What is a Thermistor?

What is a Thermistor? . Thermistors are a type of semiconductor that react like a resistor sensitive to temperature – meaning they have greater resistance than conducting materials, but lower resistance than insulating materials. To establish a temperature measurement, the measured value of a thermistor’s electrical resistance can be correlated to the temperature of the environment in which that thermistor has been situated.

The term ‘thermistor’ is a portmanteau – it is derived from the term Thermally sensitive Resistor – and these devices are a very accurate and cost-effective option for temperature measurement.
The reasons that thermistors continue to be popular for measuring temperature is: – Their higher resistance change per degree of temperature provides greater resolution – High level of repeatability and stability (±0.1

The thermistor symbols are:

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

Thermistors are easy to use, inexpensive, sturdy, and respond predictably to changes in temperature. While they do not work well with excessively hot or cold temperatures, they are the sensor of choice for applications that measure temperature at a desired base point. They are ideal when very precise temperatures are required.

Some of the most common uses of thermistors are in digital thermometers, in cars to measure oil and coolant temperatures, and in household appliances such as ovens and refrigerators, but they are also found in almost any application that requires heating or cooling protection circuits for safe operation. For more sophisticated applications, such as laser stabilization detectors, optical blocks, and charge coupled devices, the thermistor is built in. For example, a 10 kΩ thermistor is the standard that is built into laser packages.

What Are Thermistors Composed Of ?

The relationship between a thermistor’s temperature and its resistance is highly dependent upon the materials from which it is composed. Thermistor manufacturers typically determine this property with a high degree of accuracy – as this is the primary characteristic of interest to thermistor buyers.

Thermistors are made up of metallic oxides, binders, and stabilizers pressed into wafers and then cut to chip size, left in disc form, or made into another shape. The precise ratio of the composite materials governs their resistance/temperature “curve”. Manufacturers typically control this ratio with great accuracy, as it determines how the thermistor will function.

Available Thermistor Configurations

Thermistors are available in several common configurations. The three most frequently employed are the hermetically sealed flexible thermistor (HSTH series), the bolt-on/washer type, and the self-adhesive surface-mount style.

HSTH Thermistors are completely sealed within PFA (plastic polymer) jackets to protect the sensing element from moisture and corrosion. They can be used to measure the temperature of an array of liquids ranging from oils and industrial chemicals to foods.

Thermistors with bolt- or washer-mounted sensors can be installed into standard-sized threaded holes or openings. Their small thermal mass enables them to respond to temperature changes rapidly. They’re used in many applications including household appliances, water tanks, pipes, and equipment casings.

Surface-mounted thermistors come with adhesive exteriors that can easily be stuck in place on flat or curved surfaces. They can be removed and re-applied and have several commercial and industrial applications.

Temperature Range, Accuracy and Stability

Thermistors are highly accurate (ranging from ± 0.05°C to ± 1.5°C), but only over a limited temperature range that is within about 50°C of a base temperature. The working temperature range for most thermistors is between 0°C and 100°C. Class A thermistors offer the greatest accuracy, while Class B thermistors can be used in scenarios where there’s less need for exact measurement. Once the manufacturing process is complete, thermistors are chemically stable and their accuracy does not change significantly with age.

How does the thermistor “read” temperature?

A thermistor does not actually “read” anything, instead the resistance of a thermistor changes with temperature. How much the resistance changes depends on the type of material used in the thermistor.

Unlike other sensors, thermistors are nonlinear, meaning the points on a graph representing the relationship between resistance and temperature will not form a straight line. The location of the line and how much it changes is determined by the construction of the thermistor. A typical thermistor graph looks like this:

How Are Thermistors Wired?

Thermistors are very simple to wire. Most come with two-wire connectors. The same two wires that connect the thermistor to its excitation source can be used to measure the voltage across the thermistor.

Types of Thermistors

There are two types of thermistors. NTC or Negative Temperature Coefficient thermistors, and PTC or Positive Temperature Coefficient thermistors. The difference is that NTC thermistors exhibit a DECREASE in resistance as body temperature increases, while PTC thermistors exhibit an INCREASE in resistance as body temperature increases.

