What is a Temperature Sensor?

A temperature sensor is a device that measures how hot or cold an object is, providing a temperature measurement through an electrical signal in a readable form. The more common ones are thermocouples and thermal resistor temperature detectors.

In our daily lives, we should often see thermometers, water heaters, microwave ovens, refrigerators, etc. These will be applied to an important device – the temperature sensor. This article will introduce to you temperature sensors, temperature sensor principles, and types of temperature sensors.

Temperature sensor type:
In practical applications, there are many temperature sensors available, with different characteristics according to the actual application. Temperature sensors consist of two basic physical types:
1. Contact temperature sensor type
These types of temperature sensors require physical contact with the object being sensed and use conduction to monitor temperature changes. They can be used to detect solids, liquids or gases over a wide temperature range.

Probe harnesses for different types of temperature sensors

2. Non-contact temperature sensor type
These types of temperature sensors use convection and radiation to monitor temperature changes. They can be used to detect liquids and gases that emit radiant energy as heat rises and cold settles to the bottom in convection currents, or to detect radiant energy transmitted from objects in the form of infrared radiation (sun).
Contact and non-contact temperature sensors are further classified into the following temperature sensors.

The principle of temperature sensor:
1. Thermostat
A thermostat is a contact temperature sensor that consists of a bimetallic strip made of two different metals, such as aluminum, copper, nickel, or tungsten.

The difference in the linear expansion coefficients of the two metals causes them to undergo mechanical bending movements when heated.

Actual picture of thermostat

2. Bimetal thermostat
A thermostat consists of two metals with different heat levels glued together back to back. When the weather is cold, the contacts close and current flows through the thermostat. As it heats up, one metal expands more than the other, and the bonded bimetal strips bend upward (or downward), opening the contacts and preventing the flow of electricity.

Bimetal thermostat physical picture

There are two main types of bimetal strips, based primarily on their movement when subjected to temperature changes. There are “snap-action” types that produce an instantaneous “on/off” or “off/on” type action on the electrical contacts at a set temperature point, and slower “creep” types that gradually change their position as temperature changes .
Bimetal thermostat working principle diagram

Probe harness for NTC temperature sensor

Snap-acting thermostats are commonly used in our homes to control the temperature set points of ovens, irons, immersion hot water tanks, and they can also be found on walls to control home heating systems.

Crawler types typically consist of bimetallic coils or spirals that slowly unfurl or coil as the temperature changes. Generally speaking, crawler style bimetal strips are more sensitive to temperature changes than standard snap on/off types because the strips are longer and thinner, making them ideal for use on thermometers and dials, etc.

NTC temperature sensor working example diagram

3. Thermistor
Thermistors are usually made of ceramic materials, such as nickel, manganese or cobalt oxides plated in glass, which makes them easily damaged. Their main advantage over snap-action types is how quickly they respond to any changes in temperature, accuracy and repeatability.

Most thermistors have a negative temperature coefficient (NTC), which means their resistance decreases as temperature increases. However, there are some thermistors that have a positive temperature coefficient (PTC) and their resistance increases with temperature.

Thermistor physical picture

Thermistors are rated based on their resistance at room temperature (usually 25 o C), their time constant (the time it takes to react to a change in temperature), and their power rating relative to the current flowing through them. Like resistors, thermistors have resistance values at room temperature ranging from 10 megohms to a few ohms, but for sensing purposes those types measured in kiloohms are typically used.

4. Temperature sensor example No1
The resistance value of the following thermistor at 25℃ is 10KΩ, and the resistance value at 100℃ is 100Ω. Calculate the voltage drop across the thermistor when placed in series with a 1kΩ resistor to calculate the output voltage (Vout) across the 12v supply at both temperatures.
Temperature sensor example diagram

NTC resistance at 25 degrees

By changing the fixed resistor value of R2 (1kΩ in our example) to a potentiometer or preset value, a voltage output can be obtained at a predetermined temperature set point, for example a 5v output at 60°C. And by changing the potentiometer to get a specific output voltage level it can be obtained over a wider temperature range.

However, it should be noted that thermistors are nonlinear devices, and the standard resistance values of different thermistors at room temperature are different, mainly because they are made of semiconductor materials. Thermistors change exponentially with temperature and therefore have a Beta temperature constant (β) that can be used to calculate resistance at any given temperature point.

However, when used with series resistors, such as in a voltage divider network or a Wheatstone bridge type arrangement. The current obtained in response to the voltage applied to the voltage divider/bridge network is linear with temperature. The output voltage across the resistor then scales linearly with temperature.