Temperature Measurement with Thermistors

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1 Temperature Measurement with Thermistors Gerald Recktenwald Portland State University Department of Mechanical Engineering February 26, 2013 EAS 199B: Engineering Problem Solving

2 Temperature Measurement Temperature can be measured with many devices Liquid bulb thermometers Gas bulb thermometers bimetal indicators RTD: resistance temperature detectors (Platinum wire) thermocouples thermistors IC sensors Optical sensors Pyrometers Infrared detectors/cameras liquid crystals EAS 199B: Engineering Problem Solving page 1

3 IC Temperature Sensors (1) Semiconductor-based temperature sensors for thermocouple reference-junction compensation Packaged suitable for inclusion in a circuit board Variety of outputs: analog (voltage or current) and digital More useful for a manufactured product or as part of a control system than as laboratory instrumentation. Examples (circa 2010) Manufacturer Analog Devices Dallas Semiconductor Maxim National Instruments Part number AD590, AD22103, TMP35, TMP36, TMP37 DS1621, DS18B20 Max675, REF-01, LM45 LM35, LM335, LM75, LM78 EAS 199B: Engineering Problem Solving page 2

o 5.5 V) FUNCTIONAL BLOCK DIAGRAM #.-)/*01 Low Voltage Temperature Sensors " *-) TMP35/TMP36/TMP37!" # $%&'("$)*$+'+", )234+5 )23465 )234( Figure 1.

" Don t # & )*3$"9:0 confuse the TO-92-3 package with a transistor! " *-) C1/ 4 %1;<$<;$#=>?@, + 8 $B$1*$A*11:A) 7 & 4 8 C1/ #.-)/*01 Figure 2. RJ-5 (SOT-23) )*3$"9:0 %1;<$<;$#=>?

4 o 5.5 V) FUNCTIONAL BLOCK DIAGRAM #.-)/*01 Low Voltage Temperature Sensors " *-) TMP35/TMP36/TMP37!" # $%&'("$)*$+'+", )234+5 )23465 )234( Figure IC Temperature Sensors (2) FUNCTIONAL BLOCK DIAGRAM!" # $%&'("$)*$+'+", C on TMP37) re (typ) ads eration to +150 C rent s PIN CONFIGURATIONS Example: TMP36 from Analog Devices " *-) 7!" Don t # & )*3$"9:0 confuse the TO-92-3 package with a transistor! " *-) C1/ 4 %1;<$<;$#=>?@, + 8 $B$1*$A*11:A) 7 & 4 8 C1/ #.-)/*01 Figure 2. RJ-5 (SOT-23) )*3$"9:0 %1;<$<;$#=>?@, D ( 6 + $B$1*$A*11:A) e low voltage, precision centiy provide a voltage output that elsius (centigrade) temperature. o not require any external curacies of ±1 C at +25 C 5 C temperature range.!" # Figure 3. R-8 (SOIC_N) 7 & 4 E*))*2$"9:0 %1;<$<;$#=>?@, #.-)/*01 391$7F$!" # G$391$&F$" *-) G$391$4F$C1/ Figure 4. T-3 (TO-92) #.-)/*01 " *-)!" # )234+5 )23465 )234( Figure 1. PIN CONFIGURATIONS " *-) C1/ 7 & 4 )*3$"9:0 %1;<$<;$#=>?@, + 8 $B$1*$A*11:A) 7 & 4 8 C1/ #.-)/*01 Figure 2. RJ-5 (SOT-23) )*3$"9:0 %1;<$<;$#=>?@, D ( 6 + $B$1*$A*11:A)!" # Figure 3. R-8 (SOIC_N) #.-)/*01 e TMP35/TMP36/TMP37 and 7 & 4 ibration The TMP37 simplify is interfacing intended for to applications over the range of 5 C d to ADCs. 100 C All and three provides devices an are E*))*2$"9:0 EAS 199B: Engineering output scale factor Problem of 20 Solving mv/ C. The %1;<$<;$#=>?@, page 3 ation TMP37 from provides 2.7 V to 5.5 a 500 V maxiwell to below 150 C 50 with µa, reduced providing accuracy for all devices when operating 391$7F$!" # G$391$&F$" *-) G$391$4F$C1/ mv output at 25 C. Operation extends 0.1 C from in a still 5 V air. supply. In addition, a Figure 4. T-3 (TO-92) to cut the supply current to less The TMP35/TMP36/TMP37 are available in low cost 3-lead " *-) See, e.g., part number TMP36GT9Z-ND from $1.42 each (Qty 1) in Feb 2013 See tmp36-temperature-sensor/ overview for instructions on how to use the TMP36.

