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Linearization of the NTC
Thermistor Characteristic Curve
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The change in the resistance of a NTC thermistor is
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a) Linearization of NTC thermistor by paralleled resistor ��b)�������������
Signal
linearization circuitry
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NTC
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The R/T curve of linearization of a NTC thermistor by means
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The combination of a NTC thermistor and a resistor connected in
parallel will produce an S shaped characteristic curve. The best
linearization
will be obtained if the inflection is placed in the middle
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HOW TO CHOOSE AND WORK
WITH NTC THERMISTORS
Advantages of the NTC thermistor
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The NTC
thermistor
and temperature
sensor compared with other
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sensors
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measurement
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The
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1) Reliable performance;
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2) High precision, Good tolerances and interchangeability;
3) Large temperature coefficient of resistance, High accuracy
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ment and control.
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thermistor
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5) High
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thermistor��(linear)
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traditional
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Application
Notes:
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1. Please
supply���������
all characteristics
the application.
Include
resistance and
tolerance, B value, dimensions, length of wire and application temperature range etc.
2. If you are not certain of the characteristics, please provide the following data:
1) Purpose, application details
2) Environmental conditions
3) Range of temperature measurement and control
4) Dimensions
5) Testing power
6) The zero power resistance and errors at two or more temperatures
3. Insulation and housings can be added according to the requirement of users
high-dissipation coefficient. Test current can be far larger that that of an other type of
sensor which will simplify the circuitry. Special builds are available according to your
requirements (characteristics, dimensions and wire)
WARNINGS
1. Avoid sudden changes of ambient temperature of the thermistors and
temperature sensors, this could cause premature aging.
2. Excess current passing through the thermistor will cause the components
to self-heat and result in a variation of the temperature reading. This factor
should be considered before selecting. (When the heat of the component is 1/
10 of dissipation coefficient (mW/°C) the temperature variance will be 0.1°C.,
when it is 1/100 of diss.coef. the temperature difference will be 0.01°C)
3. The excess current caused by bad insulation, electrostatic induction, poor
contacts to circuitry will damage the thermistor. Pay particular attention to the
method of connection, and that too much current is not allowed to pass through
thermistor.
4. Measurements should be taken only after 5 to 7 seconds.
5. Small size and short time constant should be selected if the application
requires fast response and high precision.
6. If water, dust or ionic compounds are between the ends of the lead wire
or on the surface of insulation, the resistance will decline and become unstable
causing a difference in the temperature reading. Moisture protection and
insulation precautions should be taken to insure dryness.
2010/Sep NTC
8415 Mountain Sights Avenue • Montreal (Quebec), H4P 2B8, Canada
Tel: (514) 739-3274 • 1-800-561-7207 • Fax: (514) 739-2902
E-mail: [email protected]
Website: www.cantherm.com | Division of Microtherm
ovals
• Rated Zero Power Resistance R25
• Beta Value
• Temperature Coefficient of Zero Power Resistance�
• Dissipation Coefficient �
• Thermal time constant �
• Max. Steady State Current
• Resistance-Temperature Characteristic
• Static V-I Characteristic
B - B value
Intro
Dissipation Coefficient δ
NTC Thermistors are ceramic semi-conductor elements made from
• Basic Characteristic
and Application
of Powerand
NTC repeatable R-T curve.
metal
oxides which
have aExample
predictable
Thermistor
The
resistance changes are non-linear and exhibit a Negative
Temperature
Coefficient
therefore their resistance, at a determined
Application Guide
for
Temperaturepower,
Measurement
andas
Control
measuring
declines
the temperature of the device
• Temperature Measurement and Control
increases
versa. NTC
thermistors can be used when
• Linearization and
of the vice
NTC Thermistor
Characteristic
• Curve Application Notes and Warnings
temperature
compensation, temperature measurement or control,
or inrush surge current protection are needed.
Static V-I Characteristic
Where:
� T - The temperature coefficient of the zero power resistance at T
RT - The
zero power Coefficient
resistance at T
• Dissipation
�
dissipation coefficient is the ratio of the rate of change of
Static V-I Characteristic refers to the relationship between voltage
TThe
- Temperature
B
B
value
the dissipation
power consumption
thermistor
to the
change
of it’s
and current
whenofthe
The
coefficient is of
thea ratio
of the rate
of change
of the
power consumption of a thermistor
to the change
it’sNTC thermistor establishes the thermal
corresponding
temperature,
namely:
corresponding
temperature,
namely:
• •Static
V-I
balance
state,
because
the variable range of the relationship
Static
V-ICharacteristic
Characteristic
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Selection Criteria for Power NTC Thermistors
1. The maximum operating current of the resistor > (is greater
loop
2. Rated resistance of power NTC Thermistor R is:
1. The maximum operating current of the resistor > (is greater than) the operating curren
2. than)
Rated the
resistance
of power
NTC in
Thermistor
R is:
operating
current
actual power
between terminal voltage and current of the thermistor is very
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wide,
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voltage
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curve
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logarithm
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The dissipation coefficient is the ratio of the rate of change of the power consumption of a thermistor to the change of it’s
The
� temperature,
willwill
change
for
different
ambientambient
temperatures
and transfer mediums and should���
be
used
for
reference
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corresponding
namely:
Thevalue
valueof of
change
for different
temperatures
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The
curve
of���the
relationship
between
Igu
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Igl����������
of NTC thermistor
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purposes only.
