Standard NTC Thermistor Applications

GENERAL NOTES

Thermistors are temperature sensitive passive semiconductors which exhibit a large change in electrical resistance when subjected to a relatively minute change in body temperature. Negative Temperature Coefficient (NTC) thermistors decrease in resistance when subjected to an increase in body temperature. Their extreme sensitivity to minute temperature changes enables them to perform many unique functions heretofore impossible with standard electronic components.

NTC Resistance Temperature Characteristic Formula:

The resistance of a thermistor is solely a function of its absolute body temperature. When testing for resistance accuracy it is essential that the surrounding environmental temperature is held at a constant, and power dissipated in the thermistor is low enough to insure no "self-heating". Formula for determining resistance of RTI composition A, B, C and E NTC thermistors at any temperature within the operating limits:

Where:

R_o(T)	is the resistance at Temperature T
R_o(T_o)	is the resistance at 25°C
	is 2.718
	is the Temperature at which R is unknown expressed in °Kelvin. (273.15 + °C)
	is 298.15°K (25°C)
	is (a + bT + cT²)

	Comp. A	Comp. B	Comp. C	Comp. E
a =	2641.67	2183.03	2923.94	3197.35
b =	1.0643	6.619	4.8657	4.2865
c =	-0.0001571	-0.008048	-0.005363	-0.001836

The temperature coefficient of resistance () is expressed mathematically as follows:

which is approximately equal to ß/T²

NTC APPLICATIONS

Temperature Measurement:
When a very small amount of power is dissipated in a thermistor, its temperature will be dependent upon the surrounding ambient. Therefore, its electrical resistance becomes a function of the ambient temperature, and may be used to measure temperature variations. Because of the very high temperature coefficient of the thermistor, accurate temperature measurements can be made with a simple measuring device. Figure 1 shows a simple circuit using a microammeter in series with a thermistor connected to a potential source. The meter can be calibrated in terms of temperature.

A more sensitive method would be as shown in Figure 2 using a bridge circuit with a thermistor in one leg. Caution must be taken to insure that the power dissipated in the thermistor is held at a minimum and current flow is insufficient to cause "self- heating''.

Temperature Differential:

By placing matched thermistors in two legs of a bridge circuit as seen in Figure 3, temperature differentials as close as .001°C can be readily detected.

Temperature Control

By placing a thermistor in series with a relay coil and potentiometer as shown in Fig.4, a simple temperature controller is obtained. The potentiometer will control the switching temperature.

A more sensitive controller can be obtained by feeding the output of a thermistor bridge as shown in Figure 3 into a high gain amplifier. Sensitivity of .005°C can be sensed easily with this method.

Temperature Compensation:

Since all metals used for coil windings, etc., have a positive temperature coefficient of resistance, NTC thermistors are especially useful for compensating resistance changes in devices subjected to temperature variations. Where a copper meter coil would change 50% in resistance over a commonly used temperature range, a thermistor shunted by a resistor in series with the unit as shown in Figure 5 allows the total impedance of a circuit to be held uniform over the entire operating range. Due to the high temperature coefficient of the thermistor as opposed to the low temperature coefficient of the copper, full compensation can be achieved by using a thermistor- resistor network. This network adds less than 15% to the total impedance of the circuit. Compensation of transistor amplifiers, crystal oscillators, etc. can be achieved by using similar methods. RTI's application engineering staff is always anxious to help you solve your temperature compensation problem.

Time Delay

By placing a thermistor in series with a relay, a potentiometer, and a battery as shown in Figure 4, a simple time delay circuit is obtained. A relatively high potential is applied to the circuit. The thermistor begins to "self-heat," lowering its resistance and allowing more current to flow. The increased current further heats the thermistor, allowing still more current to flow, which in turn actuates the relay. The time required for the relay to actuate after voltage is applied can be controlled by adjusting the potentiometer.

Surge Suppression

By placing a thermistor in series with a filament string as shown in Figure 6, current surge can be eliminated. The resistance of the thermistor is higher than the total resistance of the filaments when the circuit is turned on. As current begins flowing, the thermistor "self-heats." Its resistance is reduced to a minimum and becomes insignificant to the total resistance of a circuit.

Current surges in electric motors can be held to minimum using the same concept. Figure 7 shows a typical DC motor's turn-on surge before and after the application of a RTI thermistor to the circuit.