Application Note

VISHAY BCCOMPONENTS
www.vishay.com
Resistive Products
Application Note
NTC Thermistors
APPLICATIONS
• Washing machines and dish washers
• Central-heating systems
• Air conditioning
INDUSTRIAL, TELECOMMUNICATIONS, CONSUMER
In switching, measuring and detection systems
• Process control
• Heating and ventilation
• Air conditioning
• Fire alarms
• Temperature protection in battery management/charging
systems
• LCD contrast control in flat-panel displays, mobile phones
and camcorders
• Temperature compensation of quartz oscillator frequency
in, for example, mobile phones
• Ink-jet printer head temperature detection
• Video and audio equipment
AUTOMOTIVE APPLICATIONS
NTC temperature sensors are widely used in motor vehicles.
For example:
• Inlet air-temperature control
• Transmission oil temperature control
• Engine temperature control
• Airco systems
• Airbag electronic systems
• Temperature detection of laser diode in CD players for
cars
• Frost sensors
• ABS
DOMESTIC APPLIANCES
NTC temperature sensors are in virtually all equipment in the
home where temperature plays a role. This includes
• Fridges and freezers
• Cookers and deep-fat fryers
SELECTION CHART
TOL.
ON R (± %)
OR
ON T (± °C)
Accuracy line
NTCLE203E3
NTCLE100E3
- 40 to + 125
- 40 to + 125
(1 , 2, 3, 5) %
(2, 3, 5) %
NTCLE101E3...SB0
- 40 to + 125
0.5 °C
NTCLE203E3...SB0
- 55 to + 150
0.5 °C
- 40 to + 150
- 40 to + 150
- 40 to + 150
(1, 2, 3, 5) %
(1, 2, 3, 5) %
(1, 2, 3, 5) %
- 40 to + 125
- 40 to + 125
- 40 to + 125
SMD versions
NTCS0603E3
NTCS0402E3
NTCS0805E3
Miniature accuracy line
NTCLE300E3
NTCLE201E3
NTCLE305E4
High temperature
NTCSMELFE3
NTCLG100E2
Special long-leaded
(UL2468 PVC insulation):
NTCLS100E3
NTCLP100E3
NTCLE400E3
Ring Tongue Sensors
NTCALUG02 series
NTCALUG03 series
NTCALUG01 series
Revision: 24-May-12
LEAD
B
TOL.
(± %)
RESP.
TIME
(s)
MAX.
Ø
(mm)
Ø
(mm)
L
(mm)
0.5 to 2.5
0.5 to 3.0
two-point
sensors
two-point
sensors
1.7
1.2
3.4
3.8
0.4
0.6
38 min.
17 min.
29048
29049
1.2
3.3
0.6
17 min.
29046
1.7
4.2
0.5
41
29118
1
3
1
-
-
-
-
29056
29003
29044
0.5 °C
0.5 °C
0.5 °C
1.2
1.2
0.5 to 1
1.2
1.3
0.7
2.4
2.4
1.6
AWG30
0.3
AWG32
38
38
41
29051
29051
29076
- 40 to + 150
- 40 to + 300
5%
5%
1.3
1.3
0.9
0.9
1.7
1.85
- 40 to + 85
- 40 to + 85
- 40 to + 85
3%
3%
3%
0.75 to 3
0.75 to 3
0.75 to 3
15
10
7
8
6
6
AWG24
AWG24
AWG24
400
400
400
29060
29060
29060
- 55 to + 125
- 40 to + 125
- 40 to + 150
(1, 2) %
(2, 3) %
5%
0.5
0.5 to 1.5
0.5
5
5
7.5
8.5
5.5
7.1
AWG32
AWG32
AWG24
45
70
38
29094
29114
29092
0.56 max. 25.4 min.
