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 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 Watt resistors For a 100 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 I2 R2 1k R1 We will experiment with resistor R2 to determine the VB minimum 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 1k 100 50 30 10 5 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 R2 I2 mA Power W in R2 Observation Calculated Calculated 6 071 22 071 Spring 2006 Chaniotakis and Cory 1 Experiment 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 ACH0 RTh ACH1 Vs Vo R ACH0 ACH1 Here the symbol RTh indicates a thermistor For the fixed resistor R use one of your 10k laboratory resistors We need to keep track of both the voltage Vs NTC Thermistor 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 10k thermistor This in the language of thermistors means that the thermistor has a Temperature Celcius resistance of 10k at 25 degrees 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 35000 30000 Resistance Ohm 25000 20000 15000 10000 5000 0 0 10 20 6 071 22 071 Spring 2006 Chaniotakis and Cory 30 40 50 60 70 2 You 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 Courtesy of National Instruments Used with permission 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 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 T A B RTh 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 0k 35 0C 6 5k 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 4 Experiment 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 ft or 32 8 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 thermistor RTh supply heater Rtest ground 7 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 5 Appendix 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 molecular structure is obtained by mixing metal oxides together in varying proportions to create a material with the proper resistivity Two types of Thermistors are available Negative Temperature Coefficient NTC resistance decreases with increasing temperature and Positive Temperature Coefficient PTC