Thermistors
Thermistor Characteristics
Thermistors are either NTC type, whose resistance decreases with increasing temperature, or PTC type, whose resistance increases with increasing temperature. Each type has unique features and distinct advantages.
NTC thermistors are more commonly used than PTC type, especially in temperature measurement applications. NTC thermistors are made using basic ceramics technology and semiconductor metal oxide materials (e.g., oxides of manganese, nickel, cobalt, iron, copper, and titanium). In some thermistors, the decrease in resistance is as great as 6% for each 1°C of temperature increase although 1% changes are more typical. NTC thermistors can provide good accuracy and resolution when measuring temperatures between −100°C and +300°C. If inserted into a Wheatstone bridge, a thermistor can detect temperature changes as small as ±0.005°C.
Figure 1. R–T curves of silistor and switching PTC thermistors
PTC thermistors can be divided into two categories: thermally sensitive silicon resistors (silistors) and switching PTC thermistors. Silistors exhibit a fairly uniform PTC (about +0.0077°C−1) through most of their operational range, but can also exhibit an NTC region at temperatures higher than 150°C. This type of thermistors is often used for temperature compensation of silicon semiconducting devices in the range of −60°C to +150°C. The switching PTC thermistors are made from polycrystalline ceramic materials that are normally highly resistive but become semiconductive by adding dopants. They are often manufactured using compositions of barium, lead, and strontium with additives such as yttrium, manganese, tantalum, and silica. The R–T curves of switching PTC thermistors exhibit very small NTC regions until they reach a critical temperature Tc—“Curie,” “Switch” or “Transition” temperature. After Tc, the curve exhibits a rapidly increasing PTC resistance. These resistance changes can be as much as several orders of magnitude within a temperature span of a few degrees. Figure 1 illustrates R–T curves of both silistor and switching PTC thermistors.
Most PTC thermistors’ applications are based on either the steady-state selfheated condition (voltage–current characteristic) or the dynamic self-heated condition (current–time characteristic) or a combination of both. For example, the dramatic rise in the resistance of a PTC at and above the transition temperature makes it ideal for over-current protection (resettable fuses). If an over-current condition occurs, the thermistor will self-heat beyond the transition temperature and its resistance rises dramatically. This causes the current in the overall circuit to be reduced. The key characteristics of NTC and PTC thermistors include :
- Resistance temperature (R–T) characteristics: Describe how resistance changes as temperature changes in a thermistor. Most thermistor manufacturers provide tables, R–T curves, and coefficients for their thermistor products.
- Resistance tolerance: Specifies the standard tolerances available for each thermistor type.
- Beta tolerance: Is the tolerance of a thermistor’s beta value β, determined by the composition and structure of the metal oxides being used in the sensor. For bead-type thermistors, beta tolerances are in the order of ±1% to ±5%; for metallized surface contact type, beta tolerances range from ±0.5% to ± 3%.
- Heat capacity: Is the product of the specific heat and mass of the thermistor. It represents the amount of heat required to produce a change in the body temperature of the thermistor by 1°C.
- Dissipation constant: Is the ratio of the change in power applied to a thermistor to the resulting change in body temperature due to self-heating. It is affected by lead wire materials, method of mounting, ambient temperature, and the shape of the thermistor.
- Resistance range: Defines the minimum and maximum resistance values at the reference temperature.
- Transition temperature: Is the Curie point at which the PTC thermistor’s R–T curve begins to increase sharply. PTC manufacturers often define this temperature as the point where a specified ratio exists between the minimum resistance (or at 25°C zero-power resistance) and the transition temperature resistance. For example, Thermometrics Inc. specifies the point where the resistance is twice (2×) the minimum value, whereas other manufacturers might use 10 times (10×) the minimum.
The main advantages of thermistors for temperature measurement are:
- Extremely high sensitivity. For example, a 2252 Ω thermistor has a sensitivity of
−100 Ω⋅°C-1 at room temperature. Higher resistance thermistors can exhibit a sensitivity of −10 kΩ ⋅ °C-1 or more. In comparison, a 100 Ω platinum RTD has a sensitivity of only 0.4 Ω ⋅ °C-1. - Very fast response to temperature changes.
- Relatively high resistance. Thermistors are available with base resistances (at 25°C) ranging from hundreds to millions 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, fourwire, or Wheatstone bridge configurations are necessary, although using a Wheatstone bridge can improve linearity of thermistors.
Figure 2. Comparison of R–T curves for a thermistor and an RTD. (From Potter, D., Measuring temperature with thermistors—A tutorial, Application Note 065, National Instrument Corporation, Austin, Texas, USA, 1996. With permission.)
The major disadvantages of thermistors are their high nonlinearity and limited temperature range (typically below 300°C). Figure 2 shows the R–T curve for a 2252 Ω thermistor. The curve of a 100 Ω RTD is also shown for comparison.
Table 1 compares the main characteristics of RTDs, NTC thermistors, and thermocouples.
Tabel 1. Compares the main characteristics of RTDs, NTC thermistors, and thermocouples
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. A bead-type thermistor has platinum alloy lead wires (about 0.5–5 mm in diameter) that are directly sintered into the ceramic body.
Tabel 2. Types of Bead and Metallized Surface Contact NTC Thermistors
The metallized type has metallized surface contacts (with or without radial or axial leads) for surface or spring mounting. Each type can be further characterized by differences in geometry, packaging, and/or processing techniques as shown in Table 2. Figure 3 illustrates the circuit symbol and the most common forms of thermistors.
Figure 3. Thermistors: (a) circuit symbol for NTC type (with negative sign); (b) bead; (c) rod; (d) washer; (e) surface mount; (f) disk.
Thermistors are often mounted in stainless-steel tubes to protect them from harsh environments during their operation. Thermoconductive grease or silicon sealant is typically used to improve the thermal contact between the sensor and the tube (see Figure 4). To maintain the temperature tolerance within ±0.05°C to ±1°C, thermistors are laser trimmed during the manufacturing process. Thermistors are mechanically simple and strong, providing the basis for a high reliability sensor. They are available in a large range of sizes, base resistance values, and R–T curves.
Figure 4. Internal structure of a bead-type thermistor
The fabrication of NTC thermistors uses basic ceramics technology: a mixture of two or more metal oxide powders combined with suitable binders and formed into a desired shape, dried, and sintered at an elevated temperature. By varying the types of oxides used, sintering temperature and atmosphere, a wide range of resistances and temperature coefficients can be obtained.
Thermistor Applications
NTC thermistors have been primarily used for high-resolution temperature measurements. With high sensitivity, reliability, low price, ruggedness, and ease of use, NTC thermistors have a variety of applications such as in home appliances, automobiles (to monitor coolant or oil temperature), mobile telecommunication, computers (to monitor the temperature of battery packs while charging), overheating detection in electronic equipment, medical care, and other industrial usage. PTC thermistors are often used as current-limiting devices for circuit protection as fuses. They are also used as heating elements in small temperature-controlled ovens (e.g., crystal oven).
Figure 5. A thermistor for (a) water temperature measurement; (b) flow rate measurement.
Figure 5.a shows a thermistor used to measure water temperature in Toyota automobile engines. Figure 5.b is a thermoanemometer for flow rate measurement. Two thermistor-type of temperature sensors, R0 and RS, are immersed into a moving fluid (air or liquid). R0 measures the initial temperature of the fluid. A heater, located between R0 and RS, heats the fluid and its temperature is then measured by RS. The flow rate to be measured is thus proportional to the heat loss rate.