Thermistors (1) A thermistor is an electrical resistor used to measure temperature. A thermistor designed such that its resistance varies with temperature in a repeatable way.

5 Thermistors (1) A thermistor is an electrical resistor used to measure temperature. A thermistor designed such that its resistance varies with temperature in a repeatable way. A simple model for the relationship between temperature and resistance is T = k R A thermistor with k > 0 is said to have a positive temperature coefficient (PTC). A thermistor with k < 0 is said to have a negative temperature coefficient (NTC). Photo from YSI web site: EAS 199B: Engineering Problem Solving page 4

6 Thermistors (2) NTC thermistors are semiconductor materials with a well-defined variation electrical resistance with temperature Mass-produced thermistors are interchangeable: to within a tolerance the thermistors obey the same T = F (R) relationship. Measure resistance, e.g., with a multimeter Convert resistance to temperature with calibration equation Note: The Arduino cannot measure resistance. We will use a voltage divider to measure the change in resistance with temperature. EAS 199B: Engineering Problem Solving page 5

7 Thermistors (3) Advantages Output is directly related to absolute temperature no reference junction needed. Relatively easy to measure resistance Sensors are interchangeable (±0.5 C) Disadvantages Possible self-heating error Each measurement applies current to resistor from precision current source Measure voltage drop V, then compute resistance from known current and V. Repeated measurements in rapid succession can cause thermistor to heat up Can be more expensive than thermocouples for comparable accuracy: $10 to $20/each versus $1/each per junction. Thermistors costing less than $1 each are available from electronic component sellers, e.g. Digikey or Newark. More difficult to apply for rapid transients: slow response and self-heating EAS 199B: Engineering Problem Solving page 6

8 Thermistors (4) Calibration uses the Steinhart-Hart equation Data Curve Fit T = 1 c 1 + c 2 ln R + c 3 (ln R) 3 Nominal resistance is controllable by manufacturing. Typical resistances at 21 C: kω, 20 kω, kω T ( C) Resistance (kω) EAS 199B: Engineering Problem Solving page 7

9 Resistance Measurement Resistance can be measured if a precision current source is available. I If I is known and V is measured, then R is obtained with Ohm s law R V R = V I For a typical ohmmeter, the current source and voltage measurement are inside the device, and leads connect the current source to the resistance element. I V ohmmeter leads R EAS 199B: Engineering Problem Solving page 8

10 Direct Resistance Measurement of Thermistors (1) Two-wire resistance measurement: R T = V I. Ohmmeter V Thermistor R T Resistance in the lead wires can lead to inaccurate temperature measurement. EAS 199B: Engineering Problem Solving page 9

11 Direct Resistance Measurement of Thermistors (2) Four-wire resistance measurement eliminates the lead resistance 1 Ohmmeter R lead V R lead Thermistor RT R lead R lead 1 Sketch adapted from Hints for Making Better Digital Multimeter Measurements, Agilent Technologies Corporation, EAS 199B: Engineering Problem Solving page 10

12 A Voltage Divider for Thermistors (1) Using an Arduino, we do not have ready access to a precision voltage source. We could assemble a board using high precision voltage sources, but for less effort we could just buy a temperature measurement chip like the LM334 or TMP36. 5V thermistor Analog input Instead, we will use our familiar strategy of measuring resistance with a voltage divider. 10 kω EAS 199B: Engineering Problem Solving page 11

13 Arduino code for Thermistor measurement int thermistor_reading( int power_pin, int read_pin) { int reading; } digitalwrite(power_pin, HIGH); delay(100); reading = analogread(read_pin); digitalwrite(power_pin, LOW); return(reading); float thermistor_reading_ave( int power_pin, int read_pin, int nave) { int i, reading; float sum; } digitalwrite(power_pin, HIGH); delay(10); for (i=1; i<=nave; i++) { sum += analogread(read_pin); } digitalwrite(power_pin, LOW); return(sum/float(nave)); EAS 199B: Engineering Problem Solving page 12

Temperature Measurement with Thermistors

Temperature Measurement with Thermistors Gerald Recktenwald Portland State University Department of Mechanical Engineering gerry@pdx.edu March 3, 2019 ME 121: Introduction to Systems and Control Temperature