Temperature Measurement and
Control Devices
DISC and CHIP
Style
DISC & CHIP
STYLE
Features
•
Wide Ohmic Value
Range
•
Accurate & Stable
•
Fast Thermal
Response Time
•
Tight Tolerances
•
High Sensitivity
NTC Thermistors
Negative Temperature Coefficient (NTC) thermistors are thermally sensitive
semiconductor resistors which exhibit a decrease in resistance as absolute
temperature increases. Change in the resistance of NTC thermistor can be brought
about either by a change in the ambient temperature or internally by self-heating
resulting from current flowing through the device. Most of the practical applications of
NTC thermistors are based on these material characteristics.
NTC DISC & CHIP Style Devices
RTI manufactures
DISC & CHIP
style thermistors in resistance values ranging from
1.0 ohm to 500,000 ohms. These devices are suitable for a range of resistance
values and temperature coefficients from relatively low resistance and temperature
coefficients to very high values. Precision resistance tolerances are available to 1%.
Standard resistance tolerances are from 5% to 20%. All tolerances are specified at
25°C or may be specified at any temperature within the operating temperature range
of the thermistor.
Thermistor Terminology for Temperature Measurement & Control
Devices
•
D.C
. - The dissipation constant is the ratio, normally expressed in milliwatts per
degree C (mw/°C), at a specified ambient temperature, of a change in power
dissipated in a thermistor to the resultant change in body temperature.
•
T.C.
- The thermal time constant is the time required for a thermistor to change
63.2% of the total difference between its initial and final body temperature when
subjected to a step function change in temperature under zero-power conditions
and is normally expressed in seconds (S).
•
Alpha ( ) or Temperature Coefficient or Resistance
- The temperature
coefficient of resistance is the ratio at a specified temperature, T, of the rate of
change of zero-power resistance with temperature to the zero-power resistance
of the thermistor. The temperature coefficient is commonly expressed in percent
per degree C (%/°C).
NTC
DISC & CHIP
Selection
Considerations
•
Select Req'd.
Resistance Value &
Temperature
Coefficient
•
Determine Accuracy
Req'd.
•
Review Power
Dissipation
•
Determine Operating
Temperature Range
•
Review Thermal Time
Constant
Applications
Time and temperature are two of the most frequently measured variables. There are
numerous ways of the measuring temperature electronically, most commonly by
thermocouples and negative temperature coefficient (NTC) thermistors. For general
purpose temperature measurement, NTC temperature sensors can operate over a
wide temperature range (-55 to +300°C). They are stable throughout a long lifetime,
and are small and comparatively inexpensive. Typically, they have negative
temperature coefficients between -3.3 and -4.9%/°C at 25°C. This is more than ten
(10) times the sensitivity of a platinum resistance thermometer of the same nominal
resistance. RTI's
DISC & CHIP
style thermistors are used in many applications that
require a high degree of accuracy and reliability.
Some of the most popular applications of NTC thermistors include:
• Temperature Compensation
• Temperature Measurement & Control
• Fan Motor Control
• Fluid Level & Temperature Sensors
Selection considerations for NTC
DISC & CHIP
Devices
Power dissipation is a common problem in the use of thermistors as they can only
dissipate a certain amount of power. If the power dissipated exceeds the dissipation
constant (D.C.) rating of the sensor it is likely that it will exhibit self heating. Most
thermistors dissipate from 1 to 25 mW/°C nominal. This means that the resistance
changes by an equivalent of 1°C for each D.C. rating (mW/°C) for the selected
device. To maintain a higher degree of accuracy, temperature error caused by self-
heating should be an order of magnitude less than the required sensor accuracy. For
many applications, this degree of accuracy is not required and a less stringent de-
rating may be adequate. Several options to reduce the thermistor power are to
increase the thermistor resistance, lower the source voltage and/or increase the
series resistor in the divider circuit.
As an example, if the D.C. of the thermistor selected is 5 mW/°C and the power
dissipated by the device is 20 mW/°C, then a 4°C error is induced due to the effect of
self-heating. To minimize this effect, a factor can be derived simply by taking the DC
rating times 10 (one order of magnitude lower) and use it in the power equation to
produce a good approximation of the maximum allowable power. For instance, if the
desired accuracy is 1°C, and the rated D.C. of the device selected is 5 mW/°C,
adjusting the specified D.C. rating in the power equation to 0.5 mW/°C compensates
for self-heating error and effectively predicts the maximum power the device can
dissipate without significantly affecting the desired accuracy. The resulting maximum
power that should be applied would be calculated as 1°C*0.5mW/°C = 0.5mW.
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NTC Thermistor Discs and Chips | Ametherm