In the
equation:
In the
equation:
• Dissipation Coefficient �
δ
and transfer mediums and should be used for reference
E is the loop voltage, Im is the surge current
is the loop
voltage,
Impower,
is the
surge
current
ForEconversion
power,
reversion
switch
power,
UPS power Im = 100 times operating current.
purposes
only.constant
The dissipation
constant
of a thermistor
is the in (mW/°C) required to self-heat it by 1°C
The
dissipation
of a thermistor
is the amount
of power expressed
Zero
Power
Resistance
Rt
ForFor
filament,
heater, etc.
add thereversion
loop Im = 30power,
times operating
conversion
power,
switchcurrent.
power, UPS power Im
above
ambient
temperature.
amount
of
power
expressed
in
(mW/°C)
required
to
self-heat
it
NTC Thermistors are ceramic semi-conductor elements made from metal oxides which have a predictable and repeatable
=
100
times
operating
current.
R-T curve. The resistance changes are non-linear and exhibit a Negative Temperature Coefficient
therefore
theirambient temperature.
3. When the B value is higher, the final resistance and the temperature rise will be les
by
1°C
above
The
resistance
value measured
atpower,
the rated
temperature
using a of the device increases and vice versa. NTC
resistance,
at a determined
measuring
declines
as the temperature
For filament, heater, etc. add the loop Im = 30 times operating
The value of � will change for different ambient temperatures and transfer mediums and should be used for reference
thermistors
be used
when atemperature
measurement or control,purposes
or inrush only.
surge current
power
levelcanwhich
causes
resistancecompensation,
change thattemperature
can be ignored
4. Generally, the greater the product of the time constant and the dissipation coefficien
current.
protection
are
needed.
of
the resistor and greater surge current protection.
relative to the measurement error as a whole. Since the resistance
• Thermal time constant �
The dissipation constant of a thermistor is the amount of power expressed in (mW/°C) required to self-heat it by 1°C
values are high and the change in R values are generally great,
3. When the B value is higher, the final resistance and the
Thermal
Time Constant τ
above
ambient temperature.
the errors created by measurement and long lead wires can be
The thermal time constant is the time in seconds needed for a thermistor to register a change of 63.2% of the difference
temperature rise will be less.
between the initial temperature of the thermistor and that of its surroundings when subjected to a stepped change in
ignored.
����������������������
• Zero Power Resistance Rt
4. Generally,
the greater the product of the time constant and
temperature under zero power conditions.
The thermal time constant is the time in seconds needed for
�����������������������������������
the dissipation coefficient result in a larger thermal capacity of
a thermistor
totime
register
a change
of 63.2% of the difference
The
resistance Zero
value measured
at the rated
temperature using aR25
power level which causes a resistance
change
that can
• Thermal
constant
�
Rated
Power
Resistance
the resistor and greater surge current protection.
be ignored relative to the measurement error as a whole. Since the resistance values are high and
change
in to
R the
�between
is the
in direct
thermal capacity
(C)thermistor
of the thermistor
and of
in inverse
ratio to the dissipation coefficient, namely:
�����������������������������������
theratio
initial
temperature
of the
and that
its
values are generally great, the errors created by measurement and long lead wires can be ignored.
The NTC thermistor is especially suited for use as a temperature sensor due to its hig
surroundings
subjected
to ina seconds
steppedneeded
change
tempera-to register a change of 63.2% of the
The
thermal timewhen
constant
is the time
forin
a thermistor
difference
��������������������������������������������
operating temperature range of –55°C to +300°C, it is ideally suited for measurement a
The rated resistance of thermistor which is the zero power resisbetween
the initial
temperature
of the thermistor and that of its surroundings when subjected to a stepped change in
ture
under
zero
power
conditions.
also relatively easy to monitor and of low cost to purchase.
tance measured at 25°C and indicated on the thermistor. This is
Application
Guide for
temperature under zero power conditions.
1. The maximum operating current of the resistor > (is greater than) the operating current in actual
power loop
NTC thermistors should be selected according to the following criteria:
the most common value used to describe the resistance value of a
Temp.
