Temperature is one of the most commonly measured quantities of a device or the conditions surrounding it — and particularly for electronic components. This is because electronic devices and circuitry generate heat and require some type of thermal management.
There are several types of temperature sensors that work well for such applications and that offer different features or specifications. For example, a temperature sensor can offer an analog or digital output.
In this tutorial, we’ll cover a few of the most common types. These sensors can broadly be categorized as either:
1. Contact temperature sensors — these types require contact with an object to sense its temperature and can be used to measure the temperature of a solid, liquid, or gas.
2. Non-contact temperature sensors — detect the temperature of an object or its surroundings by using radiation or convection. These sensors are mostly used to measure liquids or gases. However, the sensors that use infrared radiations are also able to detect the temperature of solid objects.
Within these two classes of temperature sensors, there are several different types to choose from.
Here are a few…
The term “thermistor” is an abbreviation for “thermally sensitive resistor.” It’s a special type of resistor with a resistance that changes based on the temperature.
- If the resistance of a thermistor increases with a rise in temperature, it has a positive temperature coefficient and is called a PTC thermistor.
- If the resistance of a thermistor decreases with a rise in the temperature, it has a negative temperature coefficient and is called an NTC thermistor. Most thermistors are NTCs.
Thermistors have a fairly low response time to any change in temperature. They’re passive electronic devices and require no current to pass through them to generate a voltage output. The physical resistance of a thermistor can range from a few ohms, kilo-ohms, or tens of mega-ohms.
A thermistor is a contact-type of sensor that offers analog output and interface in a circuit by using a voltage divider network. The potential divider network may also be interfaced with a differential amplifier or the analog input of a microcontroller for the voltage reading.
These types of sensors are reliable, highly accurate, and durable. Thermistors rarely get damaged unless they’re subject to extremely high temperatures beyond their maximum limit. However, they can sustain physical damage because they’re typically constructed of ceramic-type semiconductor materials, such as oxides of nickel, cobalt, and/pr manganese. The semiconductor material is pressed into discs or balls and hermetically sealed.
The most important features to consider when choosing a thermistor are its:
- Range of resistance
- Resistance-temperature curve
- Time constant (i.e. how fast its resistance changes with the temperature)
- Physical resistance at room temperature
- Temperature range and power rating given the current flow
NTC thermistors generally have a non-linear resistance-temperature curve due to their exponential nature. But this can be flattened for certain ranges. Standard thermistors have a typical temperature range between -50˚ to 150˚ C, while glass-encapsulated thermistors go up to 250˚ C.
Most thermistors can easily be interfaced with Arduino or other microcontroller platforms, provided the selected board or controller has analog input.
In a microcontroller-less circuit, these sensors can be interfaced with an operational amplifier by using a voltage-divider network to obtain the binary conclusion for an application (like if the temperature is lower or higher than a certain threshold).
Resistive temperature detectors (RTD)
RTDs have a positive temperature coefficient and their resistance increases with a rise in temperature. These types of sensors have a high-purity conducting metal — such as copper, platinum, or nickel — that’s wound into a coil or a thin film deposited on a ceramic substrate.
These are precision sensors that offer an extremely linear and accurate resistance-temperature curve. However, they have poor thermal sensitivity (typically, 1Ω/˚C).
RTDs that are made of platinum are the most common ones used and are called, platinum-resistance thermometer or PTC. PTCs are expensive.
Another drawback of RTDs and PTCs is that they’re self-heating. This means that their resistance is affected by heat due to the current flowing through them, which can lead to faulty readings.
RTDs are contact-type sensors that offer an analog output. To compensate for their self-heating characteristic, RTDs are typically interfaced in a circuit using a Wheatstone Bridge network, which has a constant current source connected to it. This is to compensate for any standard errors or additional wires (used for lead compensation).
Platinum RTDs have a linear resistance-temperature curve that’s above the typical range of -200˚ to 600˚ C.
PTD100 RTD is currently the most popular RTD available in 2, 3, or 4-wire packages. It has a resistance of 100Ω at 0˚ C, which rises to 140Ω at 100˚ C.
To measure the temperature using an RTD, it must be connected in a Wheatstone Bridge with a constant current source. The voltage output is measured to determine the resistance. The temperature can, then, be derived via the linear resistance-temperature relationship for the given RTD.