DOCUMENT
NUMBER
29119
29050
Document Number: 29053
1
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APPLICATION NOTE
OPERATING
TEMP. RANGE
(°C)
PRODUCT RANGE
Application Note
www.vishay.com
Vishay BCcomponents
NTC Thermistors
RANGE SUMMARY
HOW NTC TEMPERATURE SENSORS WORK
ACCURACY LINE
NTCLE203E3 and NTCLE100E3
The flagship of our ranges. The accuracy Line sensors offer
real value for money. They have low tolerances (as low as
± 1 % on the R25-value and ± 0.5 % on the B-value) and an
operating temperature range from - 40 °C to + 125 °C. In
addition, they are very stable over a long life.
NTC temperature sensors are made from a mixture of metal
oxides which are subjected to a sintering process that gives
them a negative electrical resistance versus temperature
(R/T) relationship such as that shown in figure 1.
SURFACE MOUNT TEMPERATURE SENSORS
NTCS0402, NTCS0603 and NTCS0805 series
Our surface mount NTC sensors for temperature sensing
and compensation embody all the qualities of Vishay
BCcomponents NTC technology. The sensors come in a full
range of R25-values from 2 k to 680 k with standard
tolerances from 1 % to 5 %.
HIGH-TEMPERATURE SENSORS
NTCSMELFE3 and NTCLG100E2
This range of high-quality glass-encapsulated NTC
temperature sensors are price-competitive for general use.
Not only can theleaded sensor be used at up to 300 °C, but
their glass encapsulation makes them ideal for use in
corrosive atmospheres and harsh environments. This makes
them an attractive alternative to other more expensive
sensing methods. Two types of small glass envelopes are
available: SOD 27 for sensors with leads, and SOD 80
(‘MELF’ execution) for leadless, surface mount sensors.
AUTOMOTIVE SENSORS
NTCLE203E3...SB0
These components are designed for all automotive
applications (especially ECT sensors). Their coating is
withstanding harsh potting conditions. These components
are compliant to the AEC-Q200 norm.
APPLICATION NOTE
MINIATURE CHIP ACCURACY LINE
NTCLE201E3
NTCLE300E3
NTCLE305E4
These sensors combine the features of the accuracy line
with non-insulated or insulated leads for remote sensing
applications.
SPECIAL LONG-LEADED SENSORS
NTCLS100E3
NTCLP100E3
NTCLE400E3
For special applications we can supply three types of
long-leaded sensors: water-resistant sensors for use in
humid conditions, pipe sensors for use in corrosive
atmospheres and epoxy-coated sensors for general use.
SURFACE TEMPERATURE SENSORS
NTCALUG01
NTCALUG02
NTCALUG03
Revision: 24-May-12
MSB236 - 1
log R (Ω)
R25
B = 3740 K
B = 4570 K
0
- 25
0
25
50
75 100 125
T (°C)
Fig. 1 - Typical resistance as a function of temperature for an NTC
temperature sensor.
The relatively large negative slope means that even small
temperature changes cause a significant change in
electrical resistance which makes the NTC sensor ideal for
accurate temperature measurement and control.
The main electrical characteristics of an NTC ceramic
temperature sensor are expressed by three important
parameters and their tolerances (see below).
IMPORTANT NTC PARAMETERS
PARAMETER
DESCRIPTION
R25
The resistance of the sensor in  at the
reference temperature of 25 °C
B-value
A material constant, expressed in Kelvin

The temperature coefficient of resistance
expressed in %/K or in %/°C
RESISTANCE R25 AT 25 °C (289.15 K)
The resistance at 25 °C (substantially at room temperature)
provides a convenient reference point for thermistors.
Tolerances on R25 are due mainly to variations in ceramic
material manufacture and tolerances on chip dimensions.
Through the use of highly homogeneous material
compositions and proprietary ceramic sawing techniques
allowing precise control of chip dimensions, products are
available with tolerances on R25 lower than 1 %.
Document Number: 29053
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ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay BCcomponents
NTC Thermistors
MATERIAL CONSTANT B
B is a material constant that controls the slope of the RT
characteristic (see figure 1) which can, at least to a first
approximation, be represented by the formula:
 1

1
R T = R 25 exp  B  --- – ------------------ 


 T 298.15 
(1)
Where T is the absolute temperature of the sensor.
