Temperature-sensitive resistors

While ZnO is outstanding for the high a values that can be attained, other systems that contain barrier layers, for instance the positive temperature coefficient resistors based on BaTiC 3, also show the effect, but alternatives to ZnO have not been developed commercially. 

There are numerous uses for resistors with high values of the temperature coefficient of resistance (TCR) and they may be negative (NTC) or positive (PTC). An obvious application is in temperature indicators that use negligible power to monitor resistance changes. Compensation for the variation of the properties of other components with temperature may sometimes be possible in this case the applied power may be appreciable and the resulting effect on the temperature-sensitive resistor (TSR) must be taken into account. 

There will be a time interval between the application of a voltage to a TSR and the establishment of its equilibrium temperature and resistance. Thus NTC resistors can be used to delay the establishment of a final current and power level, while PTC units can be used to give an initially high current that falls back to a required level. PTC units can be used to maintain a comparatively constant current from a source of variable voltage since the increase in resistance resulting from power increase due to a voltage increase may be sufficient to inhibit any current increase. 

Both NTC and PTC units can be used to indicate changes in ambient conditions since the power they draw from a source depends on the heat that they can dissipate into their surroundings. PTC units have the advantage that they are unlikely to overheat since an increase in temperature cuts down the power that they need to dissipate. Precautions must be taken with NTC units to ensure that runaway conditions cannot arise, because an increase in their temperature increases the power that they can draw from a constant voltage source (see Section 5.2.2). 

In a ceramic a large temperature coefficient of resistivity can arise from three causes  

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One temperature-sensitive resistor as compensator and another one as detector are integrated into adjoining strings of a Wheatstone bridge circuit the voltage can be measured. Since both resistors are exposed to the test gas flow, disturbances caused by changes in temperature and humidity are compensated. 

For every electron excited to the antibonding conduction bund, there will remain behind a hole, or vacancy, in the valence band. The electrons in both the valence band and the conduction band will be free to move under a potential by the process shown in Fig. 7.22b. but since the number of electrons (conduction band) and holes (valence band) is limited, only a limited shift in occupancy from left-bound states to right-bound states can occur and the conductivity is not high as in a metal. This phenomenon, known as intrinsic semiconduction, is the basis of thermistors (temperature-sensitive resistors). 

Thermistors, which are metal oxide beads used as temperature-sensitive resistors, have been used in thermal conductivity detectors since the mid-1950s. They offer several advantages. 

Heat is dissipated in the middle resistor (H). The resulting temperature distribution is sensed with two temperature sensitive resistors Tu, Td located symmetrically up- and downstream with respect to the heater. The temperature difference AT as function of the flow shows a maximum which limits the usable flow range of the sensor. A typical output signal of the flow sensor is given in Fig. 20. 

In contrast to direct EMF-readout systems, electrical compensation can be used [36], The ice bath is replaced by an electrical circuit that has a temperature-sensitive resistor. The circuit is preset to compensate for variation in the reference junction temperature. 

A temperature-sensitive resistor has a high enough temperature coefficient of resistance, indicating a variation of electrical resistivity with temperature. A temperature-sensitive resistor used for the measurement of fluid flow generally takes the form of either a hot wire or a hot film. When cooled by the surrounding fluid, its electrical resistance decreases, which reflects the flow rate of the fluid. 

Kanthal 70 (70Ni-30Fe) 8450 340 640 1 30 L 17 520 15 21 +350 of resistance used in voltage regulators, timing devices, temperature-sensitive resistors, temperature-compensating devices, and low-temperature heating applications... 

Temperature-sensitive resistors (thermistors) resistance varies with temperature... 

Scanning thermal microscopy (SThM) is a contact AFM technique that allows spatial mapping of temperature or thermal conductivity across a sample surface in addition to topography. Most thermal probes utilize a temperature-sensitive resistor placed on the end of the tip. These resistor probes can be fabricated from a V-shaped Wollaston wire made of a platinum inner core and outer sheath of silver, in which the silver sheath is etched away at the V-shaped tip. Eigure 19 shows a Wollaston wire probe. In passive mode, the tip is scanned across a heated sample under constant-force feedback (contact mode) and a small current is passed through the probe to sense the tip resistance. The resistance value at any point is a measure of the local temperature, and thus a temperature map and topographic image may be produced simultaneously. 

Figure 7.2 shows a 2D detector array mounted ( hybridized ) to the ROIC to form the SCA. The SCA is common in most infrared array configurations today. In monolithic structures, the detector is an integral component of the ROIC there is no separate detector. Examples of monolithic structures are visible CMOS imagers used in commercial camcorders, cameras, and cell phones, and uncooled infrared devices that utilize temperature sensitive resistors (bolometers) as the sensing component. 

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