Physics
Thermistors
Thermistors are temperature-sensitive resistors that exhibit a large change in resistance with temperature. They are commonly used in electronic devices for temperature measurement, control, and compensation. Thermistors can be classified as either NTC (negative temperature coefficient) or PTC (positive temperature coefficient) based on how their resistance changes with temperature.
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10 Key excerpts on "Thermistors"
- Dogan Ibrahim(Author)
- 2002(Publication Date)
- Newnes(Publisher)
Chapter 5 Thermistor Temperature Sensors 5.1 Thermistor principles The name thermistor derives from the words “thermal” and “resistor”. Thermistors are temperature sensitive passive semiconductors which exhibit a large change in electrical resistance when subjected to a small change in body temperature. As shown in Fig. 5.1, Thermistors are manufactured in a variety of sizes and shapes. Beads, discs, washers, wafers, and chips are the most widely used thermistor sensor types. Disc Thermistors are made by blending and compressing various metal oxide powders with suitable binders. The discs are formed by compressing under very Fig. 5.1 Some typical Thermistors 107 108 Microcontroller-based Temperature Monitoring and Control high pressure on pelleting machines to produce round, flat ceramic bodies. An electrode material (usually silver) is then applied to the opposite sides of the disc to provide the contacts for attaching the lead wires. Sometimes a coating of glass or epoxy is applied to protect the devices from the environment and mechanical stresses. Finally, the Thermistors are subjected to a special ageing process to ensure high stability of their values. Typical coated Thermistors measure 2.0 mm to 4.0 mm in diameter. Washer-shaped Thermistors are a variation of the standard disc shaped Thermistors, and they usually have a hole in the middle so that they can easily be connected to an assembly. Bead Thermistors have lead wires which are embedded in the ceramic material. They are manufactured by combining the metal oxide powders with suitable binders and then firing them in a furnace with the leads on. After firing, the ceramic body becomes denser around the wire leads. Finally, the leads are cut to create individual devices and a glass coating is applied to protect the devices from environmental effects and to provide long-term stability. Chip Thermistors are manufactured by using a technique similar to the manufac-turing of ceramic chip capacitors.
eBook - PDF- Joseph J. Carr(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
When such Thermistors are used, how-ever, it is necessary to ensure that the temperature will not go on excursions outside of the permissible linear range. There are methods for linearizing the thermistor, and these will be discussed in a later section. Thermistors are among the oldest temperature sensors available. The temperature sensitivity of electrical resistance in silver sulfide was noted by physicist Michael Faraday in 1 8 3 3 . There are several different 72 4. Temperature Sensors R τ -1 0 0 0 +100 +200 +300 TEMPERATURE (°C) Fig. 4-1 Temperature versus resistance curves for positive and negative temperature coefficient Thermistors. types of thermistor, but the simplest is the wire element. Simple wire thermistor elements are based on two physical phenomena. First, materials tend to change physical dimensions with changes in temper-ature. Metals, for example, tend to expand when heated. Second, the resistance of a material is directly proportional to the length of the sample. Thus, when a metal is heated, it tends to expand, so its electrical resistance increases. Most metals have a positive temperature coefficient (a > 0 ) . Copper, for example, has a temperature coefficient of + 0 . 0 0 4 . HEATER (A) DIRECTLY HEATED (B) INDIRECTLY HEATED Fig. 4-2 Thermistor symbols: (A) directly heated and (B) indirectly heated. Thermistors 73 Fig. 4-3 Types of thermistor packages. Not all materials have positive temperature coefficients, however. Some materials, such as carbon and some ceramics, have a negative temperature coefficient (a = -0 . 0 0 0 3 ) . Other materials, including cer-tain metal alloys, have temperature coefficients that are nearly zero. For example, in manganin and constantan, the temperature coefficient is approximately + 0 . 0 0 0 0 2 , and for nichrome, it is + 0 . 0 0 0 1 7 . At one time, radio designers used nichrome wire to wind tuning inductors because of this property.