6
In practice, B varies somewhat with temperature and is
therefore defined between two temperatures 25 °C and
85 °C by the formula:
4
B 25  85
R 85
1
1
= ln  ---------   ------------------ – ------------------
 R 25  358.15 298.15
competitor
(2)
B25/85 (expressed in K) is normally used to characterize and
compare different ceramics. Tolerance on B (or B25/85) is
caused mainly by material composition tolerances and
sintering conditions. The latest materials offer tolerances as
low as ± 0.3 % on some specific B25/85 values.
In most cases, better fitting curves than pure exponential are
required to measure the temperature accurately; see
formula (1). That is why each NTC material curve is defined
by a 3rd order polynominal, as shown below:
2
3
R T = R 25 exp  A + B  T + C  T + D  T 
(3)
or inversely expressing T as a function of RT:
1
T = ------------------------------------------------------------------------------------------------------------------------R
2 RT
3 RT
T
A 1 + B 1 ln  --------- + C 1 ln  --------- + D 1 ln  ---------
 R 25
 R 25
 R 25
APPLICATION NOTE
0
- 50
- 25
0
25
50
75
100 125
T (°C)
Fig. 2 - Typical resistance change as a function of temperature for
a 1 % Vishay NTC temperature sensor compared to a
1 % sensor with a higher B-tolerance
The exceptionally low B-value of the Vishay
BCcomponents sensor compared with those of typical
competitors (see figure 2) gives a flatter R/R ‘butterfly’
curve which means you can get more accurate temperature
measurements using Vishay BCcomponents NTC
temperature sensors.
TEMPERATURE COEFFICIENT OF
RESISTANCE
SENSOR TOLERANCES
The total tolerances of the NTC sensor over its operating
temperature range is a combination of the tolerances on R25
and on B-value given by the formula:
(5)
Figure 2 is a graphical representation of this formula which
shows a minimum at 25 °C since this is the temperature at
which the sensor is calibrated. Above and below this
temperature, the tolerances increase due to the increasing
tolerances on B-value, giving the graph a ‘butterfly’ shape.
Revision: 24-May-12
Vishay
2
(4)
The two approximations (3) and (4) represent the real material
curves with an error smaller than 0.1 % at any given
temperature.
The values of the coefficients A, B, C, D, A1, B1, C1 and D1
are given in some datasheets as NTCLE100E3 and in the
R-T computation sheets, which can be downloaded
from the website
www.vishay.com/thermistors/curve-computation-list
R 25
R
1
1
- + B --- – ------------------------- = -----------R 25
R
T 298.15
MLC729
10
ΔR
(%)
8
The temperature coefficient of resistance  expresses the
sensitivity of a sensor to temperature changes. It is defined
as:
1 R
(6)
 = ---- x -------R T
Using formula to eliminate R this can be re-expressed as:
B
(7)
 = ------2
T
Which means that the relative tolerance on  is equal to the
relative tolerance on B-value.
THERMAL STABILITY
The stability of an NTC temperature sensor is expressed in
terms of the maximum shift in its electrical properties, R25
and B-values after it has been subjected to an extended
period at its limit operating conditions. Figure 3, for
example, shows the long-term deviation of R25 and B-value
for a standard lacquered component from the NTCLE100E3
series with an R25 of 10 k.
Document Number: 29053
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Application Note
www.vishay.com
Vishay BCcomponents
NTC Thermistors
CCB434
0.20
Shift in
B25/85
(%)
0.10
CCB094
4
Voltage
(V)
T = 25 °C
3
max.
0.05
0
average
2
63.2 %
- 0.05
min.
1
- 0.10
T = 100 °C
- 0.15
- 0.20
102
103
Time (h)
104
Fig. 3 - Aging characteristics (dry heat at 150 °C of a
NTCLE100E3 series NTC temperature sensor with an R25 of 10 k
TEMPERATURE CYCLING
Another important criterion for assessing the performance
of an NTC sensor throughout its operational life is its
resistance to thermal cycling. To assess this, products are
subjected to rapid temperature variations covering the
extremes over which they are expected to operate until
failure is induced.
