Types Of Temperature Sensors Used In Circuit With Explanation

November 29, 2022November 29, 2022 CS Electrical And Electronics

Hello guys, welcome back to our blog. Here in this article, we will discuss the types of temperature sensors used in hardware with an explanation, of which type of temperature sensor is best, and their applications.

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Types Of Temperature Sensors Used In Circuit

Every day, we use temperature sensors to regulate the temperature of buildings, and the temperature of the water, and to operate refrigerators. Additional uses for temperature sensors include consumer, medical, and industrial electronics.

Different applications could require different types of temperature sensors. The variables that differ are the substance being measured (air, mass, or liquid), the environment (inside or outside), and the temperature range. Thermocouples, RTDs (resistance temperature detectors), thermistors, and semiconductor-based integrated circuits are the four temperature sensors most frequently employed in modern electronics (IC).

Due to its importance for materials and processes at the molecular level, the temperature is the most extensively measured physical characteristic. Temperature is the intensity of heat or cold as measured against a predetermined scale. The quantity of heat energy present in a system or an object is another definition of temperature. Molecular energy and heat energy are directly correlated; molecular energy increases as heat energy decreases.

The changes that occur in materials or objects when their temperature changes are observed using temperature sensors. A change in a physical quantity that correlates to a change in temperature can be detected by temperature sensors. Any physical quantity, like voltage or resistance, might be the physical quantity. Sensors that convert electrical energy to thermal energy rely on the conductor-heating effect of a current. Sensors that convert thermal energy into electrical energy need a temperature differential to work.

01. Thermocouples

The most popular kind of temperature sensor is a thermocouple. They are employed in commercial, transportation, and residential purposes. Thermocouples offer short response times, can function over a wide temperature range, and are self-powered.

Thermocouples are created by fusing two metal wires of different compositions. A Seebeck Effect results from this. The Seebeck Effect is a phenomenon where a variation in temperature between two different conductors results in a voltage differential between the two materials. The temperature can be determined by measuring and using this voltage differential.

Different temperature ranges and sensitivities are made possible by the variety of thermocouple types and materials used in their manufacture. The allocated letters serve to distinguish the various varieties. The K type is the most popular.

Code Type	Conductor Alloys	Sensing Temperature
B	Platinum Rhodium	0 – 1820 degree C
R/S	Copper / Copper Nickel Compensating	-50 to 1750 degree C
T	Copper / Constantan	-250 to 400 degree C
N	Nicrosil / Nisil	-270 to 1300 degree C
K	Nickel Chromium / Nickel Aluminium	-180 to 1300 degree C
J	Constantan / Iron	-180 to 800 degree C
E	Constantan / Nickel Chromium	-40 to 900 degree C

The small output voltage of thermocouples, which necessitates precise amplification, along with their susceptibility to outside noise over long wires, cold junctions, and difficulty in measuring temperature are some of their drawbacks. The signal circuitry’s copper traces and thermocouple wires come together at a cold junction. Another Seebeck Effect is produced as a result, and this one requires cold junction adjustment.

The MAX31855 and MAX31856 digital output thermocouples are available from Maxim Integrated. By including a high-resolution analog-to-digital converter (ADC), low noise precision gain stage, and cold junction compensation sensor, these devices aid in signal conditioning. These tools provide precise signal conditioning solutions in a compact form to designers of thermocouple circuits. Many of the common thermocouple types are compatible with them.

02. Thermistors

Thermistors and RTDs both experience quantifiable resistance changes in response to variations in temperature. The materials used to create thermistors are typically polymers or ceramics. Thermistors often cost less than RTDs but are less precise. The majority of thermistors come in two-wire layouts.

The most popular thermistor for use in temperature measurement applications is the NTC (Negative Temperature Coefficient) thermistor. As the temperature rises, the resistance of an NTC thermistor lowers. The relationship between temperature resistance and thermistors is nonlinear. To read the data accurately, this needs considerable revision. An ADC converts the output of a voltage divider formed by a thermistor and a fixed-value resistor, which is a common method of using a thermistor.

03. RTD (Resistance Temperature Detector)

Any metal changes in resistance as a result of temperature fluctuations. RTD temperature sensors are based on this variation in resistance. A resistor with clearly defined resistance versus temperature properties is known as an RTD. The most popular and precise material used to create RTDs is platinum.

PRTDs are another name for platinum RTDs. They are frequently offered with a 100 and 1000 resistance at zero degrees. PT100 and PT1000 are the names given to them, respectively.

Because they provide a nearly linear reaction to temperature changes, are accurate and stable, produce reproducible responses, and have a wide temperature range, platinum RTDs are utilized. Because of their precision and reproducibility, RTDs are frequently utilized in precision applications.

Since RTD elements typically have higher thermal masses than thermocouples, they react to temperature changes more slowly. In RTDs, signal conditioning is crucial. Additionally, an excitation current must pass through the RTD. Calculating the resistance is possible if we are aware of the current.

**RTD (Resistance Temperature Detector)**

Two, three, and four-wire possibilities are available in configurations. When the lead length is so short that resistance doesn’t significantly impair measurement accuracy, the two-wire alternative is advantageous. An RTD probe that carries the excitation current is added using a three-wire. This makes it possible to eliminate wire resistance.

The four-wire method is the most accurate since the influence of wire resistance is eliminated by separate force and sense lines. Examples of two, three, and four-wire RTD designs are shown in the above image. The MAX31865 provides a dedicated RTD signal conditioning circuit with a 15-bit resolution for each configuration, providing a way to speed up designs for both PT100 and PT1000 RTDs.

04. Semiconductor-based ICs

Local temperature sensors and distant digital temperature sensors are the two different types of semiconductor-based temperature sensor ICs. Local temperature sensors are integrated circuits (ICs) that gauge their own die temperature using a transistor’s inherent characteristics. An external transistor’s temperature is measured by distant digital temperature sensors.

Analog or digital outputs can be used by local temperature sensors. While digital outputs come in a variety of formats, including I2C, SMBus, 1-Wire®, and Serial Peripheral Interface, analog outputs can only be either voltage or current (SPI). Local temperature sensors measure the temperature of the surrounding air or printed circuit boards. The MAX31875 is a remarkably tiny local temperature sensor that can be utilized in a variety of battery-operated and other applications.

Using a transistor’s physical characteristics, remote digital temperature sensors function similarly to local temperature sensors. The transistor is separate from the sensor chip, which is the difference. A bipolar sensing transistor is sometimes included in microprocessors and FPGAs to measure the target IC’s die temperature.

Conclusion

The most common types of temperature sensors used today are thermocouples, RTDs, thermistors, and semiconductor-based integrated circuits. Thermocouples can measure a wide range of temperatures, are affordable, and are robust. RTDs give reliable and reproducible temperature measurements (although they are smaller than thermocouples), but they are slower and need an excitation current and signal conditioning.

Thermistors are tiny and robust but less accurate than RTDs and require more data corrections to interpret temperature than RTDs do. Although semiconductor-based ICs are adaptable to implantation and may be packaged in incredibly small spaces, their operating temperature range is constrained.

Despite the fact that there are additional temperature sensor options, the four options discussed in this blog will enable the majority of designers to find a solution that will be effective for their application.

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