Measurement
τ
is
in
direct
ratio
to
the
thermal
capacity
(C)
of
the
thermistor
• Rated Zero Power Resistance R25
- The required range of temperature and Control
2. Rated resistance of power NTC Thermistor R is:
thermistor.
- The required range of resistance
�and
is inindirect
ratioratio
to thetothermal
capacity (C)
of the thermistor
and in inverse ratio to the dissipation coefficient, namely:
inverse
the dissipation
coefficient,
namely:
- The required measuring accuracy
The rated resistance of thermistor which is the zero power resistance measured at 25°C and indicated on the thermistor.
Temperature
Measurement
and Control The NTC thermistor is
- Environment
(medium
of heat transfer)
Basic
Characteristic
&
Application
Example
This is the most common value used to describe the resistance value of a thermistor.
Basic
- The
expectedsuited
time constant
especially
for use as a temperature sensor due to its high
BasicCharacteristic
Characteristicand
andApplication
ApplicationExample
Example
• Max. Steady State Current
Beta Value
of
Power NTC Thermistor
- The
dimensions
ofofPower
PowerNTC
NTCThermistor
Thermistor
levelgeometrical
of accuracy.
Within the operating temperature range of –55°C
The maximum allowable continuous current allowed to pass through the thermistor at 25 deg. C.
+300°C,
it to
is use
ideally
suited for measurement
and
control
of
A to
practical
circuit
for temperature
measurement with
a NTC
thermistor
B or beta value is an indication of the slope of the curve which
could
be a Wheatstone
Bridge
in
which a NTC
thermistor
forms
one
leg
ofcost
the
Power
PowerLoad-Temperature
Load-TemperatureCharacteristic
CharacteristicCurve
Curve
temperature,
and
is
also
relatively
easy
to
monitor
and
of
low
•
Beta
Value
bridge.
Power Load-Temperature Characteristic Curve
represents the relationship between the resistance and the
to
purchase.
In the equation:
temperature of a particular thermistor measured under zero power
If the sensor temperature changes in the balanced bridge circuit, a measurable
E is the loop voltage, Im is the surge current
B or beta value is an indication of the slope of the curve which represents the relationship between
the
resistance
• Max.
Steadyand
State State
Current Current
Max.
Steady
For conversion power, reversion power, switch power, UPS power Im = 100 times operating current. current will pass through the ammeter. In some cases a variable resistor R3 is
Resistance-Temperature
Characteristic
conditions.
Theof higher
the thermistor
Beta value
the greater
in
the temperature
a particular
measured
underthe
zerochange
power conditions.
The higher the Beta• value
the greater the
NTCand
thermistors
be selected
according
to the
For filament, heater, etc. add the loop Im = 30 times operating current.
used,
according toshould
the resistance
value of
R3 from which
wefollowing
can infer to the
change in resistance
per degree
C. can calculate the RT2 using this
resistance
per degree
C. You
temperature
measured (In the balanced state).
The
maximum
allowable
continuous
currentcurrent
allowed allowed
to pass through
the thermistor at 25 deg. C.
criteria:
The
maximum
allowable
continuous
to
pass
The
R/T
characteristic
is
the
relationship
between
the
zero
power
resistance
of
the
thermistor
and
its
temperature.
Since
3. When the B value is higher, the final resistance and the temperature rise will be less.
formula:
You can calculate the RT2 using this formula:
- The required range of temperature
this
relationship
is non-linear
described
through
the thermistor
at it25is deg.
C. by the R/T curve.
Also, NTC thermistors and sensors which are used in conjunction with relays or magn
4. Generally, the greater the product of the time constant and the dissipation coefficient
result
in a larger
thermal
capacity
- The
required
range
of resistance
appropriate
alarm
and
protection
equipment and are used in applications requiring tem
of the resistor and greater surge current protection.
R-T curve of NTC thermistor
The
required
measuring
accuracy
• Resistance-Temperature Characteristic
- Environment (medium of heat transfer)
- The expected time constant
The R/T characteristic is the relationship between the zero power resistance of the thermistor and its temperature. Since
Here: B=3380
T1=25T1=25
RT1=10Kohm
Here:
B=3380
RT1=10Kohm RT1 - The zero power resis����������������������
- The geometrical dimensions
RT1 - The zero power resistance at T1
this
relationship is non-linear it is described
by the R/T curve.
Resistance-Temp.
Characteristic
tance
atzero
T1 power
RT2resistance
- The zero
RT2 - The
at T2 power resistance at T2
�����������������������������������
R-T curve of NTC thermistor
A practical circuit to use for temperature measurement with a
Unless otherwise indicated, the B value is calculated using the zero power resistance at 25 deg.
C (298.15K)
and at 50 is the relationship between the zero
The
R/T characteristic
Unless
otherwiseThe
indicated,
B value
is calculated
using
the
deg. C (323.15K).
