A thermopile is an array of thermocouples that produces an electrical voltage when its measuring and reference junctions are at different temperatures. It uses the thermoelectric effect, but a uniform temperature alone produces no net output.

A thermopile contains several thermocouples, each made from two dissimilar conductors. A temperature difference between its junctions produces a small voltage. Designers normally connect the thermocouples in series so their voltages add. Connecting matched elements in parallel can reduce source resistance and increase current capacity, but it does not multiply the voltage. Thermopiles serve in temperature, heat-flux and infrared sensors, as well as some low-power control systems.

How Does a Thermopile Work?

A thermopile converts a temperature difference into an electric voltage through the Seebeck effect, named for Thomas Johann Seebeck. Dissimilar conductors develop different thermoelectric voltages along a temperature gradient, so a circuit with junctions at different temperatures has a net output.

A thermopile is normally a series string of thermocouples. Each pair uses materials with different thermoelectric power, and the junction orientation makes the individual voltages add.

Thermoelectric power describes voltage sensitivity to temperature. Each thermocouple has a measuring junction and a reference junction. When the two junctions are at different temperatures, the circuit develops an electromotive force. A connected load can then draw an electric current, limited by the thermopile’s internal resistance and the load resistance.

For identical thermocouple pairs over a limited temperature range, the open-circuit voltage is approximately proportional to both the temperature difference and the number of series-connected pairs.

The sensitivity is described by the Seebeck coefficient, commonly expressed in volts per kelvin (V/K) or microvolts per kelvin (µV/K). This coefficient depends on the material pair and temperature, so it is not one fixed value across every operating range.

The diagram below represents two thermocouple pairs connected in series as a simple thermopile.

The two upper junctions are at temperature T1, while the two lower junctions are at temperature T2. If the Seebeck coefficient is treated as constant over this range, the thermopile voltage ΔV is approximately proportional to the number of pairs and to ΔT, or T1 – T2. A thermal resistance layer limits heat flow between the regions and helps sustain the temperature difference. Accurate work uses the device’s calibrated, temperature-dependent response.

Diagram of a differential temperature thermopile

    T1
    |\
    | \
    |  \
    |   \
    |    \
    |     \  ΔV
    |      \
    |       \
    |        \
    |         \
    |          \
    |           \
    |            \
    |             \
    |              \
    |               \
    ------------------
        Thermal
       Resistance
        Layer
    ------------------
    |               /
    |              /
    |             /
    |            /
    |           /
    |          /
    |         /
    |        /
    |       /
    |      /  ΔV
    |     /
    |    /
    |   /
    |  /
    | /
    |/ 
   T2

Adding more identical thermocouple pairs in series increases the open-circuit voltage for the same temperature difference, but it also changes the device’s internal resistance and available load power.

Matched thermocouple strings can also be connected in parallel. This less-common arrangement lowers source resistance and can supply more current to a load, while its open-circuit voltage stays close to that of one string.

A thermopile responds to a temperature difference, not to the absolute temperature of one junction by itself. Temperature measurement therefore requires a known or measured reference-junction temperature.

In a calibrated sensor, heat passing through a known thermal layer creates a temperature difference that the thermopile can detect. The measured voltage can then indicate heat flux, the heat-transfer rate per unit area. Converting voltage to heat flux requires the sensor’s calibration factor and geometry; voltage alone is not enough.

Some thermopiles use an infrared-absorbing membrane for non-contact sensing. Incident radiation warms the active junctions relative to reference junctions attached to the sensor body.

Infrared radiation covers wavelengths from roughly 700 nm to 1 mm. Any object above absolute zero emits thermal radiation, although the spectrum and intensity depend on temperature and surface properties. A thermopile sensor absorbs part of that radiation and measures the resulting junction-temperature difference.

Types of Thermopile Sensors

A thermopile sensor uses one or more thermopile elements to measure a temperature difference, heat flow or absorbed infrared power. Additional electronics can combine that signal with a reference temperature to estimate an object’s temperature.

Infrared thermopile sensors can measure without touching the target, which is useful for moving, inaccessible or hot objects. A contact sensor can still be more accurate or faster in a given application. Thermopile performance depends on calibration, emissivity, field of view, target size, distance, ambient temperature and the sensor’s time constant.

