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Thermistor buffer lab: Alpha version (Version 0

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Thermistor buffer lab: Alpha version (Version 0.1 Rough draft date: 01/20/2021) What is the point of this lab? ? ? ? ? ? Build voltage divider circuits to perform a specific function Practice wiring opamps Explain the meaning of “loading” and why buffer amplifiers are so useful Practice interpreting the behavior of sensors Read datasheets! Equipment: ? ? ? ? ? ? ? B57164K103J 10kiloohm@25?o?C negative temperature coefficient thermistor, with insulated long leads Analog Discovery 2 with breadboard and Waveforms and jumper wires Ice 10 kiloohm resistor Blue 3mm LED 100 ohm resistor LT1677 general purpose rail-to-rail opamp (8 pin dual in line package) 1 Thermistor LED lab Important Facts about thermistors = Physical Hardware ? ? ? Thermistors resistance falls dramatically as temperature rises. Resistance-vs-temperature is not a simple straight line, it is an exponential decline. Datasheet here https://www.murata.com/~/media/webrenewal/support/library/catalog/products/thermistor /ntc/r44e.ashx?la=en-us Important facts about opamps ? The LT1677 is a good opamp, it’s about 5 dollars. Don’t melt it. Double-check your pinout on the datasheet here https://www.analog.com/media/en/technical-documentation/data-sheets/1677fa.pdf 2 Thermistor LED lab 0 ?Do not? feed the unicorns! Unicorns love rainbow spaghetti. Unicorn magic is bad for circuits; do not attract unicorns. ? ? ? ? Opamp circuits can get hairy to wire, be strict about your color coding! A suggested color code is shown here: ? Positive power red (as conventional) ? Negative power black (as conventional) ? Ground green (as in household power) ? “Signal pathway” orange, a hot color different from red ? Channel 1 yellow and channel 2 blue, the default colors of the Waveforms voltmeters ? Grey for miscellaneous support wires. Use flat, firm jumpers for power and ground (things unlikely to be moved as the design develops). This keeps them out of the way and easy to ignore. Use flexible, header jumpers for connections that will change as we add circuitry. This will Avoid wires that cross over each other 3 Thermistor LED lab Use short, flat wires to bring power from the AD2 to the power rails and other commonly used locations, and use short, flat wires to bring power from the rails to the opamp. This will make your wiring easier to read, and these connections rarely need to change as you build the circuit. BE NEAT! 4 Thermistor LED lab 1 Measure your thermistor’s resistance at three temperatures Thermistors resistance falls sharply with increasing temperature. ? Use your ohmmeter to measure the resistance of the thermistor in free air. Don’t handle it with your hands too much, that will heat it up ? Put the thermistor between your hands and warm it all the way up, then measure again. This takes ~5-10 seconds. ? Immerse the thermistor in ice water (keep the leads dry) and wait for the resistance rating to stabilize and record again. ? Check your values against the resistance-vs-temperature data in the datasheet. If they are not somewhat close, something bad has happened! Condition Temperature (degrees Celsius) Free air ~+20 Ice water 0 Between hands ~+37 5 Thermistor LED lab Thermistor Resistance (ohms) 2 Build this temperature-sensitive voltage divider Ohmmeters must have the resistance unpowered, a voltmeter-based circuit can measure the thermistor’s behavior while it is live in a circuit. A consistent relationship between temperature and resistance becomes a consistent relationship between temperature and voltage. ? Use long lead wires on your thermistor so it can be moved around without being disconnected from the circuit ? Repeat the previous three conditions Condition Temperature (degrees Celsius) Free air ~+20 Ice water 0 Between hands ~+37 Predicted Output Voltage (volts) (from voltage divider rule) Measured output voltage ? Explain why the output voltage increases when hot and decreases when cold. 6 Thermistor LED lab 3 Add a buffer to your divider circuit Wiring an opamp for the first time is hard. The buffer is the simplest opamp circuit but is still pretty challenging the first time! Be neat and tidy with your wiring to succeed! ? ? ? ? Turn off all power before wiring the opamp Double check which pin is which before turning on power. At room temperature, you should get pretty close to 2.5 volts. The other conditions should match the results from the last step. This feels like a lot of work to do nothing, but it is about to pay off. 7 Thermistor LED lab 4 Add an LED as