Beta valuethe
is not
a rigorous
constant and
is temperature
dependant within
a small
range of of the thermistor and its temperature. Since
�����������������������������������
NTC thermistor could be a Wheatstone Bridge in which a NTC
power
resistance
operating
temperatures.
The NTC
thermistor
is especially
suited for use
as a temperature sensor due to its high level of accuracy. Within the
Sketch Map
of Surge
Current
Protection
in Circuit
zero
power
resistance at 25 deg. C (298.15K) and at 50 deg. C
thermistor
one leg of
this
relationship
is
non-linear
it
is
described
by
the
R/T
curve.
operating
temperature
range
of
–55°C
to
+300°C,
it
is
ideally
suited
for
measurement
and controlforms
of temperature,
andthe
is bridge.
Sketch
Map
of
Surge
Current
Protection
in
Circuit
of
Power
NTC
Thermistor
Sketch
Map
of
Surge
Current
Protection
in
Circuit
of
Power
NTC
Thermistor
of Poweralso
NTC
Thermistor
(323.15K). The Beta value is not a rigorous constant and is temrelatively easy to monitor and of low cost to purchase.
perature dependant within a small range of operating temperatures.
If the sensor temperature
R-T curve of NTC thermistor
NTC thermistors should be selected according to the following criteria:
- The required range of temperature
changes in the balanced bridge
• Temperature Coefficient of Zero Power Resistance T �
- The required range of resistance
circuit, a measurable current will
Temp.
Coefficient
of
Zero
- The required measuring accuracy
The temperature coefficient or alpha (symbol) at a specified temperature is the average percent change of the zero power
pass through the ammeter. In
- Environment (medium of heat transfer)
resistance
per degreeResistance
C to the rated resistance
- The expected time constant
Power
T(R25).
some cases a variable resistor
- The geometrical dimensions
R3 is used, and according to the
The temperature coefficient or alpha (symbol) at a specified temA practical circuit to use for temperature measurement with a NTC thermistor
resistance value of R3 from which
could be a Wheatstone Bridge in which a NTC thermistor forms one leg of the
perature is the average percent change of the zero power resiswe can infer to the temperature
bridge.
tance per degree C to the rated resistance (R25).
measured (In the balanced state).
Introduction
α
If the sensor temperature changes in the balanced bridge circuit, a measurable
current will pass through the ammeter. In some cases a variable resistor R3 is
used, and according to the resistance value of R3 from which we can infer to the
temperature measured (In the balanced state).
Namely:
Namely:
Where:
Where:
� T - The temperature coefficient of the zero power resistance at T
RT - The zero power resistance at T
- The temperature coefficient of the zero power resistance
T -T
Temperature
B
RT- B- value
The zero power resistance at T
α
at T
T - Temperature
B - B value
• Dissipation Coefficient �
• Static V-I Characteristic
Also, NTC thermistors and
sensors which are used in
Typical Application- Power NTC Thermistor Circuit
conjunction
Also, NTC thermistors and sensors which are used in conjunction with relays or magnetic amplifier loops
of the with relays or magnetic amplifier loops
�������������������������������������������������
�������������������������������������������������
appropriate alarm and protection equipment and are used in applications requiring temperature control.
When
the
of the
appropriate
alarm and protection equipment
and are used in applications requiring temperature
control. When the temperature changes, the
resistance of the NTC thermistor will also change,
which will cause the bridge circuitry to become
unbalanced and a current will pass through the
control circuit which sense the current, so the
temperature in the controlled area will be adjusted.
������ ��� �������������� ������ �� ��� ������������ ������� ������� ��� ������� ���� ��� ��� ���������� ����������� ���
The dissipation coefficient is the ratio of the rate of change of the power consumption of a thermistor
to �������
the change
of it’s
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corresponding temperature, namely:
���������� �� ���� ����� ��� ������� ��� ������� ����� �� ����� ����������� �� ������ ��������� ������������
��� ����� �� ��� ������������ ������� ��� ��� ��� �� ��� ����������
The value of � will change for different ambient temperatures and transfer mediums and should be used for reference
��������������������������������������������
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1. The maximum operating current of the resistor > (is greater than) the operating current in actual power loop
1. The maximum operating current of the resistor > (is greater than) the operating current in actual power loop
2. Rated resistance of power NTC Thermistor R is:
2. Rated resistance of power NTC Thermistor R is:
ovals
• Rated Zero Power Resistance R25
• Beta Value
• Temperature Coefficient of Zero Power Resistance�
• Dissipation Coefficient �
• Thermal time constant �
• Max. Steady State Current
• Resistance-Temperature Characteristic
• Static V-I Characteristic
B - B value
Intro
Dissipation Coefficient δ
NTC Thermistors are ceramic semi-conductor elements made from
• Basic Characteristic
and Application
of Powerand
NTC repeatable R-T curve.
metal
oxides which
have aExample
predictable
Thermistor
The
resistance changes are non-linear and exhibit a Negative
Temperature
Coefficient
therefore their resistance, at a determined
Application Guide
for
Temperaturepower,
Measurement
andas
Control
measuring
declines
the temperature of the device
• Temperature Measurement and Control
increases
versa. NTC
thermistors can be used when
• Linearization and
of the vice
NTC Thermistor
Characteristic
• Curve Application Notes and Warnings
temperature
compensation, temperature measurement or control,
or inrush surge current protection are needed.
