NTC Thermistors for Temperature measurement
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What are NTC thermistors? NTC stands for Negative Temperature Coefficient. NTC thermistors are resistors with a negative temperature coeff...
What are NTC thermistors? NTC stands for Negative Temperature Coefficient. NTC thermistors are resistors with a negative temperature coefficient, which means that the resistance decreases with increasing temperature. They are primarily used as resistive temperature sensors and current-limiting devices. The temperature sensitivity coefficient is about five times greater than that of silicon temperature sensors (silistors) and about ten times greater than those of resistance temperature detectors (RTDs). NTC sensors are typically used in a range from −55°C to 200°C.
The non-linearity of the relationship between resistance and temperature exhibited by NTC resistors posed a great challenge when using analog circuits to accurately measure temperature, but rapid development of digital circuits solved that problem enabling computation of precise values by interpolating lookup tables or by solving equations which approximate a typical NTC curve.
1. Applications utilizing the influence of ambient temperature on resistance(self-heating negligible)
The high sensitivity of an NTC thermistor makes it an ideal candidate for temperature sensing applications. These low-cost NTC sensors are normally used for a temperature range of 40°C to+300°C.
Selection criteria for NTC thermistors are temperature range resistance range measuring accuracy environment (surrounding medium) response time dimensional requirements.One of the circuits suitable for temperature measurement is a Wheatstone bridge with an NTCthermistor used as one bridge leg.
With the bridge being balanced, any change in temperature will cause a resistance change in thethermistor and a significant current will flow through the ammeter. It is also possible to use a vari-able resistor R3and to derive the temperature from its resistance value (in balanced condition).An example of a circuit including an NTC thermistor and microcontroller is given in figure 2
NTC thermistors exhibit a distinctly non-linear R/T characteristic. If a fairly linear curve is requiredfor measurements over a (wide) temperature range, e.g. for a scale, series-connected or paral-leled resistors are quite useful. The temperature range to be covered should, however, not ex-ceed 50 K to 100 K
The combination of an NTC thermistor and a paralleled resistor has an S-shaped R/T characteris-tic with a turning point. The best linearization is obtained by laying the turning point in the middleof the operating temperature range. The resistance of the paralleled resistor can then be calculat-ed by the exponential approximation:
Virtually all semiconductors and the circuits comprised of them exhibit a temperature coefficient.Owing to their high positive temperature coefficient, NTC thermistors are particularly suitable forcompensating this undesired response to temperature changes (examples: working point stabi-lization of power transistors, brightness control of LC displays). Resistors in series or shunt plussuitable voltage dividers and bridge circuits provide an excellent and easy-to-implement compen-sation network.
It is important to match the temperature of the compensating NTC thermistor to that of the compo-nent causing the temperature response. Temperature-compensating thermistors are therefore notonly available in conventional leaded styles, but also incorporated in screw-type housings for at-tachment to heat sinks and as chip version for surface mounting.Figure 8 shows a simple circuit configuration for a thermostat
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