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Temperature Sensors

The most frequently measured environmental quantity is “Temperature” This might be expected since most of the systems are affected by temperature like physical, chemical, electronic, mechanical, and biological systems. Certain chemical effects, biological processes, and even electronic circuits execute best in limited temperature ranges. Temperature is one of the most frequently calculated variables and sensing can be made either through straight contact with the heating basis or remotely, without straight contact with the basis using radiated energy in its place. There is an ample variety of temperature sensor on the market today, including Thermocouples, Resistance Temperature Detectors (RTDs), Thermistors, Infrared, and Semiconductor Sensors.

What is a Temperature Sensor?

Usually, a temperature sensor is a thermocouple or a resistance temperature detector (RTD) that gathers the temperature from a specific source and alters the collected information into understandable type for an apparatus or an observer. Temperature sensors are used in several applications namely HV system and AC system environmental controls, medical devices, food processing units, chemical handling, controlling systems, automotive under the hood monitoring and etc.

Temperature Sensor

The most frequent type of temperature sensor is a thermometer, used to determine the temperature of solids, liquids, and gases. It is also mostly used for non-scientific purposes as it is not so accurate. The different kinds of sensors are categorized by the sensing capacity of the sensor as well as the range of applications. The different types of temperature sensors include the following.

• Thermocouples
• Thermistors
• Resistor temperature detectors
• Semiconductors
• Infrared sensors
• Thermometers

LM35 Temperature Sensor

The LM35 is one kind of commonly used temperature sensor that can be used to measure temperature with an electrical o/p comparative to the temperature (in °C). It can measure temperature more correctly compare with a thermistor. This sensor generates a high output voltage than thermocouples and may not need that the output voltage is amplified. The LM35 has an output voltage that is proportional to the Celsius temperature. The scale factor is .01V/°C.

LM35 Temperature Sensor

The LM35 does not need any exterior calibration and maintains an exactness of +/-0.4°C at room temperature and +/-0.8°C over a range of 0°C to +100°C.One more significant characteristic of this sensor is that it draws just 60 microamps from its supply and acquires a low self-heating capacity. The LM35 temperature sensor available in many different packages like T0-46 metal can transistor-like package, TO-92 plastic transistor-like package, 8-lead surface mount SO-8 small outline package.

Pin Configuration

LM35 Temperature Sensor Circuit Diagram

The LM35 temperature sensor is used to detect precise centigrade temperature. The output of this sensor changes describes the linearity. The o/p voltage of this IC sensor is linearly comparative to the Celsius temperature. The operating voltage range of this LM35 ranges from-55˚ to +150˚C and it has low-self heating. This is operated under 4 to 30 volts. The most extensively used electronic devices are operational amplifiers, which are certain kind of differential amplifiers. Temperature sensor circuit has terminals such as two inputs like non-inverting (+) and inverting (-) and only one output pin. Operational amplifier IC741 is used as a non-inverting amplifier. The variation between the i/p terminals amplifies the circuit.

LM35 Circuit Diagram

The amount produced by IC2 amplifies in an amount to the temperature by 10 mV per degree. This unstable voltage is supply to a comparator IC 741. OP Amplifier is the most generally used electronic devices today. The IC 741 op-amp is one sort of differential amplifier. We have used IC741 as a non-inverting amplifier which means pin-3 is the input and the output is not inverted. This LM35 temperature sensor circuit amplifies the difference between its input terminals. The advantages of temperature sensor include It has no effect on the medium, more accurate, It has an easily conditioned output and It responds instantly.

Applications of LM35 Temperature Sensor

The applications of LM35 temperature sensor include the following

• Measuring temperature of a particular environment and HVAC applications
• Providing thermal shutdown for a component/ circuit
• Checking Battery Temperature

This is all about temperature sensor circuit and its working principle. We believe that the information given in this article is helpful for you for a better understanding of this project. Furthermore, for any queries regarding this article or any help in implementing the electronics projects, you can feel free to approach us by connecting in the comment section below. Here is a question for you: what do you mean by a heat sensor?

Different Types of Temperature Sensors

The term “Temperature” is a physical parameter and it can be measured in units of degrees. It is a critical part of any method to measure precise temperature. The usual applications needful accurate temperature quantities include medical, electrical or electronic studies, biology research, materials research, etc. A Temperature Sensor is used to measure the heat energy amount that let us detect a physical variation in temperature, generating either an analog or digital output.

Types of Basic Physical Temperature Sensors

In general, there are two types of sensing methods are available, namely contact type temperature sensors and non-contact type temperature sensors.

Contact Type Temperature Sensor

The contact type temperature sensor is in physical contact with the substance or object. These sensors are used to measure the temperature of solids, liquids or gases.

Non-contact Type Temperature Sensor

Non-contact type temperature sensors are used to detect temperature by interrupting a part of the emitted IR energy of the substance and detecting its intensity. These sensors can be used to measure only the temperature of solids and liquids. It is not potential to use them on gases due to their transparent nature.

Types of Temperature Sensors

The different types of temperature sensors available in the market can vary from simple ON/OFF thermostatic devices to highly sensitive semiconductor devices. The two basic types of physical temperature sensors like contact and non-contact are further categorized into a voltage, resistive, and electromechanical sensors. The most commonly used temperature sensors are classified into three types such as Thermistors, thermocouples, and resistance temperature detectors.These sensors vary from each other in terms of working parameters. For modest temperature series applications, solid state sensors are also accessible which give the advantage of simple interface and incorporated signal conditioning.

