Temperature sensor

A temperature sensor is an electronic device that measures the temperature of its environment and converts the input data into electronic data to record, monitor, or signal temperature changes. 

Introduction

Temperature is the most often-measured environmental quantity. This might be expected since most physical, electronic, chemical, mechanical, and biological systems are affected by temperature. Certain chemical reactions, biological processes, and even electronic circuits perform best within limited temperature ranges. Temperature is one of the most commonly measured variables and it is therefore not surprising that there are many ways of sensing it. Temperature sensing can be done either through direct contact with the heating source or remotely, without direct contact with the sou

rce 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.

Temperature sensor types

• Resistance temperature detector (RTD)
• Negative temperature coefficient (NTC) Thermistor 
• Thermocouple
• Semiconductor based sensor 

Resistance temperature detector (RTD)

The RTD is a temperature-sensing device whose resistance changes with temperature. Typically built from platinum, though devices made from nickel or copper are not uncommon, RTDs can take many different shapes like wire wound, thin film. To measure the resistance across an RTD, apply a constant current, measure the resulting voltage, and determine the RTD resistance. RTDs exhibit fairly linear resistance to temperature curves over their operating regions and any nonlinearity is highly predictable and repeatable. The PT100 RTD evaluation board uses surface mount RTD to measure temperature. An external 2, 3, or 4-wire PT100 can also be associated with measure temperature in remote areas. The RTDs are biased using a constant current source. To reduce self-heat due to power dissipation, the current magnitude is moderately low. The circuit shown in the figure is the constant current source uses a reference voltage, one amplifier, and a PNP transistor.

 Thermistor (Negative temperature coefficient)

Similar to the RTD, the thermistor is a temperature-sensing device whose resistance changes with temperature. Thermistors, however, are made from semiconductor materials. Resistance is determined in the same manner as the RTD, but thermistors exhibit a highly nonlinear resistance vs. temperature curve. Thus, in the thermistors operating range, we can see a large resistance change for a very small temperature change. This makes for a highly sensitive device, ideal for set-point applications.

Thermocouple

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 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.

Semiconductor based sensor

They are classified into different types like Voltage output, Current output, Digital output, Resistance output silicon, and Diode temperature sensors. Modern semiconductor temperature sensors offer high accuracy and high linearity over an operating range of about 55°C to +150°C. Internal amplifiers can scale the output to convenient values, such as 10mV/°C. They are also useful in cold-junction compensation circuits for wide temperature range thermocouples.

Temperature Sensor Applications

There are Temperature Sensor applications in many industries including 

• medical, 
• motorsport, 
• HVAC, 
• agriculture, 
• industrial, 
• aerospace and 
• automotive. 

Here are some of the specific temperature sensor applications which we have come across.

• Motors– there are many different aspects of motors and most of these require temperature measurement to ensure the motor itself does not overheat.
• Surface plates – ring terminal temperature sensors are often used on surface plates as they can be mounted onto a flat surface and measure temperature effectively.
• Home appliances – kettles, toasters, washing machines, dishwashers and coffee machines will all contain temperature sensors.
• 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.
• Warming Electrical Radiators – NTC thermistors are used to control the heat on electric radiators.
• Exhaust Gas Monitoring on Motorsport Vehicles – Motorsport temperature sensors need to be highly reliable and durable to ensure performance is not compromised in this harsh environment.
• Food Production; 3D printed chocolates – temperature sensors are used to monitor the temperature of the melted chocolate for 3D printing.
• Alcohol breathalyser – thermistors are used within alcohol breathalysers to measure the temperature of the subject's breath.

Other Temperature Sensor Applications Include:

• Transit
• HVAC
• Power and Utilities
• Calibration and Instrumentation
• Heat Exchangers
• Industrial Processes
• Drilling
• Heating/cooling systems
• Laboratory
• Energy