What is Thermopile and how it functions?

Thermopile sensor

What is a Thermopile?

A thermopile is a type of electronic equipment that transforms the thermal energy to the electrical energy. It is a serially linked thermocouple array made up of two distinct materials with high thermoelectric power and opposing polarity. Parallel connections are less prevalent.

Thermopile is a temperature differential measuring device. It is capable of converting heat energy into electrical energy. It employs a number of thermocouples linked in series (or) parallel.

Thermopiles are temperature sensors that do not need any touch. A thermopile’s function is to convert the heat radiation released by an object into an electrical voltage output. The output is in the tens (or) hundreds of millivolts range.

Principle of Thermopile

Thermopile sensors are intended to detect infrared (IR) radiation from a hot object and estimate temperature from a distance. The greater the temperature of a hot object, the more IR radiation it emits.

Principle of Thermopile

Thermocouple is employed in non-contact pyrometers as a temperature sensor, and thermocouple produces thermo-emf (in mV) based on the seebeck effect theory. When the sensor is placed at an appropriate distance from the source of the heat, the output of single thermocouple is insufficient to be detected up by the detector.As a result, instrument makers provide a thermopile, which is a series-connected array of thermocouples that generates a greater electrical signal (in the 10 to 100mV range).

Principle of Thermopile 2

Thermopile working principle

In this example, 4 E-type thermocouples (chromel& constantan) are linked in series such that all of the ‘hot junctions’ voltages help each other, as well as the ‘cold junctions’ voltages. However, as with all thermocouple circuits, each cold junction voltage opposes each hot junction voltage. When the hot junctions of all thermocouples approach the same hot object and the cold junctions are maintained at the same cold junction temperature and the final output will create four times the potential difference as a single type E thermocouple. As a result, it performs like a multiplied thermocouple, producing more voltage than the single thermocouple junction should under the same temperature circumstances.

Construction of the Thermopile

The sensing element is composed of up of very small thermocouples that are installed on a silicon chip. These thermocouples are able to take in energy and then send out a signal. For correction, a reference sensor is included into the package. Thermopile infrared (IR) temperature sensors monitor non-contact temperature & are available with a variety of lens and filter options, enabling them to be used in a wide range of applications, including industrial pyrometers, climate controls, and medical equipment.

Construction of the Thermopile

There are two types of thermopile detectors:

  • Thin Film Based Thermopile and
  • Silicon Based Thermopile

The arm materials for Thin Film thermopiles are antimony (Sb) & bismuth (Bi). The arm materials in a Silicon-based thermopile may alternate between n-type & p-type Poly-Silicon (or) n-type with gold (Au) (or) aluminium (Al). Thin Film detectors have lower resistance & noise voltage than Silicon detectors, hence they have a greater signal-to-noise ratio. A Thin Film type with the same output as a Silicon-based type will have the slower time constant. Thin Film based devices have bigger active areas overall.

How does a Thermopile work?

Since the voltage output of a single thermoelectric cell is so small, many of these cells are connected in series or parallel to produce a larger signal output. The thermopile is a thermocouple stack structure.

The thermoelectric effect concept is used by Thermopile. The thermoelectric effect is defined as the process by which changes in temperature are directly converted into changes in electrical voltage and vice versa.

When the dissimilar metals (thermocouples) in a thermopile are subjected to temperature variations, a voltage is created.

Adding multiple thermocouple pairs in series to make a thermopile increases voltage output. It is possible to construct thermopiles using just a single-pair of a thermocouples (consisting of two thermocouple junctions), or it is possible to use many pairs of thermocouples.

A thermopile is a sequence of thermocouples made up of two distinct materials with high thermoelectric power and opposing polarities that are coupled in series.

The thermocouples are routed through the structure’s hot and cold zones, and the hot junctions are thermally insulated from the cold junctions.

The thermoelectric coefficient, also known as the Seebeck coefficient, is a term attributed to the voltage that is produced by the thermopile as a reaction to the temperature difference that exists across the material. It is measured in volts per kelvin (V/K) (or) millivolts per kelvin (mV/K).

How does a Thermopile work?

All thermopile sensors employ infrared radiation for heat transfer & are also used for non-contact temperature readings.