Static V-I Characteristic
Where:
� T - The temperature coefficient of the zero power resistance at T
RT - The
zero power Coefficient
resistance at T
• Dissipation
�
dissipation coefficient is the ratio of the rate of change of
Static V-I Characteristic refers to the relationship between voltage
TThe
- Temperature
B
B
value
the dissipation
power consumption
thermistor
to the
change
of it’s
and current
whenofthe
The
coefficient is of
thea ratio
of the rate
of change
of the
power consumption of a thermistor
to the change
it’sNTC thermistor establishes the thermal
corresponding
temperature,
namely:
corresponding
temperature,
namely:
• •Static
V-I
balance
state,
because
the variable range of the relationship
Static
V-ICharacteristic
Characteristic
��������������������������������������������
Selection Criteria for Power NTC Thermistors
1. The maximum operating current of the resistor > (is greater
loop
2. Rated resistance of power NTC Thermistor R is:
1. The maximum operating current of the resistor > (is greater than) the operating curren
2. than)
Rated the
resistance
of power
NTC in
Thermistor
R is:
operating
current
actual power
between terminal voltage and current of the thermistor is very
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wide,
its��������������
voltage
and������
current
curve
is often�������
represented
by
double
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logarithm
coordinates.
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The dissipation coefficient is the ratio of the rate of change of the power consumption of a thermistor to the change of it’s
The
� temperature,
willwill
change
for
different
ambientambient
temperatures
and transfer mediums and should���
be
used
for
reference
�����
����
���
������������
�������
���
��������
corresponding
namely:
Thevalue
valueof of
change
for different
temperatures
���
�����
������������
�������
������
���
�����
���
����������
The
curve
of���the
relationship
between
Igu
and
Igl����������
of NTC thermistor
�������������������������������������������������
purposes only.
In the
equation:
In the
equation:
• Dissipation Coefficient �
δ
and transfer mediums and should be used for reference
E is the loop voltage, Im is the surge current
is the loop
voltage,
Impower,
is the
surge
current
ForEconversion
power,
reversion
switch
power,
UPS power Im = 100 times operating current.
purposes
only.constant
The dissipation
constant
of a thermistor
is the in (mW/°C) required to self-heat it by 1°C
The
dissipation
of a thermistor
is the amount
of power expressed
Zero
Power
Resistance
Rt
ForFor
filament,
heater, etc.
add thereversion
loop Im = 30power,
times operating
conversion
power,
switchcurrent.
power, UPS power Im
above
ambient
temperature.
amount
of
power
expressed
in
(mW/°C)
required
to
self-heat
it
NTC Thermistors are ceramic semi-conductor elements made from metal oxides which have a predictable and repeatable
=
100
times
operating
current.
R-T curve. The resistance changes are non-linear and exhibit a Negative Temperature Coefficient
therefore
theirambient temperature.
3. When the B value is higher, the final resistance and the temperature rise will be les
by
1°C
above
The
resistance
value measured
atpower,
the rated
temperature
using a of the device increases and vice versa. NTC
resistance,
at a determined
measuring
declines
as the temperature
For filament, heater, etc. add the loop Im = 30 times operating
The value of � will change for different ambient temperatures and transfer mediums and should be used for reference
thermistors
be used
when atemperature
measurement or control,purposes
or inrush only.
surge current
power
levelcanwhich
causes
resistancecompensation,
change thattemperature
can be ignored
4. Generally, the greater the product of the time constant and the dissipation coefficien
current.
protection
are
needed.
of
the resistor and greater surge current protection.
relative to the measurement error as a whole. Since the resistance
• Thermal time constant �
The dissipation constant of a thermistor is the amount of power expressed in (mW/°C) required to self-heat it by 1°C
values are high and the change in R values are generally great,
3. When the B value is higher, the final resistance and the
Thermal
Time Constant τ
above
ambient temperature.
the errors created by measurement and long lead wires can be
The thermal time constant is the time in seconds needed for a thermistor to register a change of 63.2% of the difference
temperature rise will be less.
between the initial temperature of the thermistor and that of its surroundings when subjected to a stepped change in
ignored.
����������������������
• Zero Power Resistance Rt
4. Generally,
the greater the product of the time constant and
temperature under zero power conditions.
