Week 1 Lab_Comparator_On_Off_Control (OL)

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EET 220 Industrial Applications Professor D. Overbye Week 1 Lab “Op amp and comparator with simulated thermistor” By: Deandre Wheelington ECPI University I pledge to support the Honor System of ECPI. I will refrain from any form of academic dishonesty or deception, such as cheating or plagiarism. I am aware that as a member of the academic community it is my responsibility to turn in all suspected violators of the honor code. I understand that any failure on my part to support the Honor System will be turned over to a Judicial Review Board for determination. I will report to a Judicial Review Board hearing if summoned. Name Deandre Wheelington Date: 11 Nov 23

Abstract: Using MultiSim software, we will construct an Op-Amp comparator lab to simulate positive and negative current flow to different LED’s by adjusting a variable resistor. Adjusting the resistor displays an example of how real-life applications of a thermistor work if a given voltage source and reference voltage are supplied. Introduction: We will first cover our bases by conducting research on what a thermistor is, how it works, and the difference between a positive temp coefficient and a negative temp coefficient thermistor is. We will then build the lab and measure the voltage values at 3 different terminals on the op- amp. This will demonstrate what values of voltage are flowing through the comparator to the respective LED’s by adjusting a variable 10k resistor. Week 1 Lab Assignment Op-Amp Comparator with simulated thermistor Objectives: After performing this lab, you should be able to, 1. Differentiate between NTC and PTC type thermistors. 2. Construct a circuit using variable resistor, to simulate a NTC type thermistor. 3. Construct a circuit using operational amplifier as a comparator, to turn ON/OFF two LEDs based on the temperature. Procedure: 1. What is a thermistor? A thermistor is essentially a temperature sensitive resistor. This resistor is completely controlled by a temperature reading, and depending on the temperature, depends on how much resistance is applied. 2. What is the difference between NTC and PTC type thermistors? Research online and write below your findings.

An NTC thermistor is a “Negative temperature coefficient” thermistor. A PTC thermistor is a “Positive temperature coefficient” thermistor. NTC thermistors are the most commonly used thermistors. The way this thermistor works is, as temperature increases, the overall resistance drops from a high value to a lower value and allows current to flow through it. The higher the temperature, the less resistance, and the more current flow. If that temp decreases, resistance increases, and blocks current flow. PTC Thermistors act in the opposite fashion. As temperature increases, resistance follows, and increases with it, blocking current flow. When temp drops, so does the resistance. These thermistors are generally used as fuses, or circuit breakers. Too much current creates heat, when heat is applied, temperature increases. When this happens, the resistance follows and starts blocking current. 3. In order to convert change in resistance into change in voltage, construct the following voltage divider circuit using multisim. R1 10kΩ Key=A 100 % R2 10kΩ VCC 5.0V Variable Resistor, acts as a NTC type thermistor Figure 1: Voltage divider circuit 4. Measure the voltage across the variable resistor R1 using DMM. The measured voltage is 2.5 Volts . 5. Use the following formula to calculate the temperature in degree kelvin. In the formula, ln is natural logarithm and V is the measured voltage from step 3. a. The temperature in Kelvin is 298 degrees.

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T = ( 1 298 + 1 4038 ln ( V 5 − V ) ) − 1 b. Convert kelvin to Fahrenheit using the formula below. The temperature in Fahrenheit is 76.73 degrees . 0 F = (( 0 K – 273.15)*1.8) + 32 6. From the circuit below, the reference voltage is set at VREF = 2.2V and is connected to the non- inverting terminal of the op-amp. 7. Calculate the reference temperature in degree Fahrenheit using V = 2.2V. Use the formula in step 5(a) and then convert degree kelvin into Fahrenheit using step 5(b) formula. The reference temperature is 86.44 0 F. 8. Construct the circuit shown below using multisim. R1 10kΩ Key=A 70 % R2 10kΩ VCC 5.0V U1 741 3 2 4 7 6 5 1 VREF 2.2V VEE -5.0V RED_LED Green_LED Figure 2: Op-Amp Comparator circuit 9. Run the simulation. Answer the following questions. a. Initially which LED is ON? At 100% resistance, The green LED. b. Fill in the following table by measuring the voltages.

V non-inverting at op-amp pin 3 V inverting at op-amp pin 2 Output at op-amp pin 6 2.20 V 2.50 V -2.15 V Table 1: Voltage values when variable resistor is at 100% c. State the reasons below for such a behavior indicated under 9(a). The green LED is on at 100% resistance because 10 ohms of resistance is applied to the negative terminal of the op-amp resulting in negative current to flow to the green led diode. This would mean that the temperature is low enough that too much resistance is blocking current flow to the other LED. 10. Now, change the variable resistor to 75%, which simulating increase in temperature (since the assumption is NTC type thermistor). Answer the following questions. a. Which LED is ON? The red LED . b. Fill in the following table by measuring the voltages. V non-inverting at op-amp pin 3 V inverting at op-amp pin 2 Output at op-amp pin 6 2.20 V 2.14 V 1.84 V Table 2: Voltage values when variable resistor is at 75% c. State the reasons below for such a behavior indicated under 10(a). The red LED is illuminated due to the temperature increase of the circuit, likely caused by the current flow. This results in resistance of the 10k variable resistor dropping allowing positive current flow to hit the red LED. Conclusion: To conclude this lab, we discovered the difference a variable resistor can effect on an op-amp comparator circuit by simulating a real-life application of a thermistor. We were given a reference voltage to use at the comparator op-amp. This reference voltage was a positive value at terminal 3 of the comparator. When adjusting our thermistor (variable resistor), we noticed that the less resistance we apply to it, the higher the temperature of the circuit becomes. This then demonstrates that less resistance is a higher temp which means more positive current flow. If we apply MORE resistance, the temperature drops, and negative current flow is applied. We can see the current flowing in a positive or negative direction by watching how the LEDs react.

Finally, before we adjusted our variable resistor values, we learned the formulas needed to convert a kelvin temperature to a Fahrenheit temperature, as well as how to find the kelvin temperature given a reference voltage of a circuit. SOURCES: What is the difference between an NTC and a PTC thermistor? (n.d.). Www.sensortips.com. https://www.sensortips.com/featured/what-is-the- difference-between-an-ntc-and-a-ptc-thermistor/ Bartelt, T. L. (2011). Industrial Automated Systems: Instrumentation and Motion Control. Cengage Limited. https://ecpi.vitalsource.com/books/9781305474277 ************ End of the experiment **************

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