These tests fully demonstrate the high reliability of our
products: our soldered types (for example NTCLE300E3
types) withstanding more than 5000 cycles, and our glass
encapsulated types (NTCLG100E2) more than 100 000
cycles without failure.
APPLICATION NOTE
THERMAL TIME CONSTANT AND RESPONSE
TIME
The speed of response of an NTC sensor is characterized by
its time constant. This is the time for the sensor’s
temperature to change by 63.2 % (i.e. 1 to 1/e) of the total
change that occurs when the sensor is subjected to a very
rapid change in temperature.
The conditions under which the time constant is measured
are important. Two are normally considered:
• Ambient change: the component is initially in still air at 25 °C.
Then quickly immersed in a fluid at 85 °C. The fluid is
usually silicone oil but other fluids, e.g. water for washing
machine applications, air for tumble dryers can also be
specified.
• Power-on/power-off conditions: the component is heated
by applying electrical power in still air to an equivalent
temperature of 85 °C after which electrical power is
removed and cool-down time is measured at 63.2 % of
the temperature difference.
Figure 4 represents the typical voltage drop variation over a
boiler sensor experiencing a transition from air at 25 °C to
the temperature of boiling water. The graph shows a
response time of about 4 s when the measured voltage
corresponds to an equivalent temperature of 72.4 °C.
Revision: 24-May-12
0
1
10
20
t (s)
30
Fig. 4 - Typical output of a boiler sensor undergoing a sudden
temperature transition from 25 °C to 100 °C
ADVANCED DEVELOPMENT AND
HIGH-TECHNOLOGY MANUFACTURE
The high accuracy of our NTC temperature sensor series is
principally a result of advanced development and
high-technology manufacture.
ADVANCED DEVELOPMENT
Audits of our factory by major customers especially in the
automotive industry regularly award us top marks. This is
the result of strong commitment to development and heavy
investment in personnel and equipment. Only by such
commitment have we been able to develop new and better
materials with B-value tolerances as low as 0.3 %.
HIGH-TECHNOLOGY MANUFACTURE
Our most significant improvement in NTC temperature
sensor manufacture has come through the use of precision
sawing. This gives much better control over repetitive
R25-value than the earlier pressing or tape casting
techniques and has allowed us to achieve R25 tolerances
lower than 1 %. After manufacture, we electrically test every
one of our NTC temperature sensors at reference or other
temperatures.
COMPONENT QUALITY, OUR GUARANTEE OF
EXCELLENCE
As you expect from a world-class electronic components
manufacturer, quality is an integral part of our company’s
make-up. It is reflected in our ISO-TS 16949 approved
organizations, all of which operate according to the
principles of TQM (Total Quality Management). It is reflected
too in the way we act, think and do business. Quality, in fact,
is the essence of what we have to offer:
not just in our products but in our customer service and
customer relations as well.
Document Number: 29053
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ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay BCcomponents
NTC Thermistors
Our Quality Assurance system is based on the following
principles:
• Total quality management involving careful design and
thorough investigation of conformance and reliability
before release of new products and processes.
• Careful control of purchased materials and manufacturing
process steps. This is mainly achieved by strict
implementation of Statistical Process Control (SPC) to
detect and eliminate adverse manufacturing trends before
they become significant.
• Electrical inspection of significant characteristics with a
target of zero defects in our delivered sensors.
• Statistical inspection of outgoing batches and periodic
reliability checks aimed at collecting trend information,
which is steered towards Quality improvement.
• Quality assurance at Vishay BCcomponents goes further,
however. Batch tests under extreme climatic conditions
are designed to test our sensors to destruction. Results
clearly indicate that Vishay BCcomponents NTC sensors
provide reliable performance over a long lifetime. A fact
that has been verified by ppm figures obtained from many
years of close cooperation with major customers in all
sectors of industry. Proving conclusively that Vishay
BCcomponents NTC temperature sensors offer
unsurpassed levels of quality and reliability in the field.