To understand the basic difference between PTC and NTC thermistors we can use this linear equation for the relation between the change in temperature and resistance.

dR = k dT

Where

dR = Change in resistance
k = Temperature Coefficient
dT = Change in Temperature

The temperature coefficient can be either positive or negative and it completely changes the electrical property of the component. A thermistor having a positive coefficient is called PTC while a negative coefficient thermistor is called NTC.

PTC Thermistor

PTC or Positive Temperature Coefficient thermistor is a type of thermistor whose resistance is directly proportional to the surrounding temperature. Its resistance increases with an increase in the temperature and decreases with a decrease in the temperature.

It is made of polycrystalline ceramic material. Its resistance increases non-linearly in a curve. The increase in resistance is very small at low temperatures which increases rapidly when the temperature reaches above the switching point (T_R).

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The given symbol represents a PTC thermistor with (T⁺) to show the thermistor has a positive temperature coefficient.

Characteristic Curve

The characteristic curve of a thermistor shows the relation between the resistance (dependent – displayed on Y-axis) and the temperature (Independent – displayed on X-axis). Here is the characteristics curve of a PTC thermistor.

Thermistors have rated resistance taken at 25° C. therefore, any PTC thermistors having a rated resistance is its resistance at 25° C.

Types of PTC Thermistor

There are two types of PTC thermistors

Silistors
The silistor is a PTC thermistor made from silicon having the linear characteristic. The resistance increases linearly with the increase in the temperature.

Switching Type PTC

The switching type PTC is a non-linear thermistor. It is made of a poly-crystalline ceramic body. It has a small NTC region where the resistance decreases a little with an increase in temperature up to a certain point called critical temperature or the curie temperature. After that point, the resistance increases exponentially with a slight increase in temperature.

Advantages of PTC

Here are some advantages of PTC

It provides better protection against overloading
It can efficiently and safely start an electrical motor.
Its resistance varies linearly with temperature.
It is compact in size.
It is cheaper

Disadvantages of PTC

Here are some disadvantages of PTC .

They are not very sensitive to as compared NTC.
Its reading is affected by the self-heating effect.
It has a limited temperature range as compared to the NTC.

PTC Applications

Here are some applications of PTC .

Over-Current Protection: When the current exceeds a certain limit, it generates heat that can damage the wiring as well as the components. The resistance of the PTC increases with the temperature that can be used to prevent the current from increasing.
Inrush Current Protection: Inrush current is the starting current drawn by a motor during its startup that is very high and it can damage its windings. A PTC heats up and its resistance increases that limits the inrush current.
Motor Starting: Some motors include an auxiliary startup winding that is only used for starting the motor. Initially, the PTC resistance is very low and it allows the current to pass through this auxiliary winding. The temperature gradually increases as well as the PTC resistance. Once the motor attains certain speed, the PTC resistance increases to a point that blocks the current flow to the startup windings.
Time Delay: The PTC can switch from a low resistance state to a high resistance state after some time delay. The time can depend on the change in temperature or the voltage in use. It can be used in circuits to achieve a calculated time delay function such as in circuit breakers, timers, relays.
Temperature Control: The PTC can control a heater to maintain a temperature from raising a certain limit. The PTC breaks the supply when the temperature exceeds certain limits and switches back on when the temperature falls below a certain limit.
Electrical Fuse: The resistance can also increase with an increase in its internal temperature due to the current flowing through it. It can act as a fuse to stop the current flow in case of over current.

NTC

As its name suggests, NTC or Negative temperature coefficient thermistor has a negative coefficient k. therefore, its resistance varies inversely with the temperature. The resistance decreases with an increase in temperature and vice versa. Here is the symbol of the NTC thermistor.

NTC are made of oxides of nickel, cobalt, copper, iron and titanium. They usually operate at a very large range of temperature with very precise temperature monitoring.

They are mostly used in temperature sensing devices as opposed to PTC that are mostly used as current limiters.

Characteristics Curve

The following characteristic curve shows the comparison between the resistance and temperature of the NTC thermistor. The resistance varies non-linearly with the temperature change in a form of a curve.

NTC-Thermistor-Characteristic-Curve — NTC-Characteristic-Curve

The given graph shows a drastic change in the resistance from a few ohms to mega ohms with a change in temperature thus offering precise temperature sensing and high sensitivity.

Advantages of NTC

Here are some advantages of NTC thermistor

They are far more sensitive than PTC.
It operates at a wider range of temperatures.
It has a quicker response time with high accuracy.
They provide a precise temperature reading.
They are compact and take less space on a circuit board.