The thermal time constant is the time in seconds needed for
�����������������������������������
the dissipation coefficient result in a larger thermal capacity of
a thermistor
totime
register
a change
of 63.2% of the difference
The
resistance Zero
value measured
at the rated
temperature using aR25
power level which causes a resistance
change
that can
• Thermal
constant
�
Rated
Power
Resistance
the resistor and greater surge current protection.
be ignored relative to the measurement error as a whole. Since the resistance values are high and
change
in to
R the
�between
is the
in direct
thermal capacity
(C)thermistor
of the thermistor
and of
in inverse
ratio to the dissipation coefficient, namely:
�����������������������������������
theratio
initial
temperature
of the
and that
its
values are generally great, the errors created by measurement and long lead wires can be ignored.
The NTC thermistor is especially suited for use as a temperature sensor due to its hig
surroundings
subjected
to ina seconds
steppedneeded
change
tempera-to register a change of 63.2% of the
The
thermal timewhen
constant
is the time
forin
a thermistor
difference
��������������������������������������������
operating temperature range of –55°C to +300°C, it is ideally suited for measurement a
The rated resistance of thermistor which is the zero power resisbetween
the initial
temperature
of the thermistor and that of its surroundings when subjected to a stepped change in
ture
under
zero
power
conditions.
also relatively easy to monitor and of low cost to purchase.
tance measured at 25°C and indicated on the thermistor. This is
Application
Guide for
temperature under zero power conditions.
1. The maximum operating current of the resistor > (is greater than) the operating current in actual
power loop
NTC thermistors should be selected according to the following criteria:
the most common value used to describe the resistance value of a
Temp.
Measurement
τ
is
in
direct
ratio
to
the
thermal
capacity
(C)
of
the
thermistor
• Rated Zero Power Resistance R25
- The required range of temperature and Control
2. Rated resistance of power NTC Thermistor R is:
thermistor.
- The required range of resistance
�and
is inindirect
ratioratio
to thetothermal
capacity (C)
of the thermistor
and in inverse ratio to the dissipation coefficient, namely:
inverse
the dissipation
coefficient,
namely:
- The required measuring accuracy
The rated resistance of thermistor which is the zero power resistance measured at 25°C and indicated on the thermistor.
Temperature
Measurement
and Control The NTC thermistor is
- Environment
(medium
of heat transfer)
Basic
Characteristic
&
Application
Example
This is the most common value used to describe the resistance value of a thermistor.
Basic
- The
expectedsuited
time constant
especially
for use as a temperature sensor due to its high
BasicCharacteristic
Characteristicand
andApplication
ApplicationExample
Example
• Max. Steady State Current
Beta Value
of
Power NTC Thermistor
- The
dimensions
ofofPower
PowerNTC
NTCThermistor
Thermistor
levelgeometrical
of accuracy.
Within the operating temperature range of –55°C
The maximum allowable continuous current allowed to pass through the thermistor at 25 deg. C.
+300°C,
it to
is use
ideally
suited for measurement
and
control
of
A to
practical
circuit
for temperature
measurement with
a NTC
thermistor
B or beta value is an indication of the slope of the curve which
could
be a Wheatstone
Bridge
in
which a NTC
thermistor
forms
one
leg
ofcost
the
Power
PowerLoad-Temperature
Load-TemperatureCharacteristic
CharacteristicCurve
Curve
temperature,
and
is
also
relatively
easy
to
monitor
and
of
low
•
Beta
Value
bridge.
Power Load-Temperature Characteristic Curve
represents the relationship between the resistance and the
to
purchase.
In the equation:
temperature of a particular thermistor measured under zero power
If the sensor temperature changes in the balanced bridge circuit, a measurable
E is the loop voltage, Im is the surge current
B or beta value is an indication of the slope of the curve which represents the relationship between
the
resistance
• Max.
Steadyand
State State
Current Current
Max.
Steady
For conversion power, reversion power, switch power, UPS power Im = 100 times operating current. current will pass through the ammeter. In some cases a variable resistor R3 is
Resistance-Temperature
Characteristic
conditions.
Theof higher
the thermistor
Beta value
the greater
in
the temperature
a particular
measured
underthe
zerochange
power conditions.
The higher the Beta• value
the greater the
NTCand
thermistors
be selected
according
to the
For filament, heater, etc. add the loop Im = 30 times operating current.
used,
according toshould
the resistance
value of
R3 from which
wefollowing
can infer to the
change in resistance
per degree
C. can calculate the RT2 using this
resistance
per degree
C. You
temperature
measured (In the balanced state).
The
maximum
allowable
continuous
currentcurrent
allowed allowed
to pass through
the thermistor at 25 deg. C.
criteria:
The
maximum
allowable
continuous
to
pass
The
R/T
characteristic
is
the
relationship
between
the
zero
power
resistance
of
the
thermistor
and
its
temperature.