SELECTING AN NTC TEMPERATURE SENSOR
APPLICATION NOTE
STEP 1
Decide on the sensor series you need from the “Selection
Chart”
Your choice depends on the operating temperature range
and other criteria such as:
• Accuracy
• Product size
• Required mechanical execution i.e. naked chip, SMD,
epoxy coated, moulded, surface sensor or glass sealed
• Lead length and diameter.
STEP 2
Decide on the value of R25 you need. Refer to the
R/T characteristics of the sensor series you chose in Step 1.
In these characteristic curves, each sensor in the series is
distinguished by its R25-value. Choose an R25-value to give
a resistance at your average temperature of operation of
between 1 k and 100 k or the value that best fits your
electronic measuring circuit voltage and current range.
STEP 3
Determine the tolerance on R25. Generally, you will know the
accuracy of T at which the temperature should be
measured in your application. The relative tolerance (R/R)
on sensor resistance is then: R/R =  x T in which ’’ is
the temperature coefficient of resistance; see section
“Temperature Coefficient of Resistance”. To calculate the
relative tolerance on R25 (R25/R25), simply subtract from
R/R the R tolerance due to B-value.
Revision: 24-May-12
STEP 4
Using the R/T tables of the respective datasheets, select the
sensor from the series meeting your requirements on
25 calculated in step 3.
 R
----------- R 25 
Use the RT computation files, which can be downloaded
from the website for most of the NTC thermistors (leaded or
SMD) at www.vishay.com/thermistors/curve-computation-list
STEP 5
For other important requirements such as response time
and length of component, refer to the “Selection Chart”.
Although the standard range gives the narrowest tolerances
at 25 °C , we can on request, adapt our manufacturing
processes to provide products with the narrowest tolerance
at any temperature of your choice. Please pass your request
through your local Vishay sales organization.
EXAMPLES ON HOW TO SELECT
EXAMPLE 1
A leaded NTC sensor is required for sensing temperatures in
refrigerator and freezer compartments with a temperature
accuracy of 0.5 °C over the whole temperature range of
- 25 °C to + 10 °C. Over this temperature range, the circuit
design requires that the resistance should be maintained
between 2 kand 30 k.
STEP 1
Choose the execution. Since temperature has to be
measured with high accuracy, small diameter nickel leads
are recommended. Their low heat conductivity effectively
isolates the component from the outside world, enabling it
to accurately monitor the temperature of the freezing
compartments. From the “Selection Chart” it can be seen
that NTCLE203E3 series components are the most suitable
choice.
STEP 2
Refer to the NTCLE203E3 series datasheet specifications.
The component meeting the requirement that the resistance
should be maintained between 2 k to 30 k is a
NTCLE203E3202xB0 type (x indicating the tolerance).
STEP 3
Calculate the required tolerance on R25. Knowing that
T = ± 0.5 K and taking values for  at - 25 °C and 10 °C
from the NTCLE203E3 specifications:
R
-------- = 5.42 x 0.5 = 2.71 % at - 25 °C
R
R = 4.26 x 0.5 = 2.13 % at 10 °C
-------R
To calculate the relative tolerance on R25 (R25/R25), simply
subtract from R/R, the R tolerance due to B-value at
these two temperatures obtained from this datasheet.
Document Number: 29053
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Application Note
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Vishay BCcomponents
NTC Thermistors
R 25
------------- = 2.71 % - 1.19 % = 1.52 % at 25 °C
R 25
R
T
-------- = R ------t
t
R 25
------------- = 2.13 % - 0.31 % = 1.82 % at + 10 °C
R 25
So to assure a maximum rate of temperature rise of 1 K/min
we get (taking the  and R -values at 60 °C from the
specifications):
R
3.70
-------- = ----------- x 23 820 = - 881 /min
T
100
Take the minimum which gives an R25 tolerance of 1 %. The
selected component is therefore NTCLE203E3202FB0.
STEP 4
Not applicable.
STEP 5
Suppose now that the required R25/R25 had been less than
1 %. Though no standard product meets that requirement,
it's nevertheless possible to specify custom products with a
different reference point, e.g. 0 °C instead of 25 °C that meet
narrower tolerance specifications.
