Temperature Measurement with Thermistors
Gerald Recktenwald
Portland State University
Department of Mechanical Engineering
March 3, 2019
ME 121: Introduction to Systems and Control
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
ME 121: Introduction to Systems and Control page 1
Thermocouples: Overview
• Principle of operation
• Wire types: B, E, J, K, N, R, S, T
• Formats: prefab, homemade, fast response, slow response
• Circuit diagrams: reference junction compensation
• Good practice
ME 121: Introduction to Systems and Control page 2
Seebeck Effect (1)
Temperature gradient in a conductor induces a voltage potential
T1
T2
Voltmeter
E12
E12 = σ(T2 − T1) (1)
where σ is the average Seebeck coefficient for the range T1 ≤ T ≤ T2.
ME 121: Introduction to Systems and Control page 3
EMF Relationships for Thermocouples (2)
Nominal values of Seebeck Coefficient
Type
Metal
+ −Seebeck
Coefficient
Temperature
Range
J Iron Constantan 50µV/C −210 to +760 C
K Nickel-
Chromium
Nickel 39µV/C −270 to +1372 C
T Copper Constantan 38µV/C −270 to +400 C
σ values are small, so the voltage output from thermocouples is small, typically on the
order of 10−3 V.
ME 121: Introduction to Systems and Control page 4
Thermocouple Amplifier
Although it’s possible, and in come cases preferable, to measure thermocouple signals
with a laboratory-grade DMM, for our purposes in ME 120, 121, 122, we can use a simple
thermocouple amplifier chip.
ME 121: Introduction to Systems and Control page 5
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 Part number
Analog Devices AD590, AD22103, TMP35, TMP36, TMP37
Dallas Semiconductor DS1621, DS18B20
Maxim Max31885, Max675, REF-01, LM45
National Instruments LM35, LM335, LM75, LM78
ME 121: Introduction to Systems and Control page 6
IC Temperature Sensors (2)
Example: TMP36 from Analog Devices
Don’t confuse the TO-92-3 package with a transistor!
Low Voltage Temperature Sensors TMP35/TMP36/TMP37
Rev. E Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©1996–2008 Analog Devices, Inc. All rights reserved.
FEATURES Low voltage operation (2.7 V to 5.5 V) Calibrated directly in °C 10 mV/°C scale factor (20 mV/°C on TMP37) ±2°C accuracy over temperature (typ) ±0.5°C linearity (typ) Stable with large capacitive loads Specified !40°C to +125°C, operation to +150°C Less than 50 µA quiescent current Shutdown current 0.5 µA max Low self-heating
APPLICATIONS Environmental control systems Thermal protection Industrial process control Fire alarms Power system monitors CPU thermal management
GENERAL DESCRIPTION The TMP35/TMP36/TMP37 are low voltage, precision centi-grade temperature sensors. They provide a voltage output that is linearly proportional to the Celsius (centigrade) temperature. The TMP35/ TMP36/TMP37 do not require any external calibration to provide typical accuracies of ±1°C at +25°C and ±2°C over the !40°C to +125°C temperature range.
The low output impedance of the TMP35/TMP36/TMP37 and its linear output and precise calibration simplify interfacing to temperature control circuitry and ADCs. All three devices are intended for single-supply operation from 2.7 V to 5.5 V maxi-mum. The supply current runs well below 50 µA, providing very low self-heating—less than 0.1°C in still air. In addition, a shutdown function is provided to cut the supply current to less than 0.5 µA.
FUNCTIONAL BLOCK DIAGRAM !"#$%&'("$)*$+'+",
"*-)#.-)/*01
)234+5)23465)234(
00337-001
Figure 1.
PIN CONFIGURATIONS
7
&
4
+
8
)*3$"9:0%1;<$<;$#=>?@,
1A$B$1*$A*11:A)
"*-)
#.-)/*01
C1/
1A
!"#
00337-002
Figure 2. RJ-5 (SOT-23)
7
&
4
8
D
(
6
+
)*3$"9:0%1;<$<;$#=>?@,
1A$B$1*$A*11:A)
"*-)
#.-)/*01
1A
1A
!"#
1A
1A
C1/
00337-003
Figure 3. R-8 (SOIC_N)
7 4&
E*))*2$"9:0%1;<$<;$#=>?@,
391$7F$!"#G$391$&F$"*-)G$391$4F$C1/ 00337-004
Figure 4. T-3 (TO-92)
The TMP35 is functionally compatible with the LM35/LM45 and provides a 250 mV output at 25°C. The TMP35 reads temperatures from 10°C to 125°C. The TMP36 is specified from !40°C to +125°C, provides a 750 mV output at 25°C, and operates to 125°C from a single 2.7 V supply. The TMP36 is functionally compatible with the LM50. Both the TMP35 and TMP36 have an output scale factor of 10 mV/°C.
The TMP37 is intended for applications over the range of 5°C to 100°C and provides an output scale factor of 20 mV/°C. The TMP37 provides a 500 mV output at 25°C. Operation extends to 150°C with reduced accuracy for all devices when operating from a 5 V supply.
The TMP35/TMP36/TMP37 are available in low cost 3-lead TO-92, 8-lead SOIC_N, and 5-lead SOT-23 surface-mount packages.
Low Voltage Temperature Sensors TMP35/TMP36/TMP37
Rev. E Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©1996–2008 Analog Devices, Inc. All rights reserved.
