Transcript Thermistor Selection

Example Design – Milestone 1
Technical Research
Sensor Technology
Originally Prepared by:
Prof. Shela Aboud
Modified by:
Prof. Bitar
Measurement System
Acoustic
Biological
Chemical
Electrical
Magnetic
Mechanical
Optical
Radiant
Thermal
Input
Sensor
Processing
Output
Transducer
LCD
LEDs
7-segment
dot-matrix
alarm
etc…
Input Sensor Technology
• What types of sensors are needed on
your project?
• What sensor specifications do you
need to consider?
Sensor Selection
Example Design: Tap Temperature Sensor
Specific
General
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Environmental Conditions
Input/Output Range
Linearity
Offset
Operating Life
Output Format
Overload Characteristics
Repeatability/Hysteresis
Resolution/Accuracy
Sensitivity/Selectivity
Size/Cost/Weight
Speed of Response
Stability (long and short term)
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<50C
5 - 7 0C (4 - 8 0C)
< 1degree accuracy
waterproof
durable
inexpensive
fast
low power
5-70C
>70C
Types of Temperature Sensors
 Thermoresistive
RTD (resistive temperature detector)
thermistor
 Thermoelectric
thermocouple
 Semiconductor (IC’s)
pn diode
bipolar junction transistor
 Optical
infrared detector
 Acoustic
piezoelectric
Thermoresistive sensors
RTD (PTC)
advantages:
• temperature range
• simple interface circuits
• sensitivity
• long term stability
• inexpensive
disadvantages
•not rugged
•self-heating
NTC/PTC Thermistor
advantages
• temperature range
• sensitivity
• inexpensive
disadvantages:
• PTC less sensitive
• nonlinear
• self-heating
Thermoelectric Sensor
thermocouples
advantages:
• temperature range
• very rugged
• inexpensive
• fast depending on size
disadvantages:
• error is larger than RTD or IC sensor
• some types are very sensitive to moisture
Semiconductor IC Sensors
advantages:
• temperature range
• highly linear
• small
• accurate
• easy to interface
I  I 0 exp(qV / 2k BT )
disadvantages:
• sensitive to shock
Optical Temperature Sensors
advantages:
• thermally stable
• waterproof
• good in hostile environments
disadvantages:
• limited temperature range
Acoustic Temperature Sensors
advantages:
• thermally stable
• waterproof
• good in hostile environments
disadvantages:
• expensive
• complicated circuitry
T
ultrasound
  331.5
T
(m / s)
273.15
dry air
Sensor Comparisons
Thermoresistors
RTD
Thermistor
(NTC)
Semiconductor
Temperature IC
Analog/Digital
temperature
good range
good range
good range
cost
high cost
lower cost
inexpensive
accuracy
most precise
accurate
very accurate
durability
sensitive to strain and
shock
rugged
sensitive to shock
response time
slow
fast
fast
power
problems with selfheating
lower power
low power
NTC Thermistor
Negative Temperature Coefficient
  1 1 
RT  RT 0 exp     
  T T0  
example
material constant
zero-power resistance at temp T
Types of NTC Thermistors
 Metallized surface contact
slow response times
high power dissipations
low cost
 Bead type
fast response times
high stability/reliability
low power dissipation
more costly
• bare beads
no environmental protection.
• glass coated beads
not rugged
• glass probes
easy to handle, durable, stable
• glass rods
good for mounting on circuit boards
www.thermometrics.com
Selecting a NTC thermistor:
glass probe
NTC Thermistor: Response Time
Ts  Ta 

1 e 

P
 t /
thermal time constant:
=18 msec
initial ambient temperature
Ta=25 0C
electric power
P= 0.020 Watts
dissipation constant
=0.70 mW/0C
t = 18 – 23 msec
NTC Thermistor: Sensitivity
Temp
Coeff
1 RT

RT T
=-3.7 %/C @ 5 C
T (0C)
RT/R0
4
2.078
5
2.004
6
1.930
7
1.856
8
1.787
NTC Thermistor: Sensitivity
  1 1 
RT  RT 0 (1  X ) exp     
  T T0  
X=1%
T (0C)
RT/R0
RT/R0 min
RT/R0 max
resistor tolerance
4
2.078
2.070
2.112
5
2.004
1.994
2.034
X=5%
6
1.930
1.920
1.959
7
1.856
1.851
1.888
8
1.787
1.784
1.820
RT=(RT/RT0)RT0+/- 0.02RT0
Sensor Comparisons
Thermoresistors
RTD
Semiconductor
Temperature IC
Thermistor (NTC)
Analog
temperature
good range
good range
(-80 to 160 0C)
good range
cost
high cost
lower cost
inexpensive
accuracy
most precise
accurate
(+/- 0.02RT0)
very accurate
durability
sensitive to strain and
shock
rugged
sensitive to shock
response time
slow
fast
(18-23 msec)
fast
power
problems with selfheating
lower power
(max 0.02 W)
low power
Other
R=1kW-1MW