As a result, it is found in infrared

  • Thermometers,
  • Industrial pyrometers,
  • Lifecare devices,
  • Moving object temperature management, and
  • Other applications.

Connections between thermopiles

With a series of thermocouples connected in a chain. The two top thermocouple connections are linked at temperature T1, while the two bottom thermocouple junctions are connected at temperature T2.

The delta-V thermopile’s output voltage is proportional to the temperature differential, or T1-T2, between the thermal resistance layer & the number of thermocouple junction pairs.

Connections between thermopiles

Thermocouple vs. Thermopile

Thermocouples are thermoelectric devices, whereas thermopiles convert electrical energy into thermal energy. This distinguishes thermocouples from thermopiles.

1). Thermocouple

A thermocouple is made up of two distinct metals that are linked in series. One junction is darkened to absorb radiation in order to detect it.

A voltage is generated as the temperature of the junction rises (in comparison to another non-irradiated junction). Seebeck discovered this phenomenon, which serves as the foundation for all the thermocouple temperature sensors.


A thermocouple is a transducer composed of two dissimilar metals welded together at either end that creates a voltage proportionate to the temperature difference between two junctions, one of which is normally held at a known temperature. As a result, it is also known as a thermoelectric transducer.

A thermocouple has a pointed end and is thinner. It operates at a lower voltage ranging from 8 mV to 30 mV. It is constructed from a single pair of wires.

Thermocouple 2

It works by detecting the temperature difference between their points of connection and the point at which the thermocouple’s output voltage is detected.

The closed-circuit is constructed of a single metal. A current is created owing to the potential difference between the hot and cold connections due to the temperature difference between junctions & areas of transition from one metal to another.

Thermocouples, when paired together, may be connected in series with a junction that is located on both sides of the thermal resistance layer. This creates a thermocouple pair.

The thermocouple pair’s output will be a voltage that is proportionate to the temperature differential between the thermal resistance layer & the heat flow through the thermal resistance layer.

Thermocouples are widely utilised in research and business. It is used in many different kinds of applications, including

  • Kiln temperature computation,
  • Gas turbine emissions,
  • Diesel engines, and
  • Other industrial operations.

A thermocouple’s mathematical equation is

Vout = S x (Tx-Tref)


S – Seebeck Effect

(Tx-Tref) – Temperature difference

Vout– Thermocouple Output Voltage

2). Thermopile

The thermocouple series-combination forms an electronic device that transforms an electrical energy into a thermal energy. Thermocouples are used in a variety of applications, including temperature measurement and the control of various processes.

A thermopile delivers more voltage output than a single thermocouple. It is equipped with a series of thermocouples that are configured to measure tiny amounts of radiant heat.

The thermopile uses the IR Absorber technology to monitor temperature non-contactly.A thermocouple, on the other end, is one of a contactbased temperature sensor.

It is made up of antimony & bismuth bars alternated with any two metals with varying heat conduction capabilities. It’s connected to a sensitive astatic galvanometer.


The thermopile is larger and thicker than a thermocouple. It has a greater voltage output than a thermocouple, often in the tens (or) hundreds of mV range. It is essentially a sensor that transforms heat energy into electrical energy.

To generate electrical energy, thermopiles are also utilised to generate heat from electrical components such as

  • Solar wind,
  • Radioactive materials,
  • Laser radiation,
  • Combustion.

A thermopile with N thermocouples creates a voltage N times greater than a single thermocouple, enhancing transducer sensitivity.

Thermopiles provide the voltage output proportional to the temperature gradient or differential rather than responding to absolute temperature.

This is evident in infrared thermometers, which are often used to measure temperature. They’re also often employed in

  • Heat flux sensors,
  • Pyrheliometers, and
  • Gas hob safety controls.

A thermopile’s mathematical equation is

Vout = N x S x (Tx-Tref)


N – Number of thermocouples

S – Seebeck Effect

(Tx-Tref) – Temperature difference

Vout– Thermopile Output Voltage

SL NOThermopileThermocouple
1A thermopile is an electronic device that converts the heat energy to the electrical energy.A thermocouple is a thermoelectric device used to monitor temperature.
2Thermocouples may be coupled in either a series or parallel arrangement.Temperature sensors known as thermocouples are made up of two junctions of different metals that are welded together. These junctions are referred to as the Hot Junction and the Cold Junction.
3High Output VoltageLow Output Voltage
4Thermopile are thick in sizeThermocouple are small and compact in size
5Thermopile has low sensitivityThermocouple has high sensitivity
6Non contact temperature measurementsContact temperature measurements
7Vout = S x (Tx-Tref)Vout = N x S x (Tx-Tref)
8The thermopile has a temperature range of 25°C to +100°C.The thermocouple has a temperature sensing range of -200° to 350°C.