eBook - ePubMeasurement and Instrumentation
Theory and Application
- Alan S. Morris, Reza Langari(Authors)
- 2011(Publication Date)
- Butterworth-Heinemann(Publisher)
Equation (14.8) is such that it is not possible to make a linear approximation to the curve over even a small temperature range, and hence the thermistor is very definitely a nonlinear sensor. However, the major advantages of Thermistors are their relatively low cost and their small size. This size advantage means that the time constant of Thermistors operated in sheaths is small, although the size reduction also decreases its heat dissipation capability and so makes the self-heating effect greater. In consequence, Thermistors have to be operated at generally lower current levels than resistance thermometers and so the measurement sensitivity is less.Figure 14.9 Typical resistance–temperature characteristics of thermistor materials. As in the case of resistance thermometers, some practical experimentation is needed to determine the necessary frequency at which a thermistor should be calibrated and this must be reviewed if the operating conditions change.14.4. Semiconductor Devices
Semiconductor devices, consisting of either diodes or integrated circuit transistors, have only been commonly used in industrial applications for a few years, but they were first invented several decades ago. They have the advantage of being relatively inexpensive, but one difficulty that affects their use is the need to provide an external power supply to the sensor.Integrated circuit transistors produce an output proportional to the absolute temperature. Different types are configured to give an output in the form of either a varying current (typically 1 μA°K) or a varying voltage (typically 10 mV°K). Current forms are normally used with a digital voltmeter that detects the current output in terms of the voltage drop across a 10-KΩ resistor. Although the devices have a very low cost (typically a few dollars) and a better linearity than either thermocouples or resistance thermometers, they only have a limited measurement range from −50 to +150°C. Their inaccuracy is typically ±3%, which limits their range of application. However, they are widely used to monitor pipes and cables, where their low cost means that it is feasible to mount multiple sensors along the length of the pipe/cable to detect hot spots.
- Richard S. Figliola, Donald E. Beasley(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
Thermistors are generally used when high sensitivity, ruggedness, or fast response times are required (9). Thermistors are often encapsulated in glass, and thus can be used in corrosive or abrasive environments. The resistance characteristics of the semiconductor material may change at elevated temperatures, and some aging of a thermistor occurs at temperatures above 200 C. The high resistance of a thermistor, compared to that of an RTD, eliminates the problems of lead wire resistance compensation. A commonly reported specification of a thermistor is the zero-power resistance and dissipation constant. The zero-power resistance of a thermistor is the resistance value of the thermistor with no flow of electric current. The zero power resistance should be measured such that a decrease in the current flow to the thermistor results in not more than a 0.1% change in resistance. The dissipation constant for a thermistor is defined at a given ambient temperature as d ¼ P T T 1 ð8:13Þ where d ¼ dissipation constant P ¼ power supplied to the thermistor T, T 1 ¼ thermistor and ambient temperatures, respectively Example 8.4 The output of a thermistor is highly nonlinear with temperature, and there is often a benefit to linearizing the output through appropriate circuit, whether active or passive. In this example we examine the output of an initially balanced bridge circuit where one of the arms contains a thermistor. Consider a Wheatstone bridge as shown in Figure 8.8, but replace the RTD with a thermistor having a value of R 0 ¼ 10; 000 V with b ¼ 3680 K. Here we examine the output of the circuit over two temperature ranges: (a) 25–325 C, and (b) 25–75 C. KNOWN A Wheatstone bridge where R 2 ¼ R 3 ¼ R 4 ¼ 10; 000 V and where R 1 is a thermistor. FIND The output of the bridge circuit as a function of temperature.
eBook - PDF- Ramón Pallás-Areny, John G. Webster(Authors)
- 2012(Publication Date)
- Wiley-Interscience(Publisher)
This second group includes measurements of flow, liquid level, vacuum (Pirani Method), and gas composition analysis. In all these situations there is a change in the thermal conductivity of the environment surrounding the thermistor. This second group also includes automatic volume and power control, time delay applications, and transient suppression. Technical documentation from manufacturers usually includes some very useful ideas for different applications. The circuit in Figure 2.20a 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 microammeter. Current in the circuit is a nonlinear function of the temperature because of the thermistor, but the scale of the microammeter can be marked accordingly. Figure 2.20 shows a thermal compensation application. Here the aim is to compensate for the undesired temperature sensitivity of a copper relay coil. Copper has a positive TCR. The series addition of a resistor with a negative v-y$ —^-<2h (a) -yy (b) 1 —|ι|ι yk — (c) J[ S7 Liquid (d) Figure 2.20 Some applications of NTC Thermistors for the measurement and control of temperature and other quantities, (a) Temperature measurement, (b) Temperature com-pensation. (c) Temperature control, (d) Level control, (e) Time delay when connecting. 2.4 Thermistors 105 temperature coefficient results in the overall circuit exhibiting a negligible tem-perature coefficient. The same method can be used for deflecting coils in cathode ray tubes. Section 2.4.3 describes the function of the resistor shunting the ther-mistor. See also Problems 2.6 and 2.7. Figure 2.20c shows a simple way to perform a temperature-dependent con-trol action. When ambient temperature rises above a given threshold, the ther-mistor resistance decreases enough to allow the flow of a current capable of switching the relay.
