NTC
Thermistors are ceramic semi-conductor elements
made from metal oxides which have a predictable
and repeatable R-T curve. The resistance changes
are non-linear and exhibit a Negative Temperature
Coefficient therefore their resistance, at a determined
measuring power, declines as the temperature of
the device increases and vice versa. NTC thermistors
can be used when temperature compensation, temperature
measurement or control, or inrush surge current
protection are needed.
•
Zero Power Resistance Rt |
The
resistance value measured at the rated temperature
using a power level which causes a resistance change
that can be ignored relative to the measurement
error as a whole. Since the resistance values are
high and the change in R values are generally great,
the errors created by measurement and long lead
wires can be ignored.
|
• Rated Zero Power Resistance
R25 |
The
rated resistance of thermistor which is the zero
power resistance measured at 25°C and indicated
on the thermistor. This is the most common value
used to describe the resistance value of a thermistor.
|
• Beta Value |
B
or beta value is an indication of the slope of the
curve which represents the relationship between
the resistance and the temperature of a particular
thermistor measured under zero power conditions.
The higher the Beta value the greater the change
in resistance per degree C.
You can calculate the RT2 using this formula:
Here: B=3380 T1=25 RT1=10Kohm
RT1
- The zero power resistance at T1
RT2 - The zero power resistance at T2
Unless otherwise indicated, the B value is calculated
using the zero power resistance at 25 deg. C (298.15K)
and at 50 deg. C (323.15K). The Beta value is not
a rigorous constant and is temperature dependant
within a small range of operating temperatures.
|
• Temperature Coefficient of Zero Power
Resistance T α |
The
temperature coefficient or alpha (symbol) at a specified
temperature is the average percent change of the
zero power resistance per degree C to the rated
resistance (R25).
Namely:
Where:
α T - The temperature coefficient of the zero
power resistance at T
RT - The zero power resistance at T
T - Temperature
B - B value
|
• Dissipation Coefficient δ |
The
dissipation coefficient is the ratio of the rate
of change of the power consumption of a thermistor
to the change of it’s corresponding temperature,
namely:
The
value of δ will change for different ambient
temperatures and transfer mediums and should be
used for reference purposes only.
The
dissipation constant of a thermistor is the amount
of power expressed in (mW/°C) required to self-heat
it by 1°C above ambient temperature.
|
• Thermal time constant τ |
The
thermal time constant is the time in seconds needed
for a thermistor to register a change of 63.2% of
the difference between the initial temperature of
the thermistor and that of its surroundings when
subjected to a stepped change in temperature under
zero power conditions.
τ is in direct ratio to the thermal capacity
(C) of the thermistor and in inverse ratio to the
dissipation coefficient, namely:
|
• Max. Steady State Current |
The
maximum allowable continuous current allowed to
pass through the thermistor at 25 deg. C.
|
• Resistance-Temperature Characteristic |
The
R/T characteristic is the relationship between the
zero power resistance of the thermistor and its
temperature. Since this relationship is non-linear
it is described by the R/T curve.
R-T
curve of NTC thermistor
|
• Static V-I Characteristic |
Static
V-I Characteristic refers to the relationship between
voltage and current when the NTC thermistor establishes
the thermal balance state, because the variable
range of the relationship between terminal voltage
and current of the thermistor is very wide, its
voltage and current curve is often represented by
double logarithm coordinates.
The
curve of the relationship between Igu and Igl of
NTC thermistor
Basic Characteristic and Application
Example
of Power NTC Thermistor |
Power
Load-Temperature Characteristic Curve
Sketch
Map of Surge Current Protection in Circuit of Power
NTC Thermistor
Typical
Application- Power NTC Thermistor Circuit
Selection
Criteria for Power NTC Thermistors
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:
In the equation:
E is the loop voltage, Im is the surge current
For conversion power, reversion power, switch power,
UPS power Im = 100 times operating current.
For filament, heater, etc. add the loop Im = 30
times operating current.
3. When the B value is higher, the final resistance
and the temperature rise will be less.
4. Generally, the greater the product of the time
constant and the dissipation coefficient result
in a larger thermal capacity of the resistor and
greater surge current protection.
Application Guide for
Temperature Measurement and Control |
Temperature
Measurement and Control
The NTC thermistor is especially suited for use
as a temperature sensor due to its high level of
accuracy. Within the operating temperature range
of –55°C to +300°C, it is ideally suited
for measurement and control of temperature, and
is also relatively easy to monitor and of low cost
to purchase.
NTC thermistors should be selected according to
the following criteria:
- The required range of temperature
- The required range of resistance
- The required measuring accuracy
- Environment (medium of heat transfer)
- The expected time constant
- The geometrical dimensions
A practical circuit to use for temperature measurement
with a NTC thermistor could be a Wheatstone Bridge
in which a NTC thermistor forms one leg of the bridge.
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).
Also, NTC thermistors and sensors which are used
in conjunction with relays or magnetic amplifier
loops 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.
Linearization of the
NTC Thermistor Characteristic Curve |
The
change in the resistance of a NTC thermistor is
remarkably non-linear. If a nearly linear resistance
curve is required while measuring a wide range of
temperature, such as in a dial thermostat, a resistor
connected in series or in parallel will provide
an approximation of linearity however the temperature
range exceeds 50 to 100 Kelvin.
a) Linearization of NTC thermistor by paralleled
resistor
b) Signal voltage Ve and power consumption Pv of
a linearized NTC thermistor
The
R/T curve of linearization of a NTC thermistor
by means of a parallel connection of a resistor
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 of the operating
temperature range. Under these conditions, the resistance
of the parallel connection can be approximated by
applying an exponent:
The resistance of RT, RP which are in parallel connection
is:
In the equation:
RTM is the NTC thermistor resistance at average
temperature of TM
B is the B value of NTC thermistor
(linear) slope of the characteristic curve:
The linearization circuitry of an NTC thermistor
will reduce the accuracy.
1) Sample of a simple amplifier circuit
2) The output voltage at the load resistor R(sub
L) as a function of the temperature
Advantages
of the NTC thermistor
The NTC thermistor and temperature sensor compared
with other sensors in temperature measurement and
control applications:
1) Reliable performance;
2) High precision, Good tolerances and interchangeability;
3) Large temperature coefficient of resistance,
High accuracy
4) Low cost, especially for middle-or-low temperature
measurement and control.
5) High dissipation coefficient: Test current can
be greater than of traditional sensors, simplified
circuitry.
|
Curve Application Notes and Warnings |
Application Notes:
1. Please supply all characteristics of the application.
Include the 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
Thermistors
Canadian Thermostats & Control Devices Ltd.