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MIT 6 071J - RESISTOR NETWORKS 2

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MASSACHUSETTS INSTITUTE OF TECHNOLOGY 22.071/6.071 Introduction to Electronics, Signals and Measurement Spring 2006 Lab6. Resistor Networks 2. Experiment 1. Power rating of resistors.Power rating is a very important characteristic of electronic devices. These devices may be complex electronic systems or simple electronic components such as resistors. Most of the resistors used in our laboratory are rated for ¼ Watt. Explain briefly the meaning and importance of this rating. Draw the i-v space and indicate the region inside which you are able to safely operate your ¼ Watt resistors. For a 100Ω resistor indicate the region of operation. Let’s experiment with our resistors to see what actually happens when the stated power rating is exceeded. Lets construct the circuit shown here →Resistors R1 and R2 form a current divider network. We will experiment with resistor R2 to determine tminimum resistance that it can have without violating the power rating. Violation of the power rating will simply result in the destruction of the component. (Be careful here. The resistor will get very hot and it will eventually burn with a very characteristic odor). Set the voltage VB to +15 Volts. Experiment and complete the table below. Start with R2=1k and decrease it until you violate the power rating or until you manage to destroy a resistor. (a set of test resistors could be 1kΩ, 100Ω, 50Ω, 30Ω, 10Ω, 5Ω, etc.) It is expected that you destroy at least one resistor in this experiment. Note that the +15 V power supply is limited to 500 mA. R11kΩ R2+-VBI2he R2 (Ω) I2 (mA) Calculated Power (W) in R2 Calculated Observation 6.071/22.071 Spring 2006. Chaniotakis and Cory 1Experiment 2. Voltage Divider Thermometer. We will use a thermistor device to construct a simple temperature measuring device. In Appendix A you may find a useful summary description of the thermistor characteristics. You may also find additional information posted on the Manuals and Data Sheets section of the class web site. Construct the following circuit on your prototyping board and make the connections as indicated. RThRVoACH1+ACH1-ACH0+ACH0-Vs++-- Here the symbol RTh indicates a thermistor. For the fixed resistor R use one of your 10kΩ laboratory resistors. We need to keep track of both the voltage Vs and the output voltage Vo. Connect Vs to the +5 Volt supply. The resistance of the thermistor RTh changes as a function of temperature. The particular device that we use is called a 10kΩ thermistor. This, in the language of tmeans that the thermistor has a resistance of 10kΩ at 2degrees Celsius. The resistance versus temperature dependence is shown on the above plot. Note that the resistance decreases with increasing temperature and for this reason this is called a negative temperature coefficient (NTC) thermistor. NTC Thermistor050001000015000200002500030000350000 1020304050607Temperature (Celcius)Resistance (Ohm)0hermistors, 5 6.071/22.071 Spring 2006. Chaniotakis and Cory 2You are going to use a prepared instrument to perform these measurements. This instrument is called Voltage Divider Thermometer and it is internally pre-wired to the channels indicated above for the various signals. Download the instrument from the class web site and run it. The default interface looks like: Start instrument Stop instrument Press the Start button on the upper right hand corner of the window. Observe the various outputs. Touch the thermistor with your hand. Does the temperature reading increase? How close to 37 C (the body temperature) can you get it? Press the large STOP button located in the middle of the screen. This action will update the resistance versus temperature plot. 6.071/22.071 Spring 2006. Chaniotakis and Cory 3 Courtesy of National Instruments. Used with permission.Express RTh as a function of the voltages Vs and Vo and the resistor R. If the voltages are read into your computer by a 12 bit analog to digital converter and the fixed resistor has a tolerance of 5% , calculate the expected error in the estimation of RTh. Assume that relationship between RTh and the temperature T is given by BTARTh=+ (1.1) where A and B are constants. Determine A and B so that the above function go through the points: (T,RTh)=(25.0C, 10.0kΩ), (35.0C, 6.5kΩ) If the thermistor has a tolerance of 10% and all other processes are exact what is the estimated error in the measurement of the temperature? 6.071/22.071 Spring 2006. Chaniotakis and Cory 4Experiment 3. Construct a heater and a temperature sensor. You will use this on Friday to measure the temperature coefficient of resistors. For the assembly we will use nichrome wire as a heater, a thermistor as the temperature measuring device and a simple carbon film resistor. Nichrome is an alloy of Ni, Cr and Fe. The nichrome wire we are going to use has a resistance of 10 Ω/ft, or 32.8 Ω/m. Wrap the resistor and thermistor with 1 foot of the nichrome wire and then pot the entire thing in thermally conductive grease and place a heat-shrink tube around it to contain the assembly. The cartoon and the schematic of the heater assembly is shown below. thermistor resistor Nichrome heater RtestRThheaterthermistor7supply +ground Before you proceed with supplying power and testing your heater connect the thermistor to the CURRENT HI and CURRENT LO in order to be able to measure its resistance as the temperature increases. Also connect supply+ to VOLTAGE HI and ground to VOLTAGE LO in order to measure the supply voltage. We will heat the system with a maximum current of 0.5A through the nichrome heater. Adjust the voltage via the adjustable power supply (supply +)and observe the change in RTh. What voltage is required to achieve a current of 0.5 A? What is the maximum power dissipated by the heater? What is the temperature as measured by the thermistor? (for this measure the thermistor resistance with your multimeter and look up the corresponding temperature from the thermistor data) 6.071/22.071 Spring 2006. Chaniotakis and Cory 5Appendix A. A glance at Thermistors Thermistors are non-linear temperature dependent resistors with a high resistance temperature coefficient. They are advanced ceramics where the repeatable electrical characteristics of the molecular structure allow them to be used as solid-state, resistive temperature sensors. This


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