EXAMPLE 2
Designing a fast-charging circuit for nickel hydride cells.
During fast charging, the rate of temperature rise of the cells
must be monitored. If this reaches 1 K/min with a tolerance
of ± 10 %, the circuit must switch from fast charging to
trickle charge. Ambient temperature must be between
10 °C to 45 °C to allow fast charging and the backup cut-off
temperature (above which charging is completely switched
off) is fixed at 60 °C. Temperatures are expected to be
measured with an accuracy of ± 2 °C.
STEP 1
Surface mount products can be used for this application.
Since SMDs for relatively low temperatures are needed,
refer to the NTCS series rather than NTCSMELF series.
STEP 2
Choose the R25 of the component. From the R/T
specifications of the NTCS series, it can be seen that a type
with an R25 = 100 kis suitable i.e. NTCS0603E3104xXT.
APPLICATION NOTE
STEP 3
It is possible to choose R25 tolerance from 1 % to 5 %.
Looking in the R-T computation curve for NTCS0603
100 k, we have an accuracy at 60 °C of 1.73 °C for a
R25 tolerance of ± 5 %, an accuracy of 1.19 °C for a
R25 tolerance of ± 3 %. We choose thus a R25 tolerance of
± 5 %.
STEP 4
The optimal sized sensor with good accuracy to choose is
therefore the NTCS0603E3104JXT.
This is verified by measuring the rate of change of voltage
(dV/dt) across the sensor at constant current I. The rate of
change of resistance R/t can then be determined
(= 1/I V/t).
At the same temperature, an NTC sensor with R and
B-values at the extremes set by the sensor tolerances will
have:
A resistance of 23 820 x (1 - 6.40/100) = 22 296 
an  of - 3.70 x (1 - 1/100) = - 3.66 % K
(tolerance on  = tolerance on B25/85).
So the same R/t, i.e. - 881 /min in this extreme
component will limit the maximum rate of temperature rise
T/t to 881 x 100/3.66 x 1/22 296 = 1.082 K/min which still
falls within the tolerance of ± 10 % allowed on the rate of
temperature rise (1 K/min + 10 % = 1.1 K/min).
APPLICATION GROUPING
Applications of Vishay’s NTCs may be classified into two
main groups depending on their physical properties:
1. Temperature sensors: Applications in which the sensitive
change of the resistance versus the temperature is used,
shown in the formula:
R = fT
This group is split into two subsections:
a) The temperature of the NTC thermistor is determined
only by the temperature of the ambient medium.
b) The temperature of the NTC thermistor is also
determined by the power dissipation in the NTC
thermistor itself.
2. Time delay thermistors: Applications in which the time
dependence is decisive, when the temperature is
considered as a parameter and is written:
R = ft
This group comprises all applications which make use of
the thermal inertia of NTC thermistors.
The classifications mentioned are supported by practical
examples in figure 5 to 17.
STEP 5
Verify now that the selected component fulfils the
requirement with regard to rate of temperature rise (T/t),
from section “Temperature Coefficient of Resistance”:
Revision: 24-May-12
Document Number: 29053
6
For technical questions, contact: nlr@vishay.com
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay BCcomponents
NTC Thermistors
EXAMPLES
V
V
°C
°C
-
CCB526
Fig. 5 - Temperature measurement in industrial and medical
thermometers
Flow
direction
°C
V
Heater
bimetal
mA-meter
T1
NTC
T0
NTC
-
CCB527
CCB531
Fig. 6 - Car cooling water temperature measurement with bimetal
-
Fig. 10 - Flow measurement of liquids and gases.
The temperature difference between T1 and T0 is a measure for the
velocity of the fluid or gas.
V
°C
differential
mA-meter
+V
CCB528
Fig. 7 - Car cooling water temperature measurement with
differential mA-meter
-q
+
VO
CCB532
-V
V
-
CCB529
Fig. 11 - Temperature sensing bridge with op-amp which acts as
differential amplifier. The sensitivity can be very high.
