Some applications of NTC thermistors for the measurement and control of temperature and other quantities
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The circuit in Figure 2. 17a is suitable for measuring a temperature over a limited range, for example, that of cooling water in cars. It consists of a battery, a series adjustable resistor, a thermist...
The circuit in Figure 2. 17a is suitable for measuring a temperature over a limited range, for example, that of cooling water in cars. It consists of a battery, a series adjustable resistor, a thermistor, and a micro ammeter. Curr ent in the circuit is a nonlinear function of the temperature because of the thermistor, but the scale of the micro ammeter can be marked accordingly.
Figure 2. 17b shows a thermal compensation application. In this case the aim is to compensate for the undesired temperature sensitivity of a copper- wire galvanometer. The copper temperature coefficient is positive. The ser ies addition of a resistor with a negative temperature coefficient results in the overall circuit exhibiting a negligible temperature coefficient. The function of the resistor shunting the thermistor will be described in the next section.
Figure 2. 17c shows a simple way to perform a temperature-dependent control action. When ambient temperature rises above a given threshold, the thermistor resistance decreases enough to allow the flow of a current capable of switching the relay. The adjustable resistor permits modification of the switching point.
The circuit in Figure 2. 17d can control liquid level. The supply voltage must be high enough to heat the thermistor well above the ambient. When the liquid level reaches the thermistor which it cools, its resistance increases in value and the current is reduced, thus switching the relay.
The circuit in Figure 2.17e is intended for time delay. The relay does not switch until the thermistor is hot enough to allow a higher current to flow.
Figure 2.18 shows several applications suggested for a switching PTC. In Figure 2. 18a it is used for starting a single-phase motor. When the switch is first closed, the PTC has a low resistance and allows a high current to flow through the starting coil. When the PTC heats because of the current, its resistance increases to a very high level, thus reducing the current to a very low value.
The circuit in Figure 2. l8b is usually used for automatic degaussing, for example, in color TV sets. In these units a high degaussing current must flow when first turned on, and then it must reduce to a low value.
Transient suppression when a switch opens is useful to reduce both cont act damage and transient propagation to any nearby susceptible circuits. When the switch is opened in the circuit in Figure 2. 18c, the PTC offers a low resistance because no current was flowing through it. But as time passes, its resistance increases, and most of the power stored in the inductive load is dissipated in it instead of being dissipated in form of an arc between switch contacts.
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