Different Types of Temperature Sensors

Thermistor

The thermistor is a temperature sensing device that changes the physical resistance with the temperature. Generally, these devices are made from ceramic material semiconductor like cobalt, manganese. It is shaped into small pressed hermetically closed discs that give a comparatively fast response to any temperature variations.

Thermistor

NTC Type Thermistor (Epcos)

Due to the properties of a semiconductor material, these devices have an NTC (negative temperature coefficient), i.e. the resistance falls with the rise in temperature. But, there are also Thermistors available with PTC (positive temperature coefficient), their resistance rises with the rise in temperature.

Application of a Thermistor

The thermistor is rated by their resistance value at room temperature, the time constant, and power rating. The thermistor is a passive resistive device, that requires current to generate an o/p voltage. Generally, they are connected in series with an appropriate biasing resistor to form a potential divider network.

Application of a Thermistor

Consider a thermistor with a value of resistance 2.5KΩ at 30°C of and 30Ω at 60°C. Here, the thermistor is connected in series with a 1kΩ resistor across a 5V power supply.

Here, the output voltage of this Thermistor can be calculated as

At 30°C, RNTC = 2500Ω;

Vout=1000/ (2500+1000)*5= 1.42V

At 60°C, RNTC = 30Ω;

Vout=1000/(30+1000)*5 = 4.85V

By substituting the specific resistor value with a potentiometer, we can get an o/p voltage at a prearranged temperature.

Though, it is significant to note that typical resistance values are dissimilar at room temperature for different types of thermistors since they are non-linear. Thermistor has an exponential transform with temperature; so it has a temperature constant (Beta) that is used to analyze its resistance for a given temperature. Though in a voltage divider network, the current which is obtained from the applied voltage is linear with temperature, therefore the o/p voltage across the resistor & temperature are linearly related.

Advantages of Thermistors

The advantages of Thermistors include the following.

• The speed of response is better to changes in accuracy, temperature, and repeatability.
• Compared to RTDs, it is inexpensive
• The range of higher resistance ranges from 2,000 to 10,000 ohms
• Sensitivity is Much higher, i.e., ~200 Ω/°C within a partial temperature range of up to 300 °C

Resistive Temperature Detectors

The term RTD stands for “Resistive Temperature Detector” is an electrical resistance temperature sensor that is made of coils of metals or films like platinum whose electrical resistance is a role of temperature.

RTD- Resistance Temperature Detector

Resistive Temperature Detectors have PTC (positive temperature coefficients) and not like Thermistors, they offer precise temperature measurements since they have linear output. But, they have poor sensitivity generating a small o/p change, for instance, 1Ω/°C for a temperature change. Pt100 has been the most frequently available sensor with a typical resistance value of 100Ω at 0°C. The main drawback is its high cost.

How to Use Resistive Temperature Detectors?

Resistive temperature detectors are passive resistor devices such as Thermistors and the flow of current through the sensor to get an o/p voltage is linearly connected to the temperature. But, an error can happen in the reading due to the variation in resistance caused by the self-heating of the flow of current through the resistive wires. To overcome this difficulty, a resistive temperature detector is connected to a resistive bridge n/w with additional connecting wires for lead compensation and/or adding of a constant current source.

Advantages of RTDs

The advantages of resistive temperature detectors include the following.

• The temperature range ranges from -200 to 650°C
• It offers a high o/p for a current drop
• RTDs are more linear compared to Thermistors and thermocouples.

Thermocouples

Thermocouples are the most commonly used temperature sensors due to some features like a wide temperature range from below -200°C to over 2000°C, accurate, and are relatively low-cost.

Thermocouples

Construction and Working

The construction of a Thermocouple can be done with two different metals that are soldered together producing a slight potential difference (mV) as a function of temperature. One junction is kept at a constant temperature named the reference junction, while the other is the measuring junction. With the temperature difference between the two junctions, a voltage is established across the junction which is used to measure the temperature. The Seebeck effect can be defined as the voltage difference between the two junctions.

Construction and Working of Thermocouples

If both junctions are at the similar temperature, the potential difference through the junctions is zero, i.e. V1 = V2. However, when the junctions are at dissimilar temperatures connected in a circuit, the o/p voltage is comparable to the temperature difference between the two junctions, i.e. V1 – V2.

Types of Thermocouples

Thermocouples are existing in different materials and temperature ranges; therefore there are different kinds of thermocouples available for precise applications as set by international standards such as E, J, K, N, T, and U. Where, type J &K are the most frequently used thermocouples.

Advantages of Thermocouple

• Temperature ranges are high
• Rough, withstand shock and vibration
• Offers instant response to temperature changes

How to Use a Thermocouple?

Thermocouple gives a few millivolts output voltage for a temperature change of 10°C. So, it is mandatory to increase the output voltage.

How to Use a Thermocouple

It is essential to carefully choose an amplifier to get good drift stability for stopping recalibration of the thermocouple. This makes an Op-Amp (operational amplifier) desirable for most applications.

Thus, this is all about different types of temperature sensors.We hope that you have got a better understanding of this concept.Furthermore, any doubts regarding this concept or to implement any temperature sensor based projects please comment in the comment section below.