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Understanding Thermistor

The use of thermistors is of many purposes, as they as straightforward device that changes their resistance to temperature. A thermistor is a term derived from the word “Thermal resistor”, it is a thermally sensitive resistor, giving a change in resistance for a temperature change. Thermistors are used in many ways, enabling the temperature of the medium surrounding the device or the device itself to alter its resistance. The device then detects this from a broad temperature sensing to overload cut-outs and many more ideas. Thermistors can be found in many circuits and equipment, offering a simple and cost-effective method of basic temperature sensing.

thermistor

Today you’ll get to know the definition, applications, diagram, symbol, specification, types, history, structure and composition, and working of a thermistor. You’ll also get to know the advantages and disadvantages of thermistors in their various applications.

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What is a thermistor?

A thermistor is a resistance thermometer or a resistor whose resistance is dependent on temperature. Or we can say, thermistors are types of resistors whose resistance is strongly dependent on temperature, then that of standard resistors. Thermal and resistor are the two keywords you should take note of in the thermistor. Their construction is made of metallic oxides, pressed into a bead, disk, or cylindrical shape, and then encapsulated with an impermeable material such as epoxy or glass. Thermistors are found in many circuits and equipment, providing a simple and cost-effective method of basic temperature sensing.

Thermistors are widely used as inrush current limiters, temperature sensors (negative temperature coefficient or NTC type typically), Self-resetting overcurrent protectors, and self-regulating heating elements (positive temperature coefficient or PTC type typically). This will further explain!

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Applications

The applications of thermistors are very wide, due to their effectiveness in circuits and they are very attractive to use. However, the thermistor applications depend upon whether the thermistor is either a positive or negative temperature coefficient. For negative temperature coefficient of thermistors applications include:

  • Very low-temperature thermometers are used as resistive thermometers in very low-temperature measurements.
  • NTC thermistors are used to monitor the temperature of battery packs while charging. This is because modern batteries such as Li-ion batteries are very sensitive to overcharging. The temperature offers a very good indication of the charging state, and when the charging circle should be terminated.
  • NTC thermistors are commonly used in modern digital thermostats.
  • These types of thermistors are used as in-rush-current limiting devices in power supply circuits. They display higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These types of thermistors are usually much larger than measuring thermistors, are purposely designed for this application.

Thermistor diagram

Thermistor diagram

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For positive temperature coefficient thermistors, their applications include:

Current limiting devices: the PTC thermistors are commonly used as current-limiting devices in electronic circuits, where they are used as an alternative to a fuse. Current flowing through the device under normal conditions causes a small amount of heating. This does not offer rise to any undue effects. But if the current is larger, then rises to more heat which the device may not be able to lose to the surroundings, and the resistance increases. In return, this offers rise to more heat generation in a positive feedback effect. So, as the resistance increases, the current falls, thus protecting the device.

In addition to the applications of thermistors, thermistors provide a simple, reliable, and inexpensive method of sensing temperatures. Thus, they are found in a wide variety of devices from fire alarms to thermostats. Although they may be used on their own, they may also be used as part of a Wheatstone bridge to obtain higher degrees of accuracy. Furthermore, thermistors are used as temperature compensation devices. Most of the devices are positive temperature co-efficient, their resistance increases with increasing temperature. Thermistors with negative temperature co-efficient are applicable where stability is required. They are incorporated into the circuit to counteract the effect of the components with a positive temperature co-efficient.

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Thermistor circuit symbol

Thermistors are recognized with circuits by their circuit symbol. This symbol uses the standard resistor rectangle as its basis and then has a diagonal line through which it has a small vertical section. The diagram below shows the circuit symbol that is widely used. Other types are available but they generally follow a similar approach – typically using the old resistor symbol of a zig-zag line as the basis with the same line through it as used with the more conventional rectangular resistor.

The arrow by the T signifies that the resistance is variable based on temperature. The direction of the arrow or bar is not significant. See circuit symbol below:

thermistor symbol

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 Specifications and characteristics

Thermistors have a basic resistance specification, other factors like the temperature coefficient are also necessary. The parameters specified in the datasheets include the basic resistance, tolerance on the basic resistance, B tolerance on B thermal dissipation factor, maximum power dissipation, and operating temperature range. Thermistors are a very useful form of resistor which can be used to detect temperature. They are typically used for regulating temperature, protecting circuits, and in a variety of other ways. They can be used in fire detectors as they respond to heat very quickly and offer a reliable form of component for such components.

For the characteristics, thermistors are governed based on the following formula:

R1 = R2   –  )

Where:

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  • R1 = the resistance of the thermistor at absolute temperature T1 [oK].
  • R2 = the resistance of the thermistor at temperature T2 [oK].
  • = constant depending upon the material of the transducer (e.g., an oscillator transducer)

In the above equation, the relationship between temperature and resistance is highly nonlinear. Usually, a standard NTC thermistor has a negative thermal resistance temperature coefficient of about 0.05/oC.

