Temperature sensor

Introduction

In our daily life and different branches of industry there are many cases which require to know the temperature of environment such as inside a reactor , winding a wind electrical machines and so on. Sensors are of many types but the most commonly used type of sensors are those type of sensor which detect Temperature or heat. 

Temperature sensor is a physical device which is used for measuring the temperature of anything or any place.

In other words, Temperature Sensorsmeasure the amount of heat energy or even coldness that is generated by an object or system, allowing us to “sense” or detect any physical change to that temperature producing either an analogue or digital output. Temperature sensor is a device usually an RTD (resistance temperature detector) or a thermocouple, that collects the data about temperature from a particular source and converts the data into understandable form for a device or an observer. 

The most common type of temperature sensor is a thermometer, which is used to measure temperature of solids, liquids and gases. It is also a common type of temperature sensor mostly used for non-scientific purposes because it is not so accurate.Temperature sensors are used in many applications like HVand AC system environmental controls, food processing units, medical devices, chemical handling and automotive under the hood monitoring and controlling systems, etc. 

Working of temperature sensor 

The measurement of temperature sensor is about the hotness or coldness of an object. The working base of the sensors is voltage that read across the diode. If voltage increases, then temperature rises and there is a voltage drop between the transistor terminals of base and emitter, they are recorded by the sensor. It the difference in voltage is amplified , the analogue signal is generated by device and it is directly proportional to temperature. A temperature sensor produces an analogue or digital output whose strength depends on the temperature of the sensor. Heat is conducted to the sensing element through the sensor’s package and its metal leads. In general, a sensor in a metal package will have a dominant thermal path through the package. For sensors in plastic packages, the leads provide the dominant thermal path. Therefore a board-mounted IC sensor will do a fine job of measuring the temperature of the circuit board.

If it’s needed to measure the temperature of something other than the circuit board, it should be ensured that the sensor and its leads are at the same temperature as the object you wish to measure. This usually involves making a good mechanical (and thermal) contact by attaching the sensor and its leads to the object being measured with thermally-conductive epoxy. A three-terminal sensor needs three wires for power, ground and output signals. When sensing the temperature in a remote location, it is desirable to minimise the number of wires between the sensor and the main circuit board. In such situations you can use a two-terminal sensor. Moving to two wires means that power and signal must coexist on the same wires. For example

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.

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.

Types of temperature sensor

Temperature sensor can be done either through direct contact with the heating source, or remotely, without direct contact with the source using radiated energy instead. There are a wide variety of temperature sensors on the market today, including Thermocouples, Resistance Temperature Detectors (RTDs), Thermistors, Infrared, and Semiconductor Sensors.

Thermocouples

It is a type of temperature sensor, which is made by joining two dissimilar metals at one end. The joined end is referred to as the HOT JUNCTION. The other end of these dissimilar metals is referred to as the COLD END or COLD JUNCTION. The cold junction is actually formed at the last point of thermocouple material. If there is a difference in temperature between the hot junction and cold junction, a small voltage is created. This voltage is referred to as an EMF (electro-motive force) and can be measured and in turn used to indicate temperature.

Resistance Temperature Detector (RTD)

RTD sensor is one of the most accurate sensors. In a resistor temperature detector, the resistance is proportional to the temperature. This sensor is made from platinum, nickel, and copper metals. It has a wide range of temperature measurement capabilities as it can be used to measure temperature in the range between -270oC to +850oC. RTD requires an external current source to function properly. However, the current produces heat in a resistive element causing an error in the temperature measurements. The error is calculated by this formula:

                        Delta=P *S

Where, ‘T’ is temperature, ‘P’ is I squared power produced and ‘S’ is a degree C/mill watt Where, ‘T’ is temperature, ‘P’ is I squared power produced and ‘S’ is a degree C/mill watt.

Thermistor- 

Thermistor temperature sensor, which is relatively inexpensive, adaptable, and easy to use. It changes its resistance when the temperature changes like RTD sensor. Thermistors are made from manganese and oxides of nickel, which make them susceptible to damages. So, these materials are called ceramic materials. This thermistor offers higher sensitivity than the resistor temperature detectors. Most of the thermistors have a negative temperature coefficient. It means, when the temperature increases the resistance decreases.

Semiconductor sensors

Semiconductor sensors are the devices that come in the form of ICs. Popularly, these sensors are known as an IC temperature sensor. They are classified into different types: Current output temperature sensor, Voltage output temperature sensor, Resistance output silicon temperature, sensor Diode temperature sensors and Digital output temperature sensor. Present semiconductor temperature sensors offer high linearity and high accuracy over an operating range of about 55°C to +150°C. However, AD590 and LM35 temperature sensors are the most popular temperature sensors.

Application of temperature sensor 

There are so many application of temperature sensor in many industries including medical, Motorsport, HVAC agriculture, aerospace and automotive. Here are some of the specific temperature sensor applications:

• Motors– there are many different aspects of motors and most of these require temperature measurement to ensure the motor itself does not overheat.
• Computers– within computers there are temperature sensors to ensure the system does not overheat.
• Industrial equipment – temperature sensors used within these applications will need to be robust as the environment can be very demanding.