Types of sensors in automatic control (PT4): Thermal sensors

The thermal sensor is a device that absorbs heat from the environment, depending on the sensor’s structure that converts this amount of heat into a certain amount of electricity. So a very important factor is the “temperature of the environment to measure” and “sensing temperature of the sensor”. Specifically this is: The type of sensor you see is a protective shell, the sensor element inside the shell (semiconductor, bimetallic …) so the measurement is accurate or It does not depend on how much heat is transferred from the environment to the sensor element (one of the determinants of the price of the thermal sensor).

How to classify thermal sensors

The principle of operation of the thermal sensor is to enhance the heat exchange between the sensor and the environment under which the temperature is to be measured.

On this basis, we classify heat sensors based on their operating principle, effect and function into the following categories:

– Thermocouple.

– RTD-resitance temperature detector.

– Thermistor.

– Diode, IC.

There is also a non-contact thermometer (Pyrometer). Use infrared or laser.

The design and application of various types of thermal sensors


– Structure: Including two different metal materials, one stick welding.

– Principle: Temperature change for changing electromotive force (mV).

Advantages: Durable, high temperature measurement.

– Disadvantages: Many factors influence the wrong number. Sensitivity is not high.

– Frequent use: Heat oven, harsh environment, compressor heat meter, …

– Range: -100 D.C <1400 D.

– Consists of two different metal wires attached to one end called a hot head (or head), the other two ends called cold heads (or standard ends). When there is a difference in temperature between the hot head and the cold head, there is a V charge at the cold end. One problem is to stabilize and measure the temperature at the cold end, which depends on the material. So, let’s create a pair of temperature pairs, each of which gives a different electromotive force: E, J, K, R, S, T. Note this to select transducers and controllers for suitable.

– The thermocouple thermistor wire is not long to connect to the controller, the factor that leads to inaccuracies is this, to solve this we have to compensate for it (offset on the controller ).

Note when used:

– From the above factors when using this type of sensor we note that should not connect more wires (because the signal that the mV connector will attenuate a lot). The wire of the sensor should be ventilated (do not let the cord cling to the measuring medium). Finally, it is a good idea to carefully check the device offset.

– Note: Since the signal is the voltage (negative and positive), so note the symbol to install into the amplifier properly.


– The construction of the RTD thermal sensor consists of wire made of: Copper, Nickel, Platinum, … are wrapped according to the shape of the probe. When the temperature changes, the resistance between the two ends of the wire will change, and depending on the material, the metal will have a linearity within a certain temperature range. The most common RTD type is the Pt sensor, From Platinum. Platinum has high resistivity, anti-oxidation, high sensitivity, long range of measured temperature. There are usually types: 100, 200, 500, 1000 ohm at 0 D.C. The higher the resistance, the higher the thermal sensitivity.

– RTDs usually have 2 wires, 3 wires and 4 wires.

Thermal sensor
RTD thermocouple sensor
Note when used:

– 4-wire type RTD reduces lead resistance by 1/2, helping to limit errors.

– Usage of RTD is quite pleasant compared to Thermocouple. We can connect wires to this type of sensor (welded, wire quality, anti-interference) and can be tested by VOM.

– Because of the variation of the resistor, we do not care about the wiring.


– Structure: Made from a mixture of metal oxides: manganese, nickel, cobalt, …

– Principle: change the resistance when the temperature changes.

– Advantages: Durable, cheap, easy to manufacture.

– Disadvantages: narrow linear range.

– Commonly used: Do protective functions, squeeze in the motor windings, electronic circuits.

– Measurement: 50

Thermistor Thermistor
Thermistor thermistor is composed of a mixture of ocid powders. These powders are mixed in proportions and volumes and then compressed and heated to a high temperature. And the conductivity of this mixture will change as the temperature changes.

– There are two types of thermistor: PTC positive coefficient – resistance increases with temperature; NTC negative heat coefficient – the resistance decreases with temperature. The most commonly used type is NTC.

– Thermistors only recruit within a certain temperature range of 50-150D.C so it is rarely used as a thermal sensor. Only used for the purpose of protecting, disconnecting the heat, our doctors are often called TomTom. Every cool block has some attachments attached to the motor windings.

Note when used:

– Depending on the temperature of the environment, select the appropriate thermistor, note the two types of PTC and NTC (usually referred to as normal / open) can be easily tested with the VOM clock.

– Should be tight on the surface to be measured.

– Avoid damaging the protection sheath.

– Because the voltage variation is not interested in the wiring.


– Structure: Made from semiconductor materials.

– Principle: Polarization of semiconductors is influenced by temperature.

– Advantages: Money, easy to manufacture, high sensitivity, good anti-interference, simple processing circuit.

– Disadvantages: Not resistant to high temperatures, less durable.

– Commonly used: air temperature measurement, used in measuring devices, protection of electronic circuits.

– Measurement: -50 <150 D.C.

Semi-conductor heat sensor
Semiconductors are semiconductor sensors manufactured from semiconductors. There are types such as Diode, Transistor, IC. Their principle is based on the degree of polarization of linear P-N classes with the ambient temperature. Today, with the development of the semiconductor industry, there are many types of thermal sensors with the integration of many advantages: high accuracy, good anti-interference, stable operation, Simple, cheap, ….

It is easy to encounter these sensors in the form of diode (shape similar to Pt100), IC types such as: LM35, LM355, LM45. Their principle is that changing temperature will produce variable voltages. This voltage is distributed from a standard voltage in the circuit.

IC LM35 thermal sensor and diode temperature sensor

Recent high-end thermal ICs have also been introduced, which also support the I2C (DS18B20), which opens a new dimension in the “sensor world.”

Note when used:

– Since it is made of semiconductor components, the semiconductor heat sensor is durable, not high temperature resistant. Excessive protection can damage the sensor.

Each semiconductor sensor is linear within a certain limit, except that this sensor will lose its effect. Carefully measure the range of this sensor to achieve accuracy.

– This type of sensor is less tolerant of harsh environments: high humidity, corrosive chemicals, strong shock vibration.

RADIATION RADIATION (also known as pyrometer).

– Structure: Made from electronic circuits, optical.

– Principle: Measure the radiation properties of the heat carrier environment.

Advantages: Used in harsh environments, no contact with the measuring medium.

– Disadvantages: Accuracy is not high, expensive.

– Commonly used: Making measuring instruments for the furnace.

– Measurement: -54 <1000 D.F.

Thermal radiation thermometer sensor
Radiant thermometer is a specialized type of temperature sensor used to measure the temperature of environments where conventional sensors can not be contacted (steel furnace, strong corrosive chemicals, hard Put sensor).

– Composed of: Radiation pyrometer, light intensity pyrometer, color pyrometer. They operate based on the principle that heat carriers will have energy radiation. And the radiation energy will have a certain wavelength. The thermometer will take this wavelength and analyze it to give you the temperature of the object under test.

Note when used:

– Depending on the manufacturer’s specifications, the pyrometers have different ranges, but most of them measure at high temperatures. And because the characteristics do not come in direct contact with the object being measured, the accuracy of the pyrometer is not high, influenced by the surrounding environment (angle of measurement, hand shake, ambient light).

The above is the information about the thermal sensor and application, their benefits in life. Hope that the information we provide provides useful information for you.

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