What are Thermistors?
Thermistors (THERMally sensitive resistors), are solid state, electronic devices which detect thermal environmental changes for use in Temperature measurement, control and compensation circuitry. Quality Thermistor, Inc. manufacturers temperature sensors comprised of two basic thermistor technologies:
Negative Temperature Coefficient (NTC)
Negative Temperature Coefficient (NTC) thermistors exhibit a non-linear, decreasing electrical resistance with increasing environmental temperature. Correspondingly, NTC thermistors exhibit a non-linear increasing electrical resistance with decreasing environmental temperature. An NTC thermistors exponential and inverse Resistance / Temperature (R/T) characteristic delivers precise, repeatable thermal sensing with field interchangeable capability.
Positive Temperature Coefficient (PTC)
Positive Temperature Coefficient (PTC) thermistors exhibit a linear, increasing electrical resistance with increasing environmental temperature. Correspondingly, PTC thermistors exhibit a linear, decreasing electrical resistance with decreasing environmental temperature. PTC thermistors linear and proportional change in Resistance / Temperature (R/T) offers simplified and reliable thermal management.
Frequently Asked Questions
What is a NTC thermistor?
The word 'thermistor' is derived from the phrase THERMally-sensitive resISTOR and “NTC” stands for Negative Temperature Coefficient. This means that NTC thermistors have an inverse relationship between their electrical resistance and body temperature.
If NTC thermistors have a non-linear temperature characteristic, are they difficult to use?
No, although this resistance/temperature (R/T) characteristic is non-linear, it is predictable, repeatable, and can be reproduced to exacting specifications. As a result of Quality Thermistor' superior thermistor technology and improvemtents in electronic instrumentation, the non-linearity of the NTC thermistor characteristic no longer poses a difficulty for design engineers.
From what materials are NTC thermistors made?
NTC thermistors are small, rugged ceramic semiconductors manufactured from compositions of the oxides of metals, such as manganese, nickel, cobalt, copper, and/or iron. Each NTC thermistor composition or mix, has a specific ratio of metal oxides which defines the thermistor’s physical dimensions, R/T curve and its resistance at 25 °C.
Since the resistance changes with temperature, how are NTC thermistors specified?
The resistance value of a thermistor is typically referenced at 25 °C (abbreviated to “R25”). The most common R25 values used for thermistor applications are between 100 ohms and 100 k ohms. Other R25 values as low as 50 ohms and as high as 1 megohms can be produced and resistance values at temperature points other that 25 °C can be specified.
Are NTC thermistors senstive to temperature changes?
NTC thermistors also exhibit a relatively large change in resistance vs. temperature, typically on the order of -3 % to -6 % per °C, providing a much greater sensitivity or signal response to changes in temperature when compared to other temperature sensors, such as thermocouples and RTD’s.
What is the best tool for describing the NTC thermistor R/T characteristic?
Overall, the Steinhart-Hart equation is the most useful tool for interpolating the NTC thermistor R/T curve characteristic. The Steinhart-Hart equation is a third order polynomial which provides excellent curve fitting for specific temperature spans within the temperature range of -80 °C to 260 °C.
How does aging affect thermistor stability?
“Thermometric drift” is a specific type of drift in which the drift is the same amount of temperature at all temperatures of exposure. For example, a thermistor that exhibits a -0.02°C shift at 0°, 40° and 70°C (even though this is a different percentage change in resistance in each case) would be exhibiting thermometric drift. Thermometric drift: (1) occurs over time at varying rates, based on thermistor type and exposure temperature, and (2) as a general rule, increases as the exposure temperature increases. Most drift is thermometric.
What happens if my application exceeds the temperature rating?
Intermittent temperature incursions above and below the operating range will not affect long-term survivability. Encapsulate epoxy typically begins to break down at 150°C and the solder attaching leads to the thermistor body typically reflows at about 180°C. Either condition could result in failure of the thermistor.
Are thermistors ESD sensitive?
Per MIL-DTL-39032E, Table 1, thermistors by definition are not ESD sensitive.
What is the resolution of a thermistor?
There is no limit to the resolution of a thermistor. The limitations are in the electronics needed to measure to a specified resolution. Limitations also exist in determining the accuracy of the measurement at a specified resolution.
Are QTI thermistors RoHS compliant?
(What if I don’t want a lead free part?) Quality Thermistor maintains two separate manufacturing lines to meet the specific environmental needs of our customers. One line is dedicated to RoHS compliance and the other is maintained for traditional tin/lead parts for military, aerospace and medical applications.
Does the length of wire impact the accuracy of a thermistor?
With a thermistor, you have the benefit of choosing a higher base resistance
if the wire resistance is a substantial percentage of the total resistance. An
example of this would be a 100-ohm thermistor vs a 50,000 ohm thermistor
with 10’ of 24 AWG wire.
Total wire resistance = 10’ x 2 wires x 0.02567 ohms per foot = 0.5134 ohms