Disadvantages of NTC Thermistor

Here are some advantages of NTC

The self-heating effect can cause errors in temperature measurement.
The resistance varies non-linearly with the temperature.

NTC Applications

The NTC is mainly used for measuring the temperature. Here are some of the applications of NTC thermistors:

Digital Thermometer: NTC resistance significantly changes with a small change in temperature and it can sense a wide range of temperatures with high accuracy. This is why NTC is used in a digital thermometer.
Temperature Monitoring and Control: the operation of electronic components especially semiconductor based components greatly depends on the ambient temperature. The NTC are used for monitoring and maintaining temperature to ensure uninterrupted operation of the equipment.
Fire Alarm: Fire alarms are used in every building to detect the first sign of fire and alert the personnel inside the building as well as the concerned authorities. NTC are used in fire alarms to sense the temperature.
Inrush current protection: The inrush current is a very high motor starting current. The NTC thermistor offers high initial resistance that limits the starting current. it is far more effective than using a fixed resistor for limiting the inrush current.

Types of Thermistor based on Material

The thermistor can be shaped into three different types. Therefore, they are divided into the following types:

Bead
Disc and Chip Style
Cylindrical
Metalized surface

Bead Thermistor

As the name suggests, bead are manufactured in the shape of a bead. It is made of connecting the wire directly to the ceramic body. They offer better stability with a quick response time. Their structure allows it to operate at very high temperatures. To further protect it from mechanical damage, they are encapsulated in glass. They are the smallest in size which is why they have the quickest response time. But they have low current handling capabilities.

Disc and Chip Style Thermistors

The body of such a thermistor is shaped in the form of a disc or chip. It has a larger metal surface. Due to its larger surface, they have a slower response time and have higher current handling capabilities than the bead type.

Cylindrical Thermistor

Such thermistor’s body is pressed into a cylindrical shape. They have a larger size as compared to other types. They are robust and reliable.

Metalized Surface Thermistor

Such thermistors have metalized surface contacts instead of sintered lead wire as in the other mentioned types. They have radial or axial metal contacts that are used for directly connected or surface-mounting on a circuit board.

The thermistors are also divided based on the materials used. They improve their performance, durability and stability.

Glass Encapsulated Thermistors

The thermistors are sealed in a glass body to improve their operating temperature range. It is an air-tight glass body that improves its stability and protects it from mechanical damage. Glass encapsulated thermistors can operate at above 150° C.

PAN Thermistor

The PAN is made from a special type of metal oxide that is extremely sensitive to temperature. It has very high accuracy with a tolerance up to ±0.2°C. It has a very quick response time with great precision.

It is a type of NTC used in industries for its accurate measurement. It has an operating range of 25° to 85° C.

Precision Interchangeable Thermistors

They are the most precise thermistor manufactured based on a specific characteristics curve. They are fast and have long-term stability with high-temperature accuracy. They offer interchangeability over the range of 0°C to 70°C. They do not require individual calibration.

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Advantages and Disadvantages

Advantages

Here are some advantages

It has a quick response time
It has a higher degree of accuracy and high precision.
They operate at a wide range of temperatures especially NTC.
They are far more stable in long-term use.
It can be designed to withstand any mechanical stress
It can be designed in any shape.
It has higher sensitivity than other temperature sensors.
It has a smaller size.
They have robust designs.
It is cheaper than other sensors.
It can be used in remote locations.

Disadvantages

Here are some disadvantages of thermistor

Most thermistors have a limited temperature range especially precision thermistors having high accuracy.
The resistance varies non-linearly with respect to temperature.
Due to the self-heating effect, error may get induced in the reading.
They are fragile

Applications

Here are some applications of thermistor

The main function of a thermistor is to be used as a temperature sensor.
They are used in digital thermometers.
Thermistors are used to control and maintain the temperature of a room for domestic and office use.
They are used in automobiles to monitor the temperature.
They are used for protection against overcurrent in electrical circuits and also act as a fuse.
The Inrush thermistors are used for the safe starting of electrical motor.
They also provide time delays in an electrical circuits.

General Selection Considerations

Whether installing a new system or just replacing a device in an existing system, you should consider these key points before you make your selection to ensure the desired outcome.