Thermopile sensors differ by pixel count, thermocouple arrangement, absorber, optical filter, package and signal conditioning. Common configurations include:

• Single-element thermopile sensor: One sensing pixel contains many thermocouple junctions. Active junctions sit on a thin infrared absorber, often a micromachined membrane, while reference junctions remain thermally linked to the substrate. The pixel produces one signal for the radiation collected across its field of view. Its range and response time depend on the specific design.
• Multi-element thermopile sensor: Several sensing elements share one package. They may use different optical filters or view different zones, as in infrared gas detectors with active and reference channels. Electronics can read the elements separately or combine their outputs, depending on the application.
• Array thermopile sensor: Rows and columns of sensing pixels provide separate measurements across a scene. The output forms a low-resolution map of absorbed infrared radiation. Successive frames can be processed to estimate an object’s position or movement, subject to the array resolution, optics and calibration.
• Pyroelectric thermopile sensor: Pyroelectric detectors and thermopiles use different sensing effects. A hybrid package may contain both, but it is not a standard thermopile type. A thermopile can respond to steady radiation, while a pyroelectric element responds mainly when the received radiation changes.

Applications of Thermopile Sensors

Thermopile sensors have various applications in different fields, such as:

• Medical devices: Calibrated thermopile sensors are used in some ear and forehead thermometers. Clinical accuracy depends on the complete instrument, measurement site, operating method, environment and compliance with the device instructions.
• Industrial processes: Non-contact thermopile instruments can monitor hot surfaces or processes where a contact probe is impractical. The selected sensor must match the target temperature, wavelength band, field of view, emissivity and response-time requirement.
• Environmental monitoring: Thermopiles can measure radiative heat flux, solar radiation and infrared signatures associated with fire. Filtered thermopile detectors also support nondispersive infrared gas measurement. They do not directly measure humidity or soil moisture without another sensing principle.
• Consumer electronics: Single pixels and small arrays can support non-contact temperature sensing and simple presence or thermal-zone detection. Their use depends on the product design; a thermopile alone does not provide face recognition or biometric authentication.

How to Test Thermopiles?

In some standing-pilot gas appliances, the pilot flame heats a thermopile that supplies millivolt power to the gas control. A no-heat fault can involve the thermopile, pilot flame, wiring, switches, thermostat or valve. Gas appliances present fire, explosion and carbon-monoxide hazards, so diagnosis and repair should follow the manufacturer instructions and be performed by a qualified technician.

A qualified technician can test a thermopile with a digital multimeter set to DC millivolts. The correct terminals, lead colours, access method and appliance state vary by model. Use the appliance service manual to identify the open-circuit and loaded test points before connecting the meter.

Operate the pilot and gas control only as the service procedure directs, then allow the reading to stabilise. Compare both the open-circuit and loaded readings with the exact limits for that thermopile and gas control. Published thresholds vary by model, so 650 to 850 millivolts is not a universal pass range. A low reading can also result from a weak or misdirected pilot flame, contamination, loose connections or excessive circuit resistance.

The diagram below is a conceptual layout. It does not replace the appliance wiring diagram or service procedure.

Conceptual diagram of a thermopile millivolt test

    Gas Control Valve
    |\
    | \
    |  \
    |   \
    |    \
    |     \  Thermopile Leads
    |      \  (Red and White)
    |       \
    |        \
    |         \
    |          \
    |           \
    |            \  Multimeter
    |             \  (DC Millivolt Setting)
    |              \ 
    |               \
    ------------------
        Pilot Light
       (Heats up the
       Thermopile)

Conclusion

A thermopile combines thermocouples, normally in series, to produce a larger voltage from a temperature difference than one thermocouple would provide. It does not respond to a uniform absolute temperature, and its useful current and power depend on the connected load.

An infrared thermopile sensor converts absorbed radiation into a junction-temperature difference and then into voltage. Estimating object temperature also requires the sensor-body temperature and a calibrated model that accounts for emissivity, field of view, geometry and environmental heat flow.

Single-element, multi-element and array thermopile sensors serve different spatial and spectral measurement needs. Pyroelectric detectors use a separate physical effect; they may share a package with a thermopile but are not another thermopile configuration.

Applications include non-contact thermometers, industrial surface-temperature measurement, heat-flux instruments, infrared gas detectors, thermal arrays and millivolt gas controls. Accuracy and response depend on the complete sensor system and its calibration, not on the thermopile alone.

Testing a gas-appliance thermopile requires a DC millivolt meter, the model-specific service procedure and the correct open-circuit and loaded limits. A reading outside the specified range calls for diagnosis of the pilot flame and the entire millivolt circuit before parts are replaced.

The central principle is simple: a temperature difference produces a small thermoelectric voltage, and series-connected thermocouples add those voltages. Reliable measurement or power generation then depends on materials, reference temperature, internal resistance, optics, calibration and the load.