NTC Thermistor: Introduction

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1 Introduction NTC thermistors are resistors with large negative temperature coefficient. Change in resistance of the NTC thermistors can be brought about either externally by a change in ambient temperature or internally by self-heating resulting from a current flowing through the device. This predictable change in resistance as temperature changes is the basis for all applications of thermistors. The semi conducting material of NTC thermistors usually consists of a sintered ceramic fabricated in a variety of shapes and sizes from a mixture of oxides chosen from Mn, Co, Ni, Cu, Fe and etc.by varying the composition and the size of the semi-conducting elements, resistance values between 10 0 and 10 6 ohms at room temperature can be achieved with temperature coefficients lying between -2% and -6.5% per. THINKING ELECTRONIC INDUSTRIAL Co., LTD

2 Glossary Thermistor A thermistor is a thermally sensitive semiconductor resistor whose primary function is to exhibit an important change in electrical resistance with a change in body temperature. Negative temperature coefficient thermistor (NTC) An NTC thermistor is one in which the resistance decreases with increasing temperature. Zero-power resistance (R T ) The zero-power resistance is the value of a resistance when measured at a specified temperature, under conditions such that the change in resistance due to the internal generation of heat is negligible with respect to the total error of measurement. Rated zero-power resistance (R 25 ) The rated zero-power resistance is the nominal value at the standard temperature of 25 unless otherwise specified. B-value An index of the thermal sensitivity expressed by the formula : Or Where B: constant in Kelvins (K) R 1 : resistance in ohms (Ω) at temperature T 1 R 2 : resistance in ohms (Ω) at temperature T 2 T 1 =298.15k (+25 ), T 2 =358.15k (+85 ) The value given above for T 1 and T 2 are the preferred values. When the detail specification prescribes that the B-value shall be measured at other temperatures, the specified value (in Kelvins) shall be used for T 1 and T 2 in the calculation in place of the preferred values. Operating temperature range The operating temperature range is the range of ambient temperature for which the thermistor has been designed to operate continuously at zero-power. The limits are the upper and lower operating temperature. THINKING ELECTRONIC INDUSTRIAL Co., LTD

3 Glossary Maximum power rating (Pmax) The maximum power rating is the maximum dissipation which can be applied to the thermistor for an extended period of time, at an ambient temperature of 25 or at such temperature as may be specified in the detail specification. Thinking's maximum power derating curve is given as below. At a certain ambient temperature, the power-rating shall be derated linearly to zero. Max power rating (%) T L 0 25 T U Ambient temperature ( ) Dissipation constant (δ) The dissipation constant is the quotient (in W/K), at a specified ambient temperature in a specified medium of a change in power dissipation in a thermistor to the resultant body temperature change. Thermal time constant(τ) The time (in s) required for the temperature of a thermistor to change by 63.2% of the difference between its initial and final temperatures when subjected to a step function change in temperature under zero-power conditions. Temperature T 0 τ 63.2% Ta Time THINKING ELECTRONIC INDUSTRIAL Co., LTD

4 Glossary Table 1: Heating time constant and temperature change ratio. Code Rate of change (%) for T 0 -T a τ τ τ τ τ τ τ 99.9 Resistance/temperature characteristic The relationship between the zero-power resistance of a thermistor and its body temperature. The resistance law follow approximately the formula: Where R and R 1 are the value of the zero-power resistance of the thermistor at temperature T and T 1 respectively, expressed in Kelvins, and B is the thermal sensitivity index. Voltage/current characteristic The relationship in still air at 25 or at such a temperature as may be prescribed in the detail specification between the dc or ac rms voltage across the thermistor and the applied steady-state current. THINKING ELECTRONIC INDUSTRIAL Co., LTD

5 Application Note Temperature measurement The NTC thermistor offers a practical, low-cost solution to most temperature measurement applications. One of the circuits suitable for temperature measurement is a Wheatstone bridge with an NTC thermistor used as one bridge leg. Temperature control The NTC thermistors are the most sensitive of all temperature sensing elements they are popular in temperature control systems. By using a thermistor in a voltage comparator circuit basic on-off temperature control as well as over-temperature protection can be incorporated. Temperature compensation Many semiconductors and ICs exhibit a temperature coefficient and require temperature compensation to produce stable performance over a wide temperature range. Owing to their high positive temperature coefficient, NTC thermistors are particularly suitable for this kind of compensation. R T THINKING ELECTRONIC INDUSTRIAL Co., LTD

6 Application Note Inrush current suppressing Circuits including a capacitor, filament for a bulb, inverter for fluorescent lamp, a heater and etc., induce an inrush current more than 100 times the normal current when the circuit switch is turned on. The NTC thermistors specially developed for this application limit the current at turn-on by their relatively high cold resistance. During continuous operation, the NTC thermistors heats up and its resistance remains negligibly low. I T Liquid level sensing Thermistor liquid level sensors depend for their action on the difference between the dissipation constant of the liquid and the air or vapour above the liquid. When the thermistor is immersed in a liquid the dissipation constant increases, the temperature decreases and the voltage lying across thermistor rise. Owing to this effect NTC thermistor are able to detect the presence or absence of the liquid at one precise level. V I THINKING ELECTRONIC INDUSTRIAL Co., LTD

7 Application Note Example Temperature Sensor IC (Cost : 20±0.25) NTC thermistor (Cost : 50% down) Driving voltage controlled by temperature Voltage vs. temperature characteristics Temperature sensor IC NTC thermistor Driving voltage Temperature ( ) THINKING ELECTRONIC INDUSTRIAL Co., LTD