Thermistor

A thermistor ( portmanteau of thermal and resistor English ) is a variable electrical resistor whose value varies reproducibly with temperature. It is suitable for measuring temperature or to limit the current in the switch or continuous operation.

• 2.2.1 PTC
• 2.2.2 thermistor

General

Characteristic of a thermistor is the change in resistance when the temperature changes. A resistive material metals, semi-conducting metal oxides ( ceramics ), or silicon can be used, depending on the application, both find materials with positive and negative temperature coefficients of application.

In a first approximation the dependence of the resistance of the temperature can be described as a linear equation with the proportionality:

The factor is referred to as temperature coefficient of the first order. Depending on the sign of this coefficient is made between thermistors with negative coefficients and thermistors with positive coefficients. In hot conductors, also called NTC resistor, the resistance value decreases with increasing temperature. With thermistors, also referred to as a PTC resistor, the resistance value increases with increasing temperature.

Applications

Measurement

PTC

Pure metals are thermistors. In industrial temperature measurement measuring resistors are common and standardized with platinum. They can be used with suitable equipment to 850 ° C. You have an almost linear characteristic at a relative change in resistance of just 0.4 % per ° C.

Versions with ceramic Werkstoffem be used up to about 200 ° C. Their characteristic rises in a narrow range around their nominal response with up to 30% per ° C. Measuring Technically, they are hardly more suitable than the statement " too cold / too hot ".

Thermistor

Thermistors with negative temperature coefficient ( NTC thermistor ) have due to the underlying semiconductor effect is highly dependent flaws, as the doping of the base materials. Processing such as mixing, milling, pressing, sintering has a large influence on the properties and the long term stability. Therefore NTCs long time could be produced only with strongly scattering characteristics and have the NTCs placed in its early days the reputation of being very suitable for accurate temperature measurement.

Thermistor can be used up to about 150 ° C. They have over platinum measuring resistors a significantly higher sensitivity (guideline: At room temperature, the absolute value of magnitude larger). Your non-linear behavior is described by nonlinear equations:

• The preferred representation of the dependence can be found at the resistance thermometer.
• Another function is the Steinhart-Hart equation

• As a third possibility, the relationship can be described by a polynomial. The degree of the polynomial depends on the required accuracy; usual polynomials are third or fourth degree.

The nominal value of a sensor which indicates the electrical resistance at a reference temperature. In data sheets denominations are offered in a range over several orders of magnitude. The error limits ( manufacturing tolerances ) of the nominal values ​​are around 5%, the temperature coefficient also roughly 5%.

These sensors are also in the same par value usually not interchangeable due to the dispersion without re-recording of the characteristic curve (calibration) or adjustment of the used device.

Other characteristics are

• The self-heating coefficient - in ° C / mW - or the thermal conductivity ( dissipation factor) - stated in mW / ° C - as a measure of the self-heating due to the current
• The response time after which an abrupt change in the temperature of the measured material has been taken at a given percentage of the sensor, or cooling time constant for the duration of a part of the probe cools down after switching off the current.

These characteristics are dependent on the composition of the sample material ( air, liquid ) and the flow rate.

Current limit

PTC

PTC thermistors are used, among other things, for overload protection and current limiting in continuous operation. Some PTC resistors of barium titanate show a strong non-linear relationship between temperature and resistance value and see other things as wear-free switching elements application - comparable with bimetallic switches. These types are used for thermal protection of electric motors; in excessive heating of the engine there is a power reduction or a shutdown.

In other applications, PTC resistors can be used as a slow self-resetting fuses or to control the degaussing for color picture tubes.

Thermistor

NTC thermistors are used to limit inrush currents to provide a soft start. A thermistor in the supply line of an electrical appliance is cold before turning forwards in the instant of closing nor bad and limited purely resistive inrush. After switching it warms up by the current flow and reduces its resistance to the high initial resistance by often more than 2 orders of magnitude. The use of a thermistor prevents a current peak, for example,

• If this could trigger an electrical fuse when switching on a power supply,
• If it could harm a rectifier in the charging of a capacitor.

NTC thermistors for inrush current limiting can not be connected in parallel to increase the permissible continuous current. The smaller of the resistors would get the larger share of the power, warmed up, and the already warmer component heats up even further. As a result, one of the thermistor takes over almost the entire stream. However, a series connection is possible when it comes to increasing the run-up time.

Designs

• Leaded disks or cylinders
• Surface Mount (Surface Mounted Device (SMD ); cuboid or eg SOT -23)
• Leaded metal eyelets, pills or semiconductor typical case ( TO-92, TO- 220)
• Screw-in metal housing
• Historical PTC see iron -hydrogen resistance
• Historical Hot Head of uranium dioxide as URDOX resistor see