Thermopile sensor

Thermopile sensor

A thermopile sensor is a sensor device that employs non-contact temperature detection with the use of more than one thermocouple. It has a higher voltage output than a standard thermocouple sensor device.

It is based on the absorption of infrared light by the thing being measured. The thermopile sensor’s electrical output is directly proportional to their temperature. As a result, it is referred to as a thermoelectric transducer.

Working of the Thermopile Infrared Sensor

Several series-connected thermocouples with “hot” junctions are attached to a very thin IR absorber in a thermopile infrared (IR) sensor. A silicon chip’s micro-machined membrane serves as an absorber in most condition.

The temperature differential between the absorber & the material causes the absorber temperature to rise or fall. This determines the exchange of infrared radiation between the target & the absorber.

To determine the temperature of the object, the object must completely occupy the sensor’s field of vision.

This ensures that infrared radiation is emitted merely by the object of interest & not from its surroundings.

The use of filter & lens components will significantly improve thermopile sensor performance. A filter placed in front of the thermopile ensures that only the required IR radiation reaches the absorber. It prevents dirt and gases from contaminating the thermopile structure.

Thermopile sensor 2

Thermopile sensors are therefore intended to estimate the temperature of an item (moving or fixed) from a distance by sensing the object’s infrared radiation emission.

How Can Thermopiles Be Tested?

A pilot light in a millivolt gas fireplace warms a sensor, which is generally a thermocouple (or) thermopile.

When heat is supplied to the thermopile and thermocouple, electricity is generated. This low voltage is sent to the wall switch.

The fireplace receives electricity from the switch and turns on the flame.

After a certain period, the connections inside the switch might become rusted, corroded, or destroyed, leading the switch to begin losing this voltage.

As a result, the minimal amount of power supplied back to the fireplace is insufficient for signalling the fireplace to turn on.

After checked out the possibility of a faulty wall switch & a faulty pilot flame, it is necessary to check the thermopile.

When heated by the pilot flame, the thermopile produces voltage in the same way as a thermocouple does. It can measure the voltage produced by the thermopile using a digital multimeter.

Use of a fireplace as an example where need to examine the status of a thermopile. The thermopile leads are tested using a multimeter.

They are linked to the gas control valve, so the first step should be to locate the valve.

The primary control valve is usually positioned behind the bottom grill of the fireplace.

The thermopile sensor is located underneath the primary control valve. A wire will connect the pilot assembly to the main control valve.

It is constructed from of two wires, each of which is typically covered and shielded by the metal sheath. At its terminus, the thermopile wire typically has two separate leads, denoted by the colours red and white.

The main valve will be equipped with thermopile leads. It may measure the voltage on the thermopile’s terminals using a digital multimeter set to DC millivolts. On the meter, the DC setting will either state “DC” or display a symbol.

How to Select a Thermopile?

When selecting thermopiles, the type of metal is an important factor to consider. As a result, depending on the material type of the sensor, thermopiles with base metal may monitor a varying range of temperatures. Each kind or classification denotes a certain metal and temperature range.

  • Type-E (chromel/constantan) has a temperature range of -270° C to 1000° C.
  • The temperature range for type-J (iron/constantan) is from -210° Celsius to 1200° Celsius.
  • Type-K (chromel/alumel) is -270° C to 1372° C.
  • The temperature range for Type-N (nicrosil/nisil) ranges from -270° C to 1300 ° C.
  • Type-T (copper/constantan) functions from -270° to 400° C.
  • The temperature range for Type-B (noble & refractory metals) is 0° C to 1820° C.
  • The temperature range of Type-S (platinum 10%/rhodium) is from -50° C to 1768° C.
  • The Type-R (platinum 13%/rhodium) temperature range is from -50° C to 1768° C.
  • The temperature range of Type-W (tungsten/rhenium) is from 0° C to 2300° C.