Since
3. When the B value is higher, the final resistance and the temperature rise will be less.
formula:
You can calculate the RT2 using this formula:
- The required range of temperature
this
relationship
is non-linear
described
through
the thermistor
at it25is deg.
C. by the R/T curve.
Also, NTC thermistors and sensors which are used in conjunction with relays or magn
4. Generally, the greater the product of the time constant and the dissipation coefficient
result
in a larger
thermal
capacity
- The
required
range
of resistance
appropriate
alarm
and
protection
equipment and are used in applications requiring tem
of the resistor and greater surge current protection.
R-T curve of NTC thermistor
The
required
measuring
accuracy
• Resistance-Temperature Characteristic
- Environment (medium of heat transfer)
- The expected time constant
The R/T characteristic is the relationship between the zero power resistance of the thermistor and its temperature. Since
Here: B=3380
T1=25T1=25
RT1=10Kohm
Here:
B=3380
RT1=10Kohm RT1 - The zero power resis����������������������
- The geometrical dimensions
RT1 - The zero power resistance at T1
this
relationship is non-linear it is described
by the R/T curve.
Resistance-Temp.
Characteristic
tance
atzero
T1 power
RT2resistance
- The zero
RT2 - The
at T2 power resistance at T2
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R-T curve of NTC thermistor
A practical circuit to use for temperature measurement with a
Unless otherwise indicated, the B value is calculated using the zero power resistance at 25 deg.
C (298.15K)
and at 50 is the relationship between the zero
The
R/T characteristic
Unless
otherwiseThe
indicated,
B value
is calculated
using
the
deg. C (323.15K).
Beta valuethe
is not
a rigorous
constant and
is temperature
dependant within
a small
range of of the thermistor and its temperature. Since
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NTC thermistor could be a Wheatstone Bridge in which a NTC
power
resistance
operating
temperatures.
The NTC
thermistor
is especially
suited for use
as a temperature sensor due to its high level of accuracy. Within the
Sketch Map
of Surge
Current
Protection
in Circuit
zero
power
resistance at 25 deg. C (298.15K) and at 50 deg. C
thermistor
one leg of
this
relationship
is
non-linear
it
is
described
by
the
R/T
curve.
operating
temperature
range
of
–55°C
to
+300°C,
it
is
ideally
suited
for
measurement
and controlforms
of temperature,
andthe
is bridge.
Sketch
Map
of
Surge
Current
Protection
in
Circuit
of
Power
NTC
Thermistor
Sketch
Map
of
Surge
Current
Protection
in
Circuit
of
Power
NTC
Thermistor
of Poweralso
NTC
Thermistor
(323.15K). The Beta value is not a rigorous constant and is temrelatively easy to monitor and of low cost to purchase.
perature dependant within a small range of operating temperatures.
If the sensor temperature
R-T curve of NTC thermistor
NTC thermistors should be selected according to the following criteria:
- The required range of temperature
changes in the balanced bridge
• Temperature Coefficient of Zero Power Resistance T �
- The required range of resistance
circuit, a measurable current will
Temp.
Coefficient
of
Zero
- The required measuring accuracy
The temperature coefficient or alpha (symbol) at a specified temperature is the average percent change of the zero power
pass through the ammeter. In
- Environment (medium of heat transfer)
resistance
per degreeResistance
C to the rated resistance
- The expected time constant
Power
T(R25).
some cases a variable resistor
- The geometrical dimensions
R3 is used, and according to the
The temperature coefficient or alpha (symbol) at a specified temA practical circuit to use for temperature measurement with a NTC thermistor
resistance value of R3 from which
could be a Wheatstone Bridge in which a NTC thermistor forms one leg of the
perature is the average percent change of the zero power resiswe can infer to the temperature
bridge.
tance per degree C to the rated resistance (R25).
measured (In the balanced state).
Introduction
α
If the sensor temperature changes in the balanced bridge circuit, a measurable
current will pass through the ammeter. In some cases a variable resistor R3 is
used, and according to the resistance value of R3 from which we can infer to the
temperature measured (In the balanced state).
Namely:
Namely:
Where:
Where:
� T - The temperature coefficient of the zero power resistance at T
RT - The zero power resistance at T
- The temperature coefficient of the zero power resistance
T -T
Temperature
B
RT- B- value
The zero power resistance at T
α
at T
T - Temperature
B - B value
• Dissipation Coefficient �
• Static V-I Characteristic
Also, NTC thermistors and
sensors which are used in
Typical Application- Power NTC Thermistor Circuit
conjunction
Also, NTC thermistors and sensors which are used in conjunction with relays or magnetic amplifier loops
of the with relays or magnetic amplifier loops
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appropriate alarm and protection equipment and are used in applications requiring temperature control.
