Technical Notes
Thermistor Definition
The world thermistor is derived from its
description:
“Thermal
sensitive resistor”. Thermistors are passive
semiconductors,
which produce resistance values
dependent on
temperature.
A Negative Temperature Coefficient (NTC) thermistor
decreases in resistance as its body temperature increases. In fact, NTC
thermistors exhibit two characteristics, which make them extremely useful in a
variety of applications. Their change in resistance is predicable and its is
relatively large per degree change in temperature.
Manufacturing
Process
This is a two-step process of chip manufacturing and
thermistor assembly. Metal oxide powders into ceramic sheets process
manufactured chips. These sheets are moralized with silver to allow for
electrical contact. After moralization, the ceramic sheets are diced into
chips. Each chip is tested to meet our superior quality standards.
After a chip has been manufactured and tested, leads
are attached. The chip is trimmed to match the specified tolerance, and then a
protective coating is added. Adding housings, cables and connectors can do
further customizing of the assembly.
Thermistor quality is assured with in-process
inspection and Statistical Process Control (SPC). This process takes place at
each manufacturing and assembly step. All Finished products are 100% tested
both electrically and mechanically to guarantee all specifications are met.
Resistance-Temperature(R/T)
Curves and Negative Temperature Coefficient
Nine different materials are made, each with its own
unique and predictable resistance-temperature characteristics. These
characteristics are called “curves”. Thermistors are most often specified by
their curve and by their resistance value at 25℃.
The NTC (Negative Temperature Coefficient) is the
negative percent resistance change per degree C. Our thermistors have NTC
values at 25°C ranging from –3.9%℃ to –6.4%℃. Resistance values at 25℃ range from 300
ohms to 40 meg ohms. The tables on pages 23 through 25 detail this information.
Thermal Time
Constant
Time constant, expressed in seconds, is the time
required for a thermistor to indicate 63.2% of a newly impressed temperature.
The time constant of a thermistor is directly affected by the mass of the
thermistor and thermal coupling to the environment. An epoxy or phenolic coated
thermistor coupling to the environment. An epoxy or phenolic coated thermistor
with a 0.095” O.D. will typical have a time constant of 0.75 seconds in stirred
oil and 10 seconds in still air.
Dissipation
Constant
Dissipation constant is the power required to raise
the temperature of a thermistor 1℃ above the
surrounding environment. Power is expressed in watts. The dissipation constant
of a thermistor with a 0.095” O.D., coated with epoxy or phenolic, is typically
13 mW/℃ in stirred oil
and 2 mW/℃ in still air.
Voltage/Current
Requirements
Very low current is required for a thermistor being
used in temperature measurement, control or compensation applications. Current
levels should typically be less than 100mA for a thermistor to dissipate “zero
power”. As previously discussed, power dissipation for a thermistor in still
air is approximately 2mW/℃. Therefore, in
order to keep the thermal error (self-heat) below 0.1℃, the power dissipation must be less than 0.2mW.
Self-heating is desirable in applications such as
airflow measurement and liquid level control. Standard epoxy or phenolic coated
thermistors with a 0.095” O.D. have a maximum power rating of 30 milliwatts at
25℃ to 1 milliwatt at 100℃.
Beta
The Beta value of a thermistor is one way to
characterize its resistance temperature relationship. Beta is calculated as
follows:
βT2 / T1 =
ζn(RT2/RT1)/(1/T2
– 1/T1)
Temperature is in degrees Kelvin; RT1 is
the resistance at temperature T1; RT2 is the resistance
at temperature T2.
Steinhart-Hart Equation
The Steinhart-hart Equation is an empirically developed
Polynomial, which best represents the resistance temperature relationships of NTC thermistors. The Steinhart-Hart Equation is more accurate than previously methods; as well, it is more accurate over wider temperature ranges. To solve for temperature when resistance is known, the form of the equation is:
1/T=a+b(ζnR)+c(ζnR)3
To solve for resistance when temperature is known, the form of the equation is:
R=e(exp)[(-α/2+(α2/4+α3/27)-2)-3+(-α/2-(α2/4+α3/27)2)3]
Where alpha = (a-1/T)c and β
= b/c
For both forms of the
equation T is temperature expressed in degrees Kelvin; a, b and c can be solved
simultaneously using the following:
1/T1=a+b(ζnR1)+c(ζnR1)3
1/T2=a+b(ζnR2)+c(ζnR2)3
1/T3=a+b(ζnR3)+c(ζnR3)3
The data calculated by these
equations will be accurate to better than +0.01℃ when -40℃ is less than or
equal to 150℃ and ｜T1-T2｜ is less than or
equal to 50℃ and ｜T2-T3｜ is less than or
equal to 50℃ and T1, T2 and
T3 are evenly spaced.
Maximum Temperature
Rating/ Recommended
Operating Ranges
Our thermistors may be
intermittently cycled at temperatures from -50℃ to 150℃. Stability is achieved when the thermistors are stored at temperatures less than 50℃ and operated continuously at temperatures less than 100℃. For interchangeable thermistors, optimum stability is achieved when
the thermistors are operated at temperatures within the specified
interchangeable temperature range.
Stability
Years of experience in
thermistor manufacturing, coupled with stringent process controls, ensure that
highly stable thermistors are produced. In fact, our thermistors typically
exhibit less than 0.02℃ thermometric
drift per year when stored or operated at temperatures less than 50℃. The stability of a thermistor is greatly dependent on environmental
conditions such as humidity, excessive temperatures and thermal shock; these
effects should be minimized to guarantee stability.
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No.18, Lane 609, Sec 5, Chung Sin Road, ShanChung City, Taipei Hsien, Taiwan