Base Resistance: If you’re installing a new application, be sure to select the right base resistance based on your application requirements. If you are replacing a thermistor, be sure to match the current base resistance.
Resistance vs. Temperature Curve: If you are installing a new application, determine the correct resistance vs. temperature curve relationship. If you are replacing a device, be sure to match the information from the existing thermistor.
Thermistor Packaging: Make sure the selected packaging accommodates your application requirements.

How does a thermistor work in a controlled system?

The main use of a thermistor is to measure the temperature of a device. In a temperature controlled system, the thermistor is a small but important piece of a larger system. A temperature controller monitors the temperature of the thermistor. It then tells a heater or cooler when to turn on or off to maintain the temperature of the sensor.

In the diagram below, illustrating an example system, there are three main components used to regulate the temperature of a device: the temperature sensor, the temperature controller, and the Peltier device (labeled here as a TEC, or thermoelectric cooler). The sensor head is attached to the cooling plate that needs to maintain a specific temperature to cool the device, and the wires are attached to the temperature controller. The temperature controller is also electronically connected to the Peltier device, which heats and cools the target device. The heatsink is attached to the Peltier device to help with heat dissipation.

thermistor control — Thermistor Controlled System

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The job of the temperature sensor is to send the temperature feedback to the temperature controller. The sensor has a small amount of current running through it, called bias current, which is sent by the temperature controller. The controller can’t read resistance, so it must convert resistance changes to voltage changes by using a current source to apply a bias current across the thermistor to produce a control voltage.

The temperature controller is the brains of this operation. It takes the sensor information, compares it to what the unit to be cooled needs (called the setpoint), and adjusts the current through the Peltier device to change the temperature to match the setpoint.

The location of the thermistor in the system affects both the stability and the accuracy of the control system. For best stability, the thermistor needs to be placed as close to the thermoelectric or resistive heater as possible. For best accuracy, the thermistor needs to be located close to the device requiring temperature control. Ideally, the thermistor is embedded in the device, but it can also be attached using thermally conductive paste or glue. Even if a device is embedded, air gaps should be eliminated using thermal paste or glue.

The figure below shows two thermistors, one attached directly to the device and one remote, or distant from the device. If the sensor is too far away from the device, thermal lag time significantly reduces the accuracy of the temperature measurement, while placing the thermistor too far from the Peltier device reduces the stability.

In the following figure, the graph illustrates the difference in temperature readings taken by both thermistors. The thermistor attached to the device reacted quickly to the change in thermal load and recorded accurate temperatures. The remote thermistor also reacted but not quite as quickly. More importantly, the readings are off by a little more than half a degree. This difference can be very significant when accurate temperatures are required.

thermistor location reaction — Figure 6: Thermistor Location Response Graph

Once the placement of the sensor has been chosen, then the rest of the system needs to be configured. This includes determining the base thermistor resistance, the bias current for the sensor, and the setpoint temperature of the load on the temperature controller.

Which Thermistor resistance and bias current should be used?

Thermistors are categorized by the amount of resistance measured at ambient room temperature, which is considered 25°C. The device whose temperature needs to be maintained has certain technical specifications for optimum use, as determined by the manufacturer. These must be identified before selecting a sensor. Therefore, it is important to know the following:

What are the maximum and minimum temperatures for the device?
Thermistors are ideal when measuring a single point temperature that is within 50°C of ambient. If the temperatures are excessively high or low, a thermistor will not work. While there are exceptions, most thermistors work best in the range between -55°C and +114°C.

Since thermistors are nonlinear, meaning the temperature to resistance values plot on a graph as a curve rather than a straight line, very high or very low temperatures do not get recorded correctly. For example, very small changes in very high temperatures will record negligible resistance changes, which won’t translate into accurate voltage changes.

What is the optimum thermistor range?
Depending on the bias current from the controller, each thermistor has an optimum useful range, meaning the temperature range where small changes in temperature are accurately recorded.

The table below shows the most effective temperature ranges for Wavelength thermistors at the two most common bias currents.

It is best to choose a thermistor where the setpoint temperature is in the middle of the range. The sensitivity of the thermistor is dependent on the temperature. For example, a thermistor may be more sensitive at cooler temperatures than at warmer temperatures, as is the case with Wavelength’s TCS10K5 10 kΩ thermistor. With the TCS10K5, the sensitivity is 162 mV per degree Celsius between 0°C and 1°C, and it is 43 mV / °C between 25°C and 26°C, and 14 mV °C between 49°C and 50°C.

