ntc applications, 48K

NTC Thermistors
Temperature was one of the first physical parameters to
be measured in the process field and has been sensed in
just about every way imaginable over the years. At one
time or another, just about every physical property that
changes with respect to temperature has been used as a
basis for this measurement. Over the last few years, the
development of low cost, small controllers and associated
electronics circuitry has allowed for the cost effective
measurement and control of temperature that was not
possible before. NTC thermistor elements, either alone
or as part of temperature sensing assembly, are being
utilized more and more where the need to sense or control
temperature is needed.
NTC thermistors offer designers many advantages over
other type of sensing technologies including the highest
sensitivity to temperature changes, high signal to noise
ratio, simple operation as well as being low cost. Formerly,
the nonlinear resistance versus temperature characteristic was problematic in analog sensing circuits. Today,
however, with the advent of digital electronic controls
the translation is handled via equations in software or
lookup tables. The reliability, performance and longevity of the NTC thermistor as well as its other inherent
characteristics has made it the temperature sensing element of choice where precise measurement and control
of temperature are necessary.
NTC thermistor applications make use of the characteristics inherent in their composition. Applications are
generally broken up into two separate categories that
utilize different characteristics of the NTC thermistor.
The first category is zero power or sensing applications.
These applications utilize the resistance versus temperature characteristics of the NTC thermistor to sense or
control temperature with little power being dissipated by
the thermistor. The other general category is self-heated
applications that utilize the voltage-current characteristics
of the NTC thermistor as well as its thermal characteristics
and that of the environment.
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Temperature measurement
Temperature measurement is the most common
application for NTC thermistors. The high sensitivity of
thermistors and the ability to manufacture components
with tightly controlled temperature accuracy has made the
NTC thermistor an ideal device for low cost temperature
As the cost of digital electronics has dropped while the
degree of miniaturization has increased, the ability of
the design engineer to utilize a high precision low cost
thermistor has increased. Today’s microcontrollers allow a
number of thermistor sensors to be interfaced to a control
system allowing several locations within a building or
piece of equipment to be monitored simultaneously. An
example of such a system is shown below. Additional
zones can be monitored and controlled with the only
additional cost being that of the thermistor assembly. The
microcontroller will determine the temperature of the
thermistor by converting the analog input, either voltage
or current, to a temperature value. This is accomplished
using either a “look-up” table or by programming into
the software the equations that relate resistance to
Zone 1
Zone 1
Zone 2
Zone 2
Display Driver
Zone 3
Zone 3
Plexer input CONTROLLER
Zone 4
Zone 5
Zone 6
Zone 4
Zone 5
Zone 6
Figure 22: Microcontroller System
Another method of utilizing a thermistor to measure temperature is to use a Wheatstone bridge with the thermistor
as one leg of the bridge. The circuit in Figure 23 is one
example of a circuit that utilizes a thermistor to
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NTC Thermistors
NTC Thermistors
sense temperature. As temperature increases, the
voltage output increases. The selection of R1, R2 and R3
will determine the sensitivity of the circuit as well as the
temperature range for which the circuit is best suited.
T = 10k� , Curve “Z”
thermistor network is used to match the temperature coefficient of the NTC thermistor network to the component.
The negative temperature coefficient of the thermistor is
much larger than the positive temperature coefficient of
the compensated component. The addition of a resistor in
parallel to the NTC allows the thermistor/resistor network
to offset the response of the coil or other component.
R1 = 5k�
R2 = 5k�
R3 = 10k�
Copper Coil
Meter Circuit
Temperature alarm
A temperature alarm circuit can be designed by replacing the bridge detection meter in the above circuit with
a sufficiently sensitive relay. The alarm set point will
be determined by the values of the fixed resistors. The
selection of the relay and thermistor/resistor values are
critical to the design of the temperature alarm circuit.
The bridge output is sufficiently small below the alarm
set point which is determined by the fixed resistor legs
of the bridge circuit. At a sufficiently high temperature,
the thermistor resistance would be reduced causing an
imbalance in the circuit and sufficient current to activate
the relay.
Temperature compensation
Most electronic components’ response will vary with
respect to temperature. Often times, it is necessary or
desirable to compensate for this change in response with
respect to temperature by utilizing NTC thermistors. Some
examples of components whose response is compensated
for by NTC thermistors are crystal oscillators, infrared
LEDs and mechanical meters. Normally, a resistor/
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Figure 23: Wheatstone Bridge - Voltage Mode
PTC Component
NTC Thermistor
Figure 24: Temperature Compensation
Applications that take advantage of the voltage-current
characteristics of an NTC thermistor are said to utilize
the self-heated characteristics of the thermistor. A thermistor is said to be self-heated when the power generated
internal to the thermistor from current flow is enough to
raise the body temperature of the thermistor above that
of its surroundings. The amount of self-heat generated
will be dependent upon the amount of current flowing,
the physical parameters of the thermistor as well as the
environment surrounding the thermistor.
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NTC Thermistors
supply or other devices when the power is first turned
on. For switch-mode power supplies, the large filter
capacitors present in the circuit appear as a short circuit
at turn on. During this period a large amount of current
flows through the circuit until the capacitors have had
time to become fully charged. At that point in time, the
amount of current will have dropped substantially. An
NTC thermistor is a good choice to limit current in these
types of circuits because it has a relatively high resistance
to start and then drops in resistance due to self-heating.
The NTC surge current limiter is normally placed in series
with the diode bridge, motor or other components that
require inrush current protection. Special NTC devices
have been developed to service these types of applications.
These NTCs are generally large disc type thermistors with
broad resistance tolerances. These devices are designed
to handle the large energy surge at turn on that is inherent
in many of these applications. Generally speaking, chip
thermistors, small disc thermistors, and glass encapsulated
thermistors are not suitable for these types of applications because of their higher resistance values and low
dissipation constants. However, for applications where
the inrush currents are small and the energy surge low,
any style of NTC thermistor can function as an effective
surge limiter.
Liquid Level/Air Flow
The NTC thermistor can be used to detect the absence
or presence of a liquid by taking advantage of the difference in the amount of heat that can be dissipated by the
thermistor between a liquid versus a gas. The dissipation
factor, δ, that is listed on thermistor data sheets is based
on a specific set of conditions; typically 25˚C still air
with minimal heat sinking. The ability of the thermistor
to dissipate heat is much better in flowing air or a liquid
than it is in still air. A self-heated thermistor can dissipate
roughly 4 to 6 times the amount of power in a liquid than
it can in air. The key to a successful design for liquid
level/air flow is to ensure that the system functions over
the entire operating temperature range. For example, the
system must be able to detect the difference between the
circuit subjected to the hottest liquid as opposed to the
coldest air. For applications that require a wide operating
temperature range, the addition of a second NTC thermistor to the system that is used as a temperature sensor can
be effective. This would allow the circuit to compensate
for changes in ambient temperature.
Surge current limiting in Power Supplies
The NTC thermistor can be a cost effective device to
limit the amount of inrush current in a switching power
Alternate SCL Location
Figure 25: Surge Current Limiting Circuit For Switch Mode Power Supply.
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1028 Graphite Road, St. Marys, PA 15857
Ph: 814.834.1541 Fax: 814.834.1556
NTC Thermistors