Fig. 8 - Temperature measurement with a bridge incorporating an
NTC thermistor and a relay or a static switching device
+V
APPLICATION NOTE
V
+
VO
-q
-V
-
CCB533
CCB530
Fig. 9 - Liquid level control
Revision: 24-May-12
Fig. 12 - Basic temperature sensing configuration. The op-amp
acts as a Schmitt-trigger. The transfer characteristic is shown
in figure 13
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Application Note
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Vishay BCcomponents
NTC Thermistors
+V
CCB534
VO
Relay
-q
t
-V
Fig. 13 - Transfer characteristic of the circuit shown in figure 12
CCB537
Fig. 14
Fig. 16 - Simple thermostat
+V
-V
RP
COMP 2
-q
B
RS
AND GATE
VO
COMP 1
~
A
-q
SAWTOOTH
GENERATOR
CLOCK PULSE
GENERATOR
-V
CCB535
Fig. 14 - Temperature sensing bridge with 0 °C offset and ADC.
Due to RP and RS the voltage at A varies linearly with the NTC
thermistor temperature. The voltage at B is equal to that at A when
the NTC thermistor temperature is 0 °C. Both voltages are fed to the
comparator circuit. See also figure 15
SAWTOOTH
TEMPERATURE
0 °C REF.
CCB538
Fig. 17 - Temperature compensation in transistor circuits.
Push-pull compensation.
NTC TEMPERATURE SENSORS USED AS A
THERMAL SWITCH
A common use of an NTC temperature sensor is in one of
the bridge arms of a thermal switch circuit using an
operational amplifier such as the μA 741. Figure 18 shows a
typical thermal switch circuit for a refrigerator thermostat.
The circuit consists of a 10 VDC zener diode stabilized power
supply, a wheatstone bridge (containing the NTC
temperature sensor) and an integrated comparator circuit
controlling a triac. The circuit is designed to switch a
maximum load current of 2 A off at - 5 °C and on at + 5 °C.
C11.5 μF (40 V)
APPLICATION NOTE
VO COMP 1
R5
R4
1 MΩ
100 Ω
F1
VO COMP 2
-q
Cb
50 μF
10 V (16 V)
400
mW
R1 R3
10 120
kΩ kΩ
Z1
AND GATE
OUTPUT PULSES
D1
CCB536
1N4148
Revision: 24-May-12
2A
green
μA 741 Rg
680 Ω
RP
R6
Rd
Rh
R2
10 kΩ
182 kΩ
(1) U
Triac
BT136500D
Vm
230 V
LOAD
Cd
390 Ω 30 nF (400 V)
Fig. 15 - Pulses occurring at various points in the circuit shown in
+V
MBD944
Fig. 18 - Refrigerator thermostat using an NTC temperature sensor.
Document Number: 29053
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Application Note
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NTC Thermistors
HEAT DETECTION IN FIRE ALARMS
GLOSSARY OF TERMS
RESISTANCE
R10
R3
TR1
R1
- Dq
R5
DC
supply
12 V to 28 V
Z1
R9
R6
TR4
D1
D2
R4
Z2
TH1
Z3
TR3
- Dq
NTC1
(insulated) R8 R2 C1 C2
R11
R7
alarm
D4
NTC2
(exposed)
TR2
Z4
MBD945
Fig. 19 - Circuit diagram of a typical heat detector using a matched
pair of NTC thermistors.
FAST CHARGING CONTROL WITH NTC
TEMPERATURE SENSING
Figure 20 shows the circuit diagram of an intelligent charged
designed to charge,within 1 h, a NiCd or NiMH.
TOLERANCE ON RESISTANCE
The limits of the values that the resistance can take at the
reference temperature.
B-VALUE
The B-value (expressed in K) may be calculated using the
following formula:
ln  R 1  R 2 
-------------------------------1  T1 – 1  T2
where R1 and R2 are the nominal values of resistance at T1
and T2 respectively (T expressed in K).
TOLERANCE ON B-VALUE
The limits of the value that B can take due to process and
material variations.