- Winncy Y. Du(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
PTC manufacturers often define this temperature as the point where a specified ratio exists between the mini-mum resistance (or at 25°C zero-power resistance) and the transition tem-perature resistance. For example, Thermometrics Inc . specifies the point where the resistance is twice (2 × ) the minimum value, whereas other manu-facturers might use 10 times (10 × ) the minimum. The main advantages of Thermistors for temperature measurement are: (1) Extremely high sensitivity. For example, a 2252 Ω thermistor has a sen-sitivity of − 100 Ω ⋅ °C − 1 at room temperature. Higher resistance Thermistors can exhibit a sensitivity of − 10 k Ω ⋅ °C − 1 or more. In comparison, a 100 Ω platinum RTD has a sensitivity of only 0.4 Ω ⋅ °C − 1 . (2) Very fast response to temperature changes. (3) Relatively high resistance. Thermistors are available with base resistances (at 25°C) ranging from hundreds to mil-lions of ohms. This high resistance diminishes the effect of lead wires that can cause significant errors with low resistance devices such as RTDs. The high resistance and high sensitivity of Thermistors make their measurement circuitry and signal conditioning much simpler. No special three-wire, four-wire, or Wheatstone bridge configurations are necessary, although using a Wheatstone bridge can improve linearity of Thermistors. The major disadvantages of Thermistors are their high nonlinearity and limited temperature range (typically below 300°C). Figure 2.15 shows the R–T curve for a 2252 Ω thermistor. The curve of a 100 Ω RTD is also shown for comparison [15]. Table 2.7 compares the main characteristics of RTDs, NTC Thermistors, and thermocouples. 2.3.4.2 Thermistor Design Thermistors have two nonpolarized terminals. Based on the method by which these terminals are attached to the ceramic body, Thermistors are classified into bead and metallized surface contact types.
- Richard S. Figliola, Donald E. Beasley(Authors)
- 2019(Publication Date)
- Wiley(Publisher)
If the current sources are identical, R RTD = V DIFF ∕I Thermistors Thermistors (from thermally sensitive resistors) are ceramic-like semiconductor devices. The most common Thermistors are NTC, and the resistance of these Thermistors decreases rapidly with temperature, in contrast to the small increases of resistance with temperature for RTDs. An accurate functional relationship between resistance and temperature for a thermistor is generally assumed to be of the form R = R 0 e β(1∕T−1∕T 0 ) (8.11) The parameter β ranges from 3,500 to 4,600 K, depending on the material, temperature, and individual construction for each sensor, and thus must be determined for each thermistor. Figure 8.11 shows the variation of resistance with temperature for two common thermistor materials; the ordinate is the ratio of the resistance to the resistance at 25 ∘ C. Thermistors 272 TEMPERATURE MEASUREMENTS FIGURE 8.11 Representative thermistor resistance variations with temperature. 300 250 200 150 100 50 0 T, temperature [ C] Thermistor Resistance R(T)/R(25 C) 0.001 0.01 0.1 1 10 = 3395 K = 3900 K exhibit large resistance changes with temperature in comparison to a typical RTD as indicated by comparison of Figures 8.6 and 8.11. Equation 8.11 is not accurate over a wide range of temperature unless β is taken to be a function of temperature; typically, the value of β specified by a manufacturer for a sensor is assumed to be constant over a limited temperature range. A simple calibration is possible for determining β as a function of temperature, as illustrated in the circuits shown in Figure 8.12. Other circuits and a more complete discussion of measuring β may be found in the Electronic Industries Association Standard Thermistor Definitions and Test Methods [6]. Thermistors are generally used when high sensitivity, ruggedness, or fast response times are required [7]. Thermistors are often encapsulated in glass and thus can be used in corrosive or abrasive environments.
