NTC-resistor

The pH (or pI) term of the Nemst equation contains the electrode slope factor as a linear temperature relationship. This means that a pH determination requires the instantaneous input, either manual or automatic, of the prevailing temperature value into the potentiometer. In the manual procedure the temperature compensation knob is previously set on the actual value. In the automatic procedure the adjustment is permanently achieved in direct connection with a temperature probe immersed in the solution close to the indicator electrode the probe usually consists of a Pt or Ni resistance thermometer or a thermistor normally based on an NTC resistor. An interesting development in 1980 was the Orion Model 611 pH meter, in which the pH electrode itself is used to sense the solution temperature (see below). 

Fig. 12.75Section through zirconium oxide cell 1, zirconium oxide tube 2, NTC resistor 3, dosing electrodes 4, ceramic spacer 5, measuring electrodes 6, thermal insulation 7, heating coil 8, thermocouple Source Own files... 

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. 

The behaviour of an NTC resistor on load is complicated by its increase in internal temperature when it passes a current. As a rough approximation it can... 

The preceding two equations imply that, for high accuracy, high n values must be considered. In practice n = 3 gives, for the known thermistors, a good fit and an accuracy of about 10-3°C. For a suitable narrow range of temperatures the following simplified equation may be conveniently used to represent the pT characteristics of an NTC resistor ... 

The value of c is often negligible for NTC resistors, thus the variation of resistance as a function of temperature may be written as ... 

Varistors are semiconductors that are used as resistors having a nonohmic behavior. At low voltage differences over the resistor, these varistors behave as weakly temperature-dependent (NTC) resistors but above a critical voltage difference Vc over the material the resistance drops sharply (Figure 9.10). This is a... 

Negative temperature coefficient resistors, also called thermistors, have a negative temperature dependence in the order of several percent per degree Celsius. They are made from polycrystaUine semiconductors, consisting of oxides of chromium, manganese, iron, cobalt, and nickel. The resistance of a NTC resistor can be expressed as... 

Negative temperature coefficient (NTC) resistors for example, spinel structure oxides, manganese(ll. 111) oxide - Mn304 Positive temperature coefficient (PTC) resistors for example, aluminum oxide AI2O3, magnesium oxide - MgO... 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

When microelectronics and solid state devices developed over the last five to four decades, the development of solid-state sensors followed suit, resulting in the introduction of NTC and PTC resistors to monitor temperature, and first Reed relais and inductive sensors to determine position and distance, or tachometers for rotational measurements in washing machines and dish washers over the past two decades. 

NTC and PTC resistors for temperature determination are built into nearly all large appliances... 

Thermistors Thermistors are nonlinear temperature-dependent resistors, and normally only the materials with negative temperature coefficient of resistance (NTC type) are used. The resistance is related to temperature as... 

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. 

The first, -40 to 170 °C, covers the requirements in the passenger compartment, climate control system, and powertrain applications. The sensors are mostly based on negative thermal coefficient resistors (NTCs). Other technologies are capable of meeting the technical requirements in this temperature range (PTCs, Pt 1000 resistors, integrated circuits with linear outputs (KTY-series) or copper or nickel resistors) but none of these has a major market share so far. 

For the range up to 170 °C, most sensors are based on NTCs. These resistors are made of powder mixtures of metal oxides such as iron, zinc, cobalt, manganese, copper, and nickel. These powder mixtures are formed into discs or wafers and sintered at high temperatures between 1000 and 1400 °C. The cured product is a ceramic material. 

NTCs are available as SMD resistors (surface mounted divices), as well as in special packages like glass-coated or fitted to lead frames. The size of the NTC can be as small as 1 mm x 0.5 mm x 0.6 mm (and smaller) or there are discs of 25 mm diameter [1],... 

Fig. 7.5.6 Curve input voltage NTC with pull-up resistor...

For the temperature range up to 800 or even 1100°C sensors are not yet used in high volumes. There are several possible technologies. The first is to qualify special NTCs for these temperatures [3], the second is to use thermocouples, and the third is platinum resistors there are others. The battle has just started and so far there is no clear favorite in the market. 

Thermistors are resistors with a temperature-dependent value of their resistance. There are three types of thermistors CTT (critical temperature thermistors), NTC (negative temperature coefficient), and PTC (positive temperature coefficient) thermistors. Their thermal behavior is shown in Figure 9.7. 

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