An NTC thermistor, together with fixed resistors RT1 and
RT2, is used in a voltage divider between VCC and the current
sense resistor input VSNS of the IC. At the beginning of a new
charge cycle, the IC checks if the voltage
VTEMP = VTS - VSNS is within the limits designed by the IC
manufacturer (low temperature: 0.4 VCC and high
temperature: 0.1 VCC + 0.75 VTCO). VTCO is a cut of threshold
defined by external resistors (not represented in figure 1): If
after starting the fast charge phase, VTEMP becomess lower
than VTCO, then the return to trickle mode is operated.
TOLERANCE ON R AT A TEMPERATURE DIFFERENT TO TREF
The sum of the tolerances on resistance and tolerance due
to B-deviation.
During the fast charge period, the IC samples the voltage
VTEMP and the return to trickle mode can also be operated
when the variation in time of VTEMP is going over a threshold.
-VALUE OR TEMPERATURE COEFFICIENT
Variation of resistance (in %/K) for small variations of
temperature (1 °C or 1 K) around a defined temperature.
This is called the T/t termination: each 34 s, VTEMP has
fallen by 16 mV ± 4 mV compared to the value measured two
samples earlier, then the fast charge is terminated.
For further information refer to Application Note
“Fast Charging Control with NTC Temperature Sensing”
(doc. 29089)
VCC
APPLICATION NOTE
Also called nominal resistance. Formerly specified at only
one temperature, or sometimes at two or maximum three.
Now new technologies allow the specification of resistance
values on all applicable temperature ranges for several
types.
PACK +
VTCO
RT2
TS
BQ2005
RT1
R-TOLERANCE DUE TO B-DEVIATION
Due to the tolerance on the B-value, the limits of the value
that R can take at a certain temperature increase with the
difference of that temperature to the reference temperature.
MAXIMUM POWER DISSIPATION AND ZERO POWER
Maximum power which could be applied without any risk of
failure. The maximum dissipation of an NTC thermistor is
derated in function of ambient temperature. At low
temperatures a certain dissipation can generate high
voltages across the sensor which are not allowed.
Zero-power is practically limited to less than 1 % of
maximum specified power dissipation only for low
self-heating by measuring current.
DISSIPATION FACTOR
N
T
C
SNS
PACK -
Due to electrical power dissipated in the NTC thermistor, its
average body temperature will rise. The dissipation factor
equals the electrical power that is needed to raise the
average body temperature of the NTC with 1 K. It is
expressed in mW/K. The smaller the dissipation factor, the
more sensitive the NTC thermistor is for self-heating by
current injection.
Fig. 20 - BQ2005
Revision: 24-May-12
Document Number: 29053
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Application Note
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Vishay BCcomponents
NTC Thermistors
HOW TO MEASURE NTC THERMISTORS
The published
temperature T.
RT-values
are
measured
at
the
stirred, there is still a temperature gradient in the fluid.
Measure the temperature as close as possible to the NTC.
The published B-value at 25 °C is the result of the
measurement at 25 °C and that at 85 °C. Hence, these
values should be used when checking.
• After placing the NTC in the thermostatic bath, wait until
temperature equilibrium between the NTC and the fluid is
obtained. For some types this may take more than 1 min.
Make sure that the NTC sensor is at an adequate depth
below the fluid level, as ambient temperature can be
conducted though wires or clamps to the sensing
element.
The following general precautions have to be taken when
measuring NTC thermistors:
• Never measure thermistors in air; this is quite inaccurate
and can give deviations of more than 1 K. For
measurements at room temperature or below, use low
viscosity silicone oil, purified naphta or some other
non-conductive and non-aggressive fluid. For higher
temperatures use oil, preferably silicon oil.
APPLICATION NOTE
• Use a thermostatic liquid bath with an accuracy and
repeatability of better than 0.1 °C. Even if the fluid is well
Revision: 24-May-12
• Keep the measuring power as low as possible, otherwise
the NTC will be heated by the measuring current.
Miniature NTC thermistors are especially sensitive in this
respect. Measuring power of less than 5 % of the
dissipation factor in the used medium is recommended,
which gives self-heating of less than 0.05 °C.
Document Number: 29053
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THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000