a load to the buffer circuit To be useful, sensors need to be controlling something that does some useful work! The opamp allows the low-current (microamps) measurement circuit control a load that needs much more current (milliamps) to operate. ? Heat and cool the thermistor in the three standard conditions and observe the output of the LED. ? Play with the circuit by putting the thermistor in another thermal condition, such as blowing on it or touching it to something very hot and record the outcome Condition Free air Ice water Between hands _____________ 8 Thermistor LED lab LED description (qualitative) 5 Reflection Questions ? What would happen if we swapped the positions of the thermistor and the fixed resistor? ?When prototyping this lab, the leads of the thermistor were not insulated and the ice water shorted the thermistor. What would we observe if the water shorted the thermistor? ? Explain why relatively small changes in temperature can produce rather large changes in brightness. ?This similar circuit without a buffer will NOT WORK. Why not? 9 Thermistor LED lab Feedback What was good about this lab and should be retained and magnified in future revisions? What was frustrating or confusing about this lab and needs to be reduced or changed in future revisions? 10 Thermistor LED lab Instructor Details Credits Version 0.1 Initial concepting and drafting: Brian Faulkner Initial draft thoughts: Luke Weber, Steve Holland, Tim Johnson Learning Objectives ? ? ? ? Build voltage divider circuits to perform a specific function Practice wiring opamps Explain the meaning of “loading” and why buffer amplifiers are so useful Practice interpreting the behavior of sensors Instructor Guidance, common student problems ? ? Opamp wiring will cause MANY errors on the first try. Have them check one node at a time Students not being careful with icewater and spilling Hardware Choices I went with the LT1677 because it was being used in electronics and I wanted to match, but any rail-to-rail amp is pretty much fine for this experiment. A 10k thermistor is just a standard, and a high enough resistance that a loading effect would be very large, too large for an LED. I went with blue because its forward voltage is about in the middle of the voltage range, to get a wide brightness variation within the temperature range available with no fancy temperature producing materials. 11 Thermistor LED lab Appendix Test circuit for those having trouble with their opamp Some equations for a linearization scheme, would need more temperature points. Keeping here for now Inspiration from this page here : ?https://www.edn.com/linearize-thermistors-with-new-formula/ https://users.physics.unc.edu/~sean/Phys351/techresource/data_sheets/thermistor%20app%20 notes.pdf 12 Thermistor LED lab A 55-turn coil of radius Ro = 5.60 cm carries a current l1 = 125 mA. Running through the coil at is a long straight wire carrying a current l2 = 2.44 A. Determine the magnetic field at the center of the coil, point P, in terms of i, j, k notation. 11 Reflection Questions ? If we swapped the positions of the thermistor and fixed resistor; then the operation of Thermistor will be reversed, since with increasing temperature the resistance of Thermistor will decrease, and thus the output of the buffer or OPAMP will also decrease while the LED light may be weak or even doesn't give light, and vice versa. ? If the water shorted the thermistor; the thermistor resistance will appear as a short circuit, and thus all voltage will be applied across the fixed resistance giving a very large value at the output and the LED will be turned on all time irrelated of the temperature applied to the thermistor. ? Relatively small changes in temperature can produce larger changes in brightness, as the Buffer amplifier uses the small current generated as a result of variation in thermistor resistance at its input to give larger current in the order of milliamps at the output circuit of the LED, which causes larger changes in the brightness of the LED. ? The given circuit will not work, since at high temperature the thermistor resistance will be low in the order of 2-8 kilo ohm) causing an electric current less than or close to 1mA to pass through the circuit which is not sufficient to turn on the LED and also the voltage drop across the LED will be very high, while in case of low temperature the Thermistor resistance will be larger in value causing a small current and smaller drop across the LED which may result in very low current passing through the circuit which is also not sufficient for the LED to turn on which requires at least 30-40mA to turn on.