- Vera Button(Author)
- 2015(Publication Date)
- Academic Press(Publisher)
0 are determined by the proportions and quality control of the components used, sintering process, etc. Resistance of Thermistors can be affected by oxides composition, quality control of components, thermal treatment, thickness and diameter (disc type), distance between wire leads (bead type), impurity doping (silicon single crystal type), substrate material (thin film type), heterogeneity in diffusion of the contact material in the case of Thermistors with metallized surface (chip type), etc. The resistance tolerance of Thermistors is reduced producing them in “matched units pair,” which consists of an arrangement of two Thermistors (drop type, chip or thin film) connected in series or in parallel that results in better tolerance for a given temperature range. Accurate temperature sensing within wide temperature range can be obtained by mounting both high and low resistance Thermistors in the same casing.Thermistors, mainly NTC type, of different formats, sizes, and casings are used in medical electronic thermometers for monitoring the patient temperature with skin probes, for central temperature measurement (e.g., esophageal and rectal) with internal probes, and for direct temperature measurement (e.g., inside blood vessels and heart chambers) with invasive probes with mini and microsensors inserted in catheters. Thermistors are also used for liquid immersion temperature measurements (blood, saline, drug solutions, etc.).The main advantage of the use of Thermistors as temperature transducers is their high sensibility to small temperature changes. Thermistors are less expensive than RTD, copper or nickel extension wires can be used and they become more stable with use. Some disadvantages may also be mentioned, such as their limited working temperature range, the initial accuracy drift, the lack of standards for replacement, and the possibility of loss of calibration, if they are used beyond temperature ratings.4.2.2.2 Thermoelectric temperature transducers
4.2.2.2.1 Thermocouple
A thermocouple is a temperature sensor that consists of two wires of different metallic materials put in thermal contact (Figure 4.19
eBook - ePub- Cecil L. Smith(Author)
- 2011(Publication Date)
- Wiley-AIChE(Publisher)
Compared to platinum RTDs, the resistance of Thermistors is much greater and is more sensitive to temperature. The nature of the sensitivity is the basis for the following classifications:Negative temperature coefficient (NTC) . The resistance of the thermistor decreases with temperature. The Thermistors used for industrial temperature measurement are of this type.Positive temperature coefficient (PTC) . These are sometimes referred to as “switching PTC Thermistors” because their resistance increases abruptly at a certain temperature. This makes them ideally suited for initiating actions (such as a shutdown) to avoid equipment damage due to elevated temperatures. One application is to protect the windings in electric motors from thermal damage. However, they have no application to process temperature measurement.The DIN 43760 standard for the 100-Ω platinum RTD contributed to the industrial acceptance of RTDs for temperature measurement. Unfortunately, no such standard has appeared for Thermistors. Thermistors are differentiated by their zero-power resistance, which is the DC resistance at a specified temperature (usually 25 °C) with negligible self-heating.Resistance-Temperature CharacteristicFigure 2.25 shows a plot of the thermistor resistance as a function of temperature for a specific commercial product (this one was chosen only because the resistance-temperature data could be downloaded over the Internet). This graph of resistance as a function of temperature clearly shows thatFigure 2.25 Resistance of a commercial thermistor as a function of temperature (R vs. T ).- Thermistor resistance at 25 °C is much greater than 100 Ω. There is an advantage for this: Lead wire resistance is insignificant in comparison. The resistance of the thermistor can be determined using two lead wires and a current source.
- The resistance decreases rapidly with temperature. Thermistors are capable of detecting temperature changes on the order of 0.001 °C. But with such large resistance changes, temperature spans have to be narrow (like 20 °C).
- The relationship is highly nonlinear. This presents great difficulties for analog systems but not for digital systems. The nonlinear problem is often overemphasized. For this reason, we will examine some formulations that very effectively address the nonlinear issues. These will involve logarithms. While this can lead to cardiac arrest for the designers of analog circuits, designers of digital systems barely take notice.
eBook - ePubReal-Time Environmental Monitoring
Sensors and Systems - Textbook
- Miguel F. Acevedo(Author)
- 2023(Publication Date)
- CRC Press(Publisher)
active, i.e., require energy to operate.Examples of resistive sensors are potentiometers, resistive temperature detectors, LDR, Thermistors, liquid level sensors, strain gages, resistive gas sensors, liquid conductivity sensors, and resistive hygrometers.Thermistors: Temperature Response
Thermistors can be of two types: NTC (negative temperature coefficient) and PTC (positive temperature coefficient). Those of NTC type are made from semiconductor material and resistance decreases gradually with temperature. In opposite fashion, a PTC, made from ceramic, will have a resistance that increases quickly with temperature.We will focus on NTC Thermistors for which the resistance decreases non-linearly with temperature. A general model is the B parameter equationR =(3.16)R 0exp(B)()T−− 1T 0− 1where T is the temperature in K, R is the thermistor resistance in Ω, T0 is the nominal value of T (25°C = 298 K), R0 is the nominal value of R at T0 , and B is a parameter in K (Rudtsch and von Rohden 2015 ). Figure 3.15 shows an example of the model response for B = 4100 K, T0 = 25°C, and R0 = 10 kΩ.The B parameter equation can be inverted to calculate temperature given resistance by rearranging terms and taking logarithm of both sides of the equationFIGURE 3.15 Thermistor resistance vs. temperature using the B parameter model.(3.17)1 T=1+T 01 Bln(R)R 0Denotinganda 0= 1 /T 0, we obtaina 1= 1 / B(3.18)1 T=a 0+a 1ln(R)R 0This equation can be considered the first-order approximation n =
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