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Construction and structure

In the construction of thermistors, two or more semiconductor powders made of metallic oxides are mixed with a binder to form a slurry. Small drops of this slurry are formed over lead wires and then dried in a sintering furnace. During this process, the slurry will shrink onto the lead wires to make an electrical connection. The processed metallic oxide is sealed by putting a glass coating on it. This glass coating offers a waterproof property to the devices, helping to improve their stability.

Thermistors are available in a variety of shapes and sizes, and they can be made from a variety of materials depending on the intended application. Also, the temperature to which they will operate is another factor. In their physical look or shape, thermistors can come as flat discs, used in applications where they need to be in contact with a flat surface. They can also be made in form of beads or even rods for use in temperature probes. The most actual design of a thermistor is very dependent upon the suitability of the application.

Generally, metallic oxide thermistors are used for temperatures that range between 200 – 700 K. These types of thermistors are made from a fine powder version of the material that is compressed and sintered at high temperatures. Common materials used for these thermistors include Manganese oxide, nickel oxide, cobalt oxide, copper oxide, and ferric oxide.

On the other hand, semiconductor thermistors are used for much lower temperatures. Germanium thermistors are more widely used than their silicon counterparts and are used for temperatures that range below 100 K, i.e., within 100 degrees of absolute zero. Silicon thermistors can be used at temperatures up to 250oK. Above this temperature, a positive temperature coefficient is set in.

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Types of thermistors

The two types of thermistors are Negative Temperature Coefficient (NTC) Thermistor and Positive Temperature Coefficient (PTC) Thermistor.

NTC thermistors:

In these types of thermistors, when the temperature increases, resistance decreases, and then when temperature decreases, resistance increases. Therefore, in NTC thermistors, temperature and resistance are inversely proportional. This is the most common type of thermistor. The relationship between resistance and temperature in an NTC thermistor is governed by the following equation:

RT = R0   –  )

Where:

  • RT = the resistance at temperature T (K).
  • R0 = is the resistance at temperature T0 (K).
  • T0 = is the reference temperature (normally 250C).
  • = the constant that its value is dependent on the characteristics of the material. The nominal value is taken as 4000.

In NTC thermistors, if the value is high, then the resistor-temperature relationship is very good. Thus, a higher value determines the higher variation in resistance for the same rise in temperature, hence an increased sensitivity and accuracy is given by the thermistor.

With expression one, the resistance temperature coefficient can be obtained. You should know this is the expression for the sensitivity of the thermistor.

aT =  –  = –                           (2)

The above equation shows aT to be a negative sign which indicates the negative resistance-temperature characteristics of the NTC thermistor. If  = 4000 K and T = 298 K, then the aT = – 0.0045/0K.  This is much higher than the platinum RTD and will be able to measure the very small changes in the temperature. Although, alternative forms of heavily doped thermistors are available but at a high cost. They have a positive temperature co-efficient.

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PTC thermistor:

In PTC types of thermistors, that is, positive temperature coefficient thermistors have the reverse relationship between temperature and resistance. So, when the temperature increases, the resistance increases, and when the temperature decreases, resistance decreases. Therefore, in PTC resistors, temperature and resistance are inversely proportional. These types of thermistors are not common as that of the negative temperature coefficient, but they are frequently used as a form of circuit protection. Similar to the function of fuses, PTC thermistors can be used as a current-limiting device.

Current passing through a device causes a small amount of resistive heating. So, if the current is large enough to generate heat then the device can lose to its surroundings then the device heats up. In PTC thermistors, the heating up will also cause its resistance to increase which creates a self-reinforcing effect that drives the resistance upwards, thus limiting the current. This is how it acts as a current limiting device – protecting the circuit.

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Working of a thermistor

The working principle of thermistors is less complex and can easily understand. The principle is just that their resistance is dependent on their temperature. An ohmmeter is used to measure the resistance of a thermistor. Knowing the exact relationship between how changes in the temperature will affect the resistance of the thermistor. So, by measuring the thermistor’s resistance, its temperature can be derived. The amount to which the resistance changes depends on the type of material used in the thermistor. The relationship between a thermistor’s temperature and resistance is non-linear.

Watch the video below to learn about the working of thermistors:

Advantages and disadvantages of thermistors

Advantages:

Below are the advantages of thermistors in their various applications:

  • Durability
  • Highly sensitive
  • Portability
  • Lower cost
  • Last longer
  • Linear output
  • Best for measuring range of temperature
  • Best for measuring single point temperature
  • Widest operating temperature range

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Disadvantages:

Despite the good advantages of thermistors, some limitations still occur. Below are the disadvantages of thermistors in their various applications:

  • Non-linear output
  • Limited temperature range
  • Slow response time
  • Some have low sensitivity

Conclusion

Thermistors are resistance thermometers or a resistor whose resistance is dependent on temperature. The word is derived from the terms thermal and resistor, and Their construction is made of metallic oxides, pressed into a bead, and disk. That is all for this article, where the definition, applications, symbol, specification, types, history, structure and composition, working, advantages and disadvantages of thermistors are being discussed.

I hope you get a lot from the reading, if so, kindly share with other students. Thanks for reading, see you next time!

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