A thermocouple is the most commonly used contact-type temperature sensor. They’re compact, inexpensive, simple to use, and provide a quick response time to temperature changes. These sensors offer the widest temperature range, which is between -200˚ and 2,000˚ C. A thermocouple is made of two wires of dissimilar metals, electrically bonded by two junctions. The metals, for example, might be copper and constantan.
One junction is maintained at a constant temperature for reference and it’s called the cold junction. The other one is used to measure the temperature and it’s called the hot junction. Since the temperatures at both junctions are typically different, it’s used as the potential between them to measure the actual temperature.
The junction between the two metals creates a thermoelectric effect, where a constant potential forms of a few milli-volts. This voltage difference between the junctions is called the “seeback effect.” Essentially, it’s like a voltage gradient between the two.
When both junctions are the same temperature, they have zero voltage difference. When both junctions are at different temperatures, a voltage proportional to the temperature difference is generated. The difference in voltage increases as the temperature difference between the two junctions rises — and until a peak voltage difference is generated. This peak voltage is determined by the characteristics of each metal.
The measurement of a thermocouple’s voltage output requires an amplifier. There’s typically only a few milli-volts of difference as per a 10˚ C rise in temperature. Chopper and instrumentation amplifiers are commonly used because they offer superior drift stability with very high gains.
The voltage output measured from a thermocouple can be applied to the analog input of a microcontroller or a regular amplifier circuit for a logical conclusion.
Thermocouples are made from a variety of metals and have different temperature ranges depending upon the metal combination used for their construction. As a result, these sensors are listed and available based on standard codes and lead colors.
Thermocouples are chosen for an application based on their temperature range. R, J, and T types are commonly used. Although thermocouples are inexpensive, they have poor accuracy (tolerance 0.5˚ to 5˚ C) and a non-linear temperature curve. Engineers typically have to match the sensor used up with a lookup table to determine the temperature conversion, control, and compensation.
A thermostat is an electro-mechanical temperature sensor that’s constructed by bonding two different metals to form a bi-metallic strip. When this strip is exposed to heat, it bends due to the different linear expansions of the two metals. The metals can be nickel, aluminum, tungsten, or copper.
Thermostats are often used as electrical switches or to control an electrical switch in thermostatic controls. Thermostatic switches are controlled by movement and can be:
1. A snap-action type — which offers an instant ON/OFF operation that’s widely used in ovens, hot water tanks, electric irons, and other domestic heating appliances.
2. A creep-action type — used as dials or gauges, and provides gradual changes in temperature. These types are assembled as bi-metallic coils or spirals and are more sensitive to temperature changes.
Thermostats are available for a wide range of temperatures, however they have poor reliability due to large hysteresis. Typically, these sensors are only used in control applications, where a precise temperature set-point is used to operate as a switch.
Semiconductor-based temperature sensors are dual-integrated circuits. Two identical diodes with temperature-sensitive voltage-current characteristics are integrated to detect temperature changes.
Overall, these sensors offer low accuracy (tolerance between 1˚ to 5˚ C), slow responsiveness (between 5 and 60 seconds), and a narrow temperature range (between -70˚ to 150˚ C).
Many semiconductor-based temperature sensors now come with internal amplifiers, generating an output of about 10mV/˚ C. These sensors have better accuracy and high linearity. The semiconductor types are generally used with thermocouples for cold-junction temperature compensation.
Infrared temperature sensors
Infrared sensors are a non-contact type of temperature sensor. They’re photo-sensitive devices that detect infrared (IR) radiations from the surrounding area or an object to measure heat.
Thermopiles are one of the most popular types of non-contact temperature sensors. They’re used for measuring heat, as well as gas concentrations.
Thermopiles are often used in industrial process control, medical temperature readers, heat alarms, microwave ovens, road ice detection, and automobiles.
Thermocouples are the most common contact-type temperature sensors because they’re inexpensive, offer the widest temperature range, and have acceptable characteristics for most applications.
For better accuracy, precision, and response, however, RTDs and thermistors are preferred. The most important considerations when choosing a contact-type temperature sensor are its size, cost, temperature range, and accuracy. These factors also depend on the application.
Non-contact temperature sensors are typically based on sensing infrared radiations. The most important factors to consider when choosing non-contact temperature sensors are cost, reliability, accuracy, and the application for use. Some popular applications that use non-contact temperature sensors are thermal imaging, infrared thermometers, and infrared scanning.
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