When
the
of the
appropriate
alarm and protection equipment
and are used in applications requiring temperature
control. When the temperature changes, the
resistance of the NTC thermistor will also change,
which will cause the bridge circuitry to become
unbalanced and a current will pass through the
control circuit which sense the current, so the
temperature in the controlled area will be adjusted.
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The dissipation coefficient is the ratio of the rate of change of the power consumption of a thermistor
to �������
the change
of it’s
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corresponding temperature, namely:
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The value of � will change for different ambient temperatures and transfer mediums and should be used for reference
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1. The maximum operating current of the resistor > (is greater than) the operating current in actual power loop
1. The maximum operating current of the resistor > (is greater than) the operating current in actual power loop
2. Rated resistance of power NTC Thermistor R is:
2. Rated resistance of power NTC Thermistor R is:
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Linearization of the NTC
Thermistor Characteristic Curve
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The change in the resistance of a NTC thermistor is
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remarkably
non-linear. If a nearly linear resistance
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curve
is required while measuring a wide range of
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temperature, such as in a dial thermostat, a resistor
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provide
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approximation
linearity
however
the
temperature
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range���������
exceeds� 50
to�����
100 Kelvin.
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a) Linearization of NTC thermistor by paralleled resistor ��b)�������������
Signal
linearization circuitry
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voltage
Ve and
power
consumption
Pv of��������
a linearized NTC
NTC��thermistor
reduce the
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thermistor
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NTC
of an
accuracy.
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The R/T curve of linearization of a NTC thermistor by means
of a
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parallel
connection
of a����������
resistor ��� � �������� ��������� �����
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The combination of a NTC thermistor and a resistor connected in
parallel will produce an S shaped characteristic curve. The best
linearization
will be obtained if the inflection is placed in the middle
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of
the operating temperature range. Under these conditions, the
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resistance
of the
connection
be approximated
by
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Sample
of a simple
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applying
an exponent:
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2)
The
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the temperature
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HOW TO CHOOSE AND WORK
WITH NTC THERMISTORS
Advantages of the NTC thermistor
�� ��� ���������
The NTC
thermistor
and temperature
sensor compared with other
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sensors
in
temperature
measurement
and control applications:
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The
resistance
of
which
are
in parallel
connection
is: ����� �� ��� �������������� ������
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���RT,
��RP
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1) Reliable performance;
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2) High precision, Good tolerances and interchangeability;
3) Large temperature coefficient of resistance, High accuracy
4) Low cost, especially for middle-or-low temperature measure�� ��� ���������
�����
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ment and control.
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��at
��
In ��the
equation:
RTM is����������
the NTC����
thermistor
resistance
average
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5) High
dissipation coefficient: Test current can be greater than of
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temperature
TM��������������
B is����������
the B value
of NTC�����������
thermistor��(linear)
slope ��������� �� �� ���
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traditional
sensors, simplified circuitry.
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��� curve:
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of��the
characteristic
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Application
Notes:
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���the
���������
1. Please
supply���������
all characteristics
the application.
Include
resistance and
tolerance, B value, dimensions, length of wire and application temperature range etc.
2. If you are not certain of the characteristics, please provide the following data:
1) Purpose, application details
2) Environmental conditions
3) Range of temperature measurement and control
4) Dimensions
5) Testing power
6) The zero power resistance and errors at two or more temperatures
3. Insulation and housings can be added according to the requirement of users
high-dissipation coefficient. Test current can be far larger that that of an other type of
sensor which will simplify the circuitry. Special builds are available according to your
requirements (characteristics, dimensions and wire)
WARNINGS
1. Avoid sudden changes of ambient temperature of the thermistors and
temperature sensors, this could cause premature aging.
2. Excess current passing through the thermistor will cause the components
to self-heat and result in a variation of the temperature reading. This factor
should be considered before selecting. (When the heat of the component is 1/
10 of dissipation coefficient (mW/°C) the temperature variance will be 0.1°C.,
when it is 1/100 of diss.coef. the temperature difference will be 0.01°C)
3. The excess current caused by bad insulation, electrostatic induction, poor
contacts to circuitry will damage the thermistor. Pay particular attention to the
method of connection, and that too much current is not allowed to pass through
thermistor.
4. Measurements should be taken only after 5 to 7 seconds.
5. Small size and short time constant should be selected if the application
requires fast response and high precision.
6. If water, dust or ionic compounds are between the ends of the lead wire
or on the surface of insulation, the resistance will decline and become unstable
causing a difference in the temperature reading. Moisture protection and
insulation precautions should be taken to insure dryness.
2010/Sep NTC
8415 Mountain Sights Avenue • Montreal (Quebec), H4P 2B8, Canada
Tel: (514) 739-3274 • 1-800-561-7207 • Fax: (514) 739-2902
E-mail: [email protected]
Website: www.cantherm.com | Division of Microtherm
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