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Thermistor Definition
The
world thermistor is derived from its description:
values
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.
OCTSENS TECH CO., LTD.
4F., No. 22, Ln 75, Sec. 2, Jhongjua Rd.,
Jhongjheng Dist., Taipei City 10065, Taiwan, R.O.C.
Tel: 886-2-23883573 Fax: 886-2-23883575