What is a Thermistor?

A thermistor (also known as a thermal resistor) is a type of resistor whose electrical resistance changes as the temperature changes. A thermistor is particularly sensitive to temperature changes even though the resistance of all resistors will fluctuate slightly with temperature. Thermistors function as a circuit’s passive element. They are a reliable, economical, and accurate method of measuring temperature. The preferred sensor for many different applications, thermistors do not perform well in extremely hot or cold temperatures. When a precise temperature reading is needed, thermometers are the best option. The thermistor symbol is displayed as follows:

Uses of Thermistors

There are many uses for thermometers. They are frequently used as thermistor thermometers to measure temperature in a variety of liquid and ambient air environments. Thermistors are frequently used for the following purposes:

• Automotive applications (to gauge the temperatures of the coolant and oil in cars and trucks)
• Digital thermometers (thermostats)
• Circuit protection (i.e. surge protection)
• Household appliances (like microwaves, fridges, and ovens)
• To determine electrical materials’ thermal conductivity
• Rechargeable batteries (ensure the correct battery temperature is maintained)
• Temperature adjustment (maintaining resistance to account for effects brought on by variations in temperature in another area of the circuit)
• Useful in many basic electronic circuits (e.g. as part of a beginner Arduino starter kit)
• Used in wheatstone bridge circuits

Thermistor Working Principle

The resistance of a thermistor is temperature-dependent, which is how it functions. An ohmmeter can be used to determine a thermistor’s resistance. The temperature of a thermistor can be determined by measuring its resistance if we are aware of the precise relationship between changes in temperature and the thermistor’s resistance. The thermistor’s material composition determines how much the resistance varies. The temperature and resistance of a thermistor have a non-linear relationship. Below is a typical thermistor graph:

We could easily align the resistance measured by the ohmmeter with the temperature shown on the graph if we had a thermistor with the aforementioned temperature graph. We can therefore determine the temperature of the thermistor by drawing a horizontal line across from the resistance on the y-axis and a vertical line downward from where this horizontal line intersects the graph.

Types of thermistor

There are two types of thermistors:

• Negative Temperature Coefficient (NTC) Thermistor
• Positive Temperature Coefficient (PTC) Thermistor

NTC Thermistor

When the temperature rises, resistance falls in an NTC thermistor. Resistance also rises as temperature drops. Therefore, the relationship between temperature and resistance in an NTC thermistor is inverse. These thermistor types are the most prevalent.

The following formula governs the relationship between resistance and temperature in an NTC thermistor:

Where:

• R₀ is the resistance at temperature T₀ (K)
• R_T is the resistance at temperature T (K)
• β is a constant, its value is dependant on the characteristics of the material. The nominal value is taken as 4000.
• T₀ is the reference temperature (normally 25^oC)

The resistor-temperature relationship will be excellent if the value of β is high. As a result, you have increased the sensitivity (and consequently accuracy) of the thermistor. A higher value of β indicates a higher variation in resistance for the same rise in temperature.

From the expression (1), we can obtain the resistance temperature co-efficient. This is nothing but the expression for the sensitivity of the thermistor.

Above we can clearly see that the αT has a negative sign. This negative sign indicates the negative resistance-temperature characteristics of the NTC thermistor.

If β = 4000 K and T = 298 K, then the α_T = –0.0045/^oK. This is much higher than the sensitivity of platinum RTD. This would be able to measure the very small changes in the temperature.

Alternative, highly doped thermistors with a positive temperature coefficient are now commercially available (at a high cost). The thermistors are unquestionably a non-linear sensor because the expression (1) prevents even a small temperature range from being approximated linearly by the curve.

PTC Thermistor

The relationship between temperature and resistance in a PTC thermistor is the opposite. The resistance rises as the temperature rises. Resistance also drops off as temperature rises. As a result, resistance and temperature are inversely proportional in a PTC thermistor. PTC thermistors are frequently used as a form of circuit protection even though they are not as common as NTC thermistors. PTC thermistors can serve as a current-limiting device, much like fuses do.

Whenever current flows through a device, a small amount of resistive heating results. The device heats up if the current is sufficient to produce more heat than it can dissipate to the environment. A PTC thermistor’s resistance will rise as a result of this heating. As a result, the resistance rises and limits the current through a self-reinforcing effect. It safeguards the circuit by serving as a current-limiting device in this way.

Thermistors Characteristics

The following is the relationship that determines a thermistor’s characteristics:

Where:

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

It is clear from the equation above that resistance and temperature have a very nonlinear relationship. The usual negative thermal resistance temperature coefficient of an NTC thermistor is 0.05/^oC.

Thermistors Construction

A slurry is created by combining two or more semiconductor powders made of metallic oxides with a binder to create a thermistor. This slurry condenses into tiny drops that cover the lead wires. We must place it in a sintering furnace to dry it out. The slurry will shrink onto the lead wires during this process, creating an electrical connection. This refined metallic oxide is sealed by being covered in a glass coating. The thermistors are given a waterproof quality by this glass coating, which contributes to an increase in their stability.

The market offers a variety of thermistors in various sizes and shapes. Beads with a diameter of 0.15 to 1.5 millimeters are used as smaller thermistors. Thermistor material can also be pressed under high pressure into flat cylindrical shapes with a diameter ranging from 3 millimeters to 25 millimeters to create disks and washers.

A thermistor typically ranges in size from 0.125 mm to 1.5 mm. Thermistors that are offered for sale have nominal temperatures of 1K, 2K, 10K, 20K, 100K, etc. This number represents the resistance at a 25 °C temperature. Thermistors come in a variety of models, including bead, rod, disc, and others. The main benefits of thermistors are their compact size and affordable price. Although the size reduction also reduces its ability to dissipate heat and increases the self-heating effect, this size advantage means that the time constant of thermistors operated in sheaths is small. The thermistor may sustain long-term damage from this effect. Thermistors must be operated at low electric current levels in comparison to resistance thermometers to prevent this, which lowers measurement sensitivity.

Thermistor vs Thermocouple

The following are the main distinctions between a thermistor and a thermocouple:

Thermistors:

• A more narrow range of sensing (55 to +150^oC – although this varies depending on the brand)
• Nonlinear relationship between the sensing parameter (resistance) and temperature
• Sensing parameter = Resistance
• Good for sensing small changes in temperature (it’s hard to use a thermistor accurately and with high resolution over more than a 50^oC range).
• The sensing circuit is simple and doesn’t need amplification & is very simple
• NTC thermistors have a roughly exponential decrease in resistance with increasing temperature
• Accuracy is usually hard to get better than 1^oC without calibration

Thermocouples:

• Have a wide range of temperature sensing (Type T = -200-350^oC; Type J = 95-760°C; Type K = 95-1260°C; other types go to even higher temperatures)
• Can be very accurate
• Thermocouple voltage is relatively low
• Sensing parameter = voltage generated by junctions at different temperatures
• Have a linear relationship between the sensing parameter (voltage) and temperature

Thermistor vs RTD

Thermistors and resistance temperature detectors (RTDs) are closely related. RTDs and thermistors both have variable resistance that varies with temperature.

The kind of material they are made of is the primary distinction between the two. RTDs are typically made of pure metals, whereas thermocouples are frequently made of ceramic or polymer materials. Thermistors perform better than other devices in almost every way.

Compared to RTDs, thermometers are more precise, less expensive, and have quicker response times. The temperature range is the only real area where a thermistor falls short of an RTD. Compared to thermistors, RTDs can measure temperature over a wider range.

Other than this, there is no benefit to using a thermistor instead of an RTD.