Temperature sensors are used to measure temperature in circuits which control a wide variety of equipment. Various processes require temperature monitoring for effective control. Such processes include manufacturing processes, transportation, security, maintenance, and other types of processes during which monitoring the thermal characteristics of devices is necessary or advisable. Temperature sensors are widely used in many fields, such as household electrical appliances and medical appliances. They are also used in mobile communication equipment, for example, cellular phones. Many different types of temperature sensors are commercially available. Temperature sensors having temperature-dependent properties which can be measured electrically include resistors, semiconductor devices such as diodes, and thermocouples. A resistance thermometer has a sensing resistor having an electrical resistance varying with temperature. The temperature sensor is driven by a constant current source in order to develop voltages across the changing resistance of the sensing resistor. The thermistor is a temperature sensitive resistor and is generally composed of semi-conductor materials. The integrated circuit temperature sensor has also been used to measure temperatures. The integrated circuit sensor typically employs an integrated diode whose output characteristics are dependent upon temperature. Temperature sensing in high temperature environments such as automotive vehicle exhaust systems, is typically done using thermistors constructed of materials capable of withstanding the high temperature environment.

A thermistor is an electronic device which utilizes the change of resistance when the temperature changes, and is widely used as a temperature sensor and a compensator for an electronic circuit. Thermistors are ceramic semiconductors which exhibit large changes in electrical resistance with corresponding changes in temperature. Thermistors have an extremely high temperature coefficient of resistance and precise resistance versus temperature characteristics. Because of their sensitivity, accuracy, and stability, thermistors are generally accepted to be the most advantageous sensor for many applications including temperature measurement, compensation, and control. Thermistors having positive temperature coefficients (PTC thermistors) as well as thermistors having negative temperature coefficients (NTC thermistors) are used. The NTC thermistor has a negative temperature coefficient and whose resistivity decreases with increasing temperature, and is applied in the form of a temperature sensor to temperature compensation elements, etc. NTC thermistors are widely being used for the purposes of temperature detection and temperature compensation. NTC thermistors used for temperature measurement and compensation are usually made from various compositions including the oxides of manganese, nickel, cobalt, copper, iron, and other metals to form a ceramic semiconductor material. The PTC thermistor has a positive temperature coefficient and whose resistivity increases sharply at a certain specific temperature. A PTC chip thermistor may be incorporated into the circuit of an electronic device so as to generate heat when an overcurrent with intensity greater than a specified level flows therethrough, thereby increasing its resistance due to its positive resistance-temperature characteristic and keeping the intensity of the current flowing into the electronic device below a certain level. The PTC thermistor is widely applied to temperature control elements, overcurrent control elements, motor-starting elements, constant-temperature heat generators, etc. 

Thermistor Terminology

Negative Temperature Coefficient (NTC)

An NTC thermistor is one in which the zero-power resistance decreases with an increase in

body temperature.

Zero-Power Resistance (RT)

The zero-power resistance is the DC resistance value of a thermistor measured at a specified

temperature with power dissipated by the thermistor low enough that any further decrease in

power will result in not more than 0.1 % (or one-tenth of the specified measurement tolerance, whichever is smaller) change in resistance.

Rated zero power resistance (R25)

The zero power resistance is measured under the standard temperature of 25°C.

B value (unit:K)

B value is a constant describing the physical characteristic of the NTC thermistor material, also

called thermistor coefficient.

That is: β = ln(R1/R2)/(1/T1-1/T2)

R1-Resistance at Temperature T1

R2-Resistance at Temperature T2

B value is usually determined by zero-power resistance at 25°C / 85°C in American market and 25°C / 50°C in Asia market.

Maximum Operating Temperature

The maximum operating temperature of a thermistor is the maximum body temperature at

which the thermistor will operate for an extended period of time with acceptable stability of

its characteristics. This temperature can be the result of internal or external heating, or both,

and should not exceed the maximum value specified.

Maximum Power Rating

The maximum power rating of a thermistor is the maximum power which a thermistor will

dissipate for an extended period of time with acceptable stability of its characteristics.

Dissipation Constant

The dissipation constant is the ratio, (expressed in milliwatts per degree C) at a specified

ambient temperature, of a change in power dissipation in a thermistor to the resultant body

temperature change.

Thermal Time Constant

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.

Zero-Power Temperature Coefficient Of Resistance (alpha α)

Zero-power temperature coefficient of resistance is the slope of the R-T curve at any given temperature is used to express the point. It is a measure of the rate of change in resistance of the thermistor at a specific temperature. Alpha is expressed in -%/°C. As the R-T curve is not linear, alpha is greater at lower temperatures than at higher temperatures.

Alpha is useful for determining what tolerances are required for an application. For example,

the alpha value at 25°C for a particular NTC was –4.0%/°C, if the application requires a temperature accuracy ±0.5°C, then the NTC zero-power resistance at 25°C tolerance would need to specified as ±2.0%. (4.0%*0.5)

Tolerance on Resistance

This is a method of measuring precision in NTC thermistors. Tolerance is the percentage of variation in resistance at a specific temperature. Tolerance is always stated as a percentage at a specified temperature. The industry standard is to use 25°C as the base temperature, unless another temperature is specified.

The maximum operating temperature is the maximum body temperature at which the thermistor will operate for an extended period of time with acceptable stability of its characteristics. This temperature is the result of internal or external heating, or both, and should not exceed the maximum value specified.
.
The maximum power rating of a thermistor is the maximum power which a thermistor will dissipate for an extended period of time with acceptable stability of its characteristics.

The temperature-wattage characteristic of a thermistor is the relationship at a specified ambient temperature between the thermistor temperature and the applied steady state wattage.

The current-time characteristic of a thermistor is the relationship at a specified ambient temperature between the current through a thermistor and time, upon application or interruption of voltage to it.

The stability of a thermistor is the ability of a thermistor to retain specified characteristics after being subjected to designated environmental or electrical test conditions.

Note:

1. Custom orders are welcome.

2. If the model required is not listed, please submit the following information so our engineers can determine the correct product for you:

a. Operating temperature range, working criteria and dielectric requirements for the sensor (i.e.: air, water, oil, etc.).

 b. Required temperature reaction time (thermal time constant).

 c. Outline size (include drawing).

3. For your convenience, we have our own processing equipment for the terminations. Interface terminals can be  processed and the guard sleeve can be assembled to your requirements.

4. We can develop temperature sensors with different types of specifications and outlines to meet your needs.

Thanks for visiting, if you need more information please contact our sales man.