Advantages of Thermopiles

  • It is not need to use an external power source with it.
  • It provides a consistent reaction to the DC radiation that has been removed from the temperature-measuring bodies.
  • It has features of responsiveness that are consistent.
  • The Thermopile is a non-contact temperature sensing device that use infrared radiation as a means of transferring heat.
  • These are also offered in more compact sizes.
  • It has a lower overall price.
  • Because it makes use of a number of different thermocouple devices, it is able to produce a higher o/p voltage.

Disadvantages of Thermopiles

  • Because they have a static charge, any that have not been used should be kept in conductive material so that they are protected from static discharges and static fields.
  • These components are susceptible to damage when subjected to stress, which might result in the supply’s polarity being inverted.
  • These should not be directly exposure to moisture (or) sunlight because doing so may cause the device’s performance to deteriorate or be negatively affected by corrosion.
  • These objects need regular maintenance.
  • Dirty or greasy fingers should not be used to operate this object since the dust on your fingertips will impact the functionality of the device. To ensure optimal functionality, must clean the components using cotton swabs (or) alcohol.
  • If want an accurate reading of the temperature of an object, the field of vision of the thermopile should be totally occupied by that object.

Application of the Thermopile

  • Hand-Held Temperature Measurements without Contact
  • Temperature measurement without contact in industrial applications & process control
  • Tympanic Thermometers
  • Refrigeration Leak Detection Using Infrared Radiometry
  • Analysis of CO, CO2, and HC in Automotive Exhaust Gas
  • Anaesthesia, incubator CO & CO2, and blood alcohol analysis are all part of the medical gas analysis.
  • Alcohol Breathalyzers
  • Horizon sensors for planes, satellites, and hobbyists
  • HVAC and lighting control for commercial buildings
  • Scanners of thermal lines
  • HVAC control in a vehicle
  • Vehicle Occupancy Sensing
  • Measuring the temperature of household appliances
  • Fire and explosion detection
  • Tunnel fire detection
  • Aircraft Detection of Flames and Fire
  • Monitoring of blood glucose levels
  • Detection & early warning of black ice
  • Security Human detection and presence
  • Utilised for the purpose of detecting fires in aeroplanes.
  • The process temperature is monitored by it.

Some Useful Questions

1). What unique features does the thermopile possess?

Thermopile sensors are able to determine the precise quantity of thermal energy that is being radiated and produce a proportionate voltage using thermoelectricity. A thermopile sensor uses common connections and has the ability to monitor temperatures over a broad range. Thermopiles can often be purchased for a low price and may also be used in the form of a thermopile detector.

2). How is thermopile gets formed?

A thermopile is constructed using electrically coupled thermocouple junction pairs in a series configuration. The temperature of one of the thermocouple connections, referred to as the active junction that increases as it takes in heat radiation and becomes an active junction.

3). What type of material is thermopile made of?

Bismuth and antimony are often utilised as the thermocouple materials in thermopiles because of their comparatively high thermoelectric coefficients. A thermoelectric coefficient is a measurement of the magnitude of the induced voltage that is produced in response to a temperature differential.

4). What type of accuracy does thermopile have?

It delivers an accuracy of  ±0.2 °C in the range of 36°C to 38°C, which is acceptable for use in medical applications.

5). What is meant by thermopile sensitivity?

The output voltage of the thermopile detector is compared to the amount of uniform light that is falling on it to determine its level of sensitivity. The sensitivity that is defined by homogeneous incident light is not the same as the sensitivity that is defined when a light spot that has a tiny beam diameter compared to the photosensitive region is illuminated on it.

6). What is a thermopile’s temperature?

Thermopiles are able to identify any object or body with a surface temperature above absolute zero (-273.15°C) by detecting out the electromagnetic radiation that it emitted out. This radiation has a wide spectrum distribution that is affected by the surface temperature of emitting substance and is defined by Planck’s radiation law.

7). Why do connect the thermocouples in a thermopile in series?

In a thermopile, thermocouples are typically connected in series to increase the voltage output. Voltage output therefore rises with a relatively slight change in temperature. This enhances thermocouple accuracy.