change. However it is necessary to take more measurements at other unloaded output voltages to

determine any loading effects. To do this, reduce the Pot output voltage in 2 volt increments while measuring with the DVM for J1 open and closed for each increment.

2 determine any loading effects. To do this, reduce the Pot output voltage in 2 volt increments while measuring with the DVM for J1 open and closed for each increment. Repeat the above all over again for a 1K ohm load. Use the following table to record all these measurements for both load resistor values. Shut off the Supply when finished. Load Resistor = 1 Switch J1 open Load Resistor = 1 Switch J1 closed Load Resistor = 1K Switch J1 open Load Resistor = 1K Switch J1 closed Using the results from the above table to Plot a curve of loaded Pot voltage verses no load voltage for both resistor loads. Note the simulated results can be automated. Check out the Power Point Hints # 1 and Circuit fileon my class web page.

3 Part 2: In this part and the next two the properties of three sensors will be briefly investigated. The first is the Thermistor. There are basically two types, a PTC(Positive Temperature Coefficient) Thermistor and a NTC(Negative Temperature Coefficient) Thermistor. In this Lab the NTC type is being used. Thermistors are temperature variable resistors used to detect a temperature or its variation. Set up the following Lab bench Test Circuit to investigate the Thermistors operation. 2 V1 1V R1 1kΩ 1 R2 1kΩ 96 % 3 Lab NTC Thermistor Rp 11kΩ 95 % XMM1 Thermistor (NTC) Test Circuit Note the center variable tap on the potentiometer R2, is jumpered to one of its sides which turns it into a variable resistor who's value can be varied from zero to 1K ohms. Once you have completed the circuit set the DVM to measure DC Volts. Also, set the Power Supply for 1 volts, turn up the Current Limit off of zero and then turn on the supply. Now adjust the variable resistor, R2 until the DVM reads as close to 5 volts as possible. Do not touch this R2

adjustment after this since it is critical that it not change. This will be the Reference Voltage of the Thermistor at room temperature which will assume to be 7 degrees F.

4 adjustment after this since it is critical that it not change. This will be the Reference Voltage of the Thermistor at room temperature which will assume to be 7 degrees F. Next pinch the Thermistor tightly between your thumb and index finger for about a minute then record the DVM reading as you did before. This is approximately equal to 85 degrees F. Finally try to breathe a hot breath from deep in your lungs, record the lowest Thermistor voltage reading after a few attempts. Shut off the Supply when finished then disconnect the positive lead from the circuit. Then use the Ohmmeter function of the DVM to measure the resistance of the Pot R2, be careful not to touch R2's adjustment control since we don't want its value to change. Room Temp. Finger Temp. Breath Temp. across Thermistor Temp. Degrees F 7 85 Pot R2's Resistance Calculate Rt XXXXXXXXXXXXXXX XXXXXXXXXXXXXXX Plot the first two readings, Voltage verses Temperature. Assume the graph is linear, use it to estimate the temperature of your breath. Part 3: The next sensor that well be investigated will be a Photoresistor or LDR(Light Dependant Resistor). The Photoresistor is basically a resistor who's resistance decreases as the intensity of

5 light falling on it increases. It s resistance can vary from 5 ohms (full light)to 5 Meg ohms (total darkness)the response is not linear but exponential. Use the same circuit arrangement as in part 2 except replace R1 with a 1K resistor, R2 with a 1K Pot connected as a variable resistor and the Thermistor with a Photresistor(LDR) Rp. 2 V1 1V R1 1.kΩ 1 R2 1kΩ 84 % 3 Photo-resistor Rp 2.8KΩ 95 % XMM1 Photoresistor Test Circuit Turn on the Supply and set it to 1 volts, DC if it was changed. Bend the Photoresistors leads down so that it lays on the protoboard, the photoresistor side facing up towards the overhead room lights. Adjust R2 until the DVM reads approximately 5 volts. This voltage will be the reference. Now Cover the Photoresistor with the palm of your hand putting the cell in complete darkness. Read the voltage when the photoresistor is in darkness. Then use the Ohmmeter function to measure the resistance of the Pot R2, be careful not to touch R2's adjustment control since we don't want its value to change. Basically follow the same procedure as in Part 2 with the Thermistor. Record the data in the table below.. Turn off the Supply when you are finished. LDR in room Light LDR in Total Darkness Pot R2's Resistance Calculate Rp XXXXXXXXXXXXXXXX

6 JOR 1. Explain why the Potentiometers output voltage is not linear under certain load conditions? 2. What general rule can you make that would apply for a load resistor/potentiometer combination that would allow for essentially linear operation. 3. From the measured value of the Potentiometers resistance, calculate the circuit current. Using the measured DVM voltages, calculate the Thermistor resistances for both test conditions. Hint for finding the current and resistance. Note, I=(V2-V3)/(R1+R2)=(1 - Vdvm)/(5K + R2m) R2m=Measured value of R2. Rt=Vdvm/I or Rt=(5K + R2m)(Vdvm)/(1 - Vdvm) Rt = (5K + R2m)/((1/Vdvm) - 1) 4. Now that you have calculated the two resistances for the defined temperatures. Assume the curve of Temperature vs Resistance is linear, what would be the resistance at 93 degrees F? 5. Using the same procedure as the thermistor section, Calculate the two resistance of the Photocell for the light settings defined?

ECE 2010 Laboratory # 3 J.P.O Rourke

ECE 2010 Laboratory # 3 J.P.O Rourke ECE Laboratory # 3 J.P.O Rourke Prelab: Simulate the circuits in Part through 4 of this Laboratory. Use the given Multisim circuits that follow Lab 3 on the Website. You do not need to look up any sensors