What are the upper and lower voltage limits of the sensor input of the temperature controller?
The voltage limits of the sensor feedback to a temperature controller are specified by the manufacturer. The ideal is to select a thermistor and bias current combination that produces a voltage inside the range allowed by the temperature controller.

Voltage is related to resistance by Ohm’s Law. This equation is used to determine what bias current is needed. Ohm’s Law states that the current through a conductor between two points is directly proportional to the potential difference across the two points and, for this bias current, is written as:

V = I_BIAS x R

Where:
V is voltage, in Volts (V)
I_BIAS is the current, in Amperes or Amps (A)
I_BIAS means the current is fixed
R is resistance, in Ohms (Ω)

The controller produces a bias current to convert the thermistor resistance to a measurable voltage. The controller will only accept a certain range of voltage. For example, if a controller range is 0 to 5 V, the thermistor voltage needs to be no lower than 0.25 V so that low end electrical noise does not interfere with the reading, and not higher than 5 V in order to be read.

Assume the use of the above controller and a 100 kΩ thermistor, such as Wavelength’s TCS651, and the temperature the device needs to maintain is 20°C. According to the TCS651 datasheet, the resistance is 126700 Ω at 20°C. To determine if the thermistor can work with the controller, we need to know the usable range of bias currents. Using Ohm’s Law to solve for I_BIAS, we know the following:

V / R = I_BIAS

0.25 / 126700 = 2 µA is the lowest end of the range
5.0 / 126700 = 39.5 µA is the highest end

Yes, this will work, if the temperature controller bias current can be set between 2 µA and 39.5 µA.

When selecting a thermistor and bias current, it is best to choose one where the voltage developed is in the middle of the range. The controller feedback input needs to be in voltage, which is derived from the thermistor resistance.

Since people relate to temperature most easily, the resistance often needs to be changed to temperature. The most accurate model used to convert thermistor resistance to temperature is called the Steinhart-Hart equation.

What is the Steinhart-Hart Equation?

The Steinhart-Hart equation is a model that was developed at a time when computers were not ubiquitous and most mathematical calculations were done using slide rules and other mathematical aids, such as transcendental function tables. The equation was developed as a simple method for modeling thermistor temperatures easily and more precisely.

The Steinhart-Hart equation is:

1/T = A + B(lnR) + C(lnR)2 + D(lnR)3 + E(lnR)4…

Where:
T is temperature, in Kelvins (K, Kelvin = Celsius + 273.15)
R is resistance at T, in Ohms (Ω)
A, B, C, D, and E are the Steinhart-Hart coefficients that vary depending on the type of thermistor used and the range of temperature being detected.
ln is Natural Log, or Log to the Napierian base 2.71828

The terms can go on infinitely but, because the error is so small, the equation is truncated after the cubed term and the squared term is eliminated, so the standard Steinhart-Hart equation used is this:

1/T = A + B(lnR) + C(lnR)3

One of the pleasures of computer programs is that equations that would have taken days, if not weeks, to solve are done in moments. Type “Steinhart-Hart equation calculator” in any search engine and pages of links to online calculators are returned.

How is the Steinhart-Hart Equation used?

This equation calculates with greater precision the actual resistance of a thermistor as a function of temperature. The more narrow the temperature range, the more accurate the resistance calculation will be. Most manufacturers provide the A, B, and C coefficients for a typical temperature range.

Who are Steinhart and Hart?

John S. Steinhart and Stanley R. Hart first developed and published the Steinhart-Hart equation in a paper called “Calibration curves for thermistors” in 1968, when they were researchers at Carnegie Institution of Washington. Steinhart went on to become Professor of Geology and Geophysics, and Marine Studies at the University of Wisconsin-Madison and Stanley R. Hart became a Senior Scientist at Woods Hole Oceanographic Institution.

FAQs

What is the difference between thermistors and RTDs?
are highly sensitive and operate in a narrower range, while RTDs (Resistance Temperature Detectors) offer higher accuracy over a broader range.

How are thermistors calibrated?
Calibration involves comparing the readings to a known temperature standard and adjusting accordingly.

Are thermistors interchangeable between devices?
Not always. Ensure compatibility based on specifications like resistance range and sensitivity.