FEATURES Low voltage operation (2.7 V to 5.5 V) Calibrated directly in °C 10 mV/°C scale factor (20 mV/°C on TMP37) ±2°C accuracy over temperature (typ) ±0.5°C linearity (typ) Stable with large capacitive loads Specified !40°C to +125°C, operation to +150°C Less than 50 µA quiescent current Shutdown current 0.5 µA max Low self-heating
APPLICATIONS Environmental control systems Thermal protection Industrial process control Fire alarms Power system monitors CPU thermal management
GENERAL DESCRIPTION The TMP35/TMP36/TMP37 are low voltage, precision centi-grade temperature sensors. They provide a voltage output that is linearly proportional to the Celsius (centigrade) temperature. The TMP35/ TMP36/TMP37 do not require any external calibration to provide typical accuracies of ±1°C at +25°C and ±2°C over the !40°C to +125°C temperature range.
The low output impedance of the TMP35/TMP36/TMP37 and its linear output and precise calibration simplify interfacing to temperature control circuitry and ADCs. All three devices are intended for single-supply operation from 2.7 V to 5.5 V maxi-mum. The supply current runs well below 50 µA, providing very low self-heating—less than 0.1°C in still air. In addition, a shutdown function is provided to cut the supply current to less than 0.5 µA.
FUNCTIONAL BLOCK DIAGRAM !"#$%&'("$)*$+'+",
"*-)#.-)/*01
)234+5)23465)234(
00337-001
Figure 1.
PIN CONFIGURATIONS
7
&
4
+
8
)*3$"9:0%1;<$<;$#=>?@,
1A$B$1*$A*11:A)
"*-)
#.-)/*01
C1/
1A
!"#
00337-002
Figure 2. RJ-5 (SOT-23)
7
&
4
8
D
(
6
+
)*3$"9:0%1;<$<;$#=>?@,
1A$B$1*$A*11:A)
"*-)
#.-)/*01
1A
1A
!"#
1A
1A
C1/
00337-003
Figure 3. R-8 (SOIC_N)
7 4&
E*))*2$"9:0%1;<$<;$#=>?@,
391$7F$!"#G$391$&F$"*-)G$391$4F$C1/ 00337-004
Figure 4. T-3 (TO-92)
The TMP35 is functionally compatible with the LM35/LM45 and provides a 250 mV output at 25°C. The TMP35 reads temperatures from 10°C to 125°C. The TMP36 is specified from !40°C to +125°C, provides a 750 mV output at 25°C, and operates to 125°C from a single 2.7 V supply. The TMP36 is functionally compatible with the LM50. Both the TMP35 and TMP36 have an output scale factor of 10 mV/°C.
The TMP37 is intended for applications over the range of 5°C to 100°C and provides an output scale factor of 20 mV/°C. The TMP37 provides a 500 mV output at 25°C. Operation extends to 150°C with reduced accuracy for all devices when operating from a 5 V supply.
The TMP35/TMP36/TMP37 are available in low cost 3-lead TO-92, 8-lead SOIC_N, and 5-lead SOT-23 surface-mount packages.
See, e.g., part number TMP36GT9Z-NDfrom www.digikey.com.$1.42 each (Qty 1) in Feb 2013
See http://learn.adafruit.com/
tmp36-temperature-sensor/
overview for instructions on how to usethe TMP36.
ME 121: Introduction to Systems and Control page 7
Thermistors (1)
A thermistor is an electrical resistor used to
measure temperature. A thermistor is 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:www.ysitemperature.com
The T = F (R) relationship for thermistors is nonlinear. The temperature coefficient k
is the slope in the curve over a narrow range of temperatures and resistances.
ME 121: Introduction to Systems and Control page 8
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
indicate the change in resistance as a change in voltage.
ME 121: Introduction to Systems and Control page 9
Thermistors (3)
Advantages
• Output is directly related to absolute temperature – no reference junction needed.
• Relatively easy to measure resistance or convert resistance to voltage with a voltage
divider.
• High precision thermistors have interchangeable tolerances of ±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
• Precision thermistors are 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 due to slow(er) response and self-heating
ME 121: Introduction to Systems and Control page 10
Thermistors (4)
Calibration uses the Steinhart-Hart equation
T =1
c1 + c2 lnR + c3(lnR)3
Nominal resistance is controllable by
manufacturing.
Typical resistances at 21 C:
10 kΩ, 20 kΩ, . . . 100 kΩ. 5 10 15 20 25 300
5
10
15
20
25
30
35
40
45
50
Resistance (kΩ)
T (°
C)
Data
Curve Fit
ME 121: Introduction to Systems and Control page 11
Resistance Measurement
Resistance can be measured if a precision current
source is available.
If I is known and V is measured, then R is obtained
with Ohm’s law
R =V
I
R V
I
For a typical ohmmeter, the current source and voltage
measurement are inside the device. The leads connect
the internal current source to the external resistance
element being measured.
RVI
leadsohmmeter
ME 121: Introduction to Systems and Control page 12
Direct Resistance Measurement of Thermistors (1)
Two-wire resistance measurement: RT =V
I.
Ohmmeter
Thermistor
RTV
Resistance in the lead wires can contribute to inaccuracy in the temperature measurement.
ME 121: Introduction to Systems and Control page 13
Direct Resistance Measurement of Thermistors (2)
Four-wire resistance measurement eliminates the lead resistance1
Ohmmeter
Rlead
ThermistorRleadRT
Rlead
Rlead
V
1Sketch adapted from Hints for Making Better Digital Multimeter Measurements, Agilent Technologies Corporation,www.agilent.com.
ME 121: Introduction to Systems and Control page 14
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.
Instead, we will use our familiar strategy of measuring
resistance with a voltage divider.
thermistor
10 kΩ
5V
Analog input
ME 121: Introduction to Systems and Control page 15
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));
ME 121: Introduction to Systems and Control page 16