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Thermoelectric detectors
Thermoelectricity
The thermoelectric effect is the direct conversion of temperature differences to
electric voltage and vice versa
A thermoelectric device creates voltage when there is a different temperature on each side.
Conversely, when a voltage is applied to it, heat is transferred from one side to the other, creating
a temperature difference.
This effect can be used to generate electricity, measure temperature or change the temperature of
objects.
Because the direction of heating and cooling is determined by the polarity of the applied
voltage, thermoelectric devices can be used as temperature controllers.
The term "thermoelectric effect" encompasses three separately identified effects: the Seebeck
effect, Peltier effect, and Thomson effect.
1) Seebeck effect: The Seebeck effect is the conversion of heat directly into electricity at the
junction of different types of wire.
2) Peltier effect: When an electric current is passed through a circuit of a thermocouple, heat is
evolved at one junction and absorbed at the other junction. This is known as Peltier Effect.
The Peltier effect is the presence of heating or cooling at an electrified junction of two different
conductors
3) Thomson effect: As per the Thomson effect, when two unlike metals are joined together
forming two junctions, the potential exists within the circuit due to temperature gradient along
the entire length of the conductors within the circuit.
In most of the cases the emf suggested by the Thomson effect is very small and it can be
neglected by making proper selection of the metals.
The Peltier effect plays a prominent role in the working principle of the thermocouple.
i)Thermocouple
The general circuit for the working of thermocouple is shown in the figure 1 above. It comprises
of two dissimilar metals, A and B. These are joined together to form two junctions, p and q,
which are maintained at the temperatures T 1 and T2 respectively. Remember that the
thermocouple cannot be formed if there are not two junctions. Since the two junctions are
maintained at different temperatures the Peltier emf is generated within the circuit and it is the
function of the temperatures of two junctions.
If the temperature of both the junctions is same, equal and opposite emf will be generated at both
junctions and the net current flowing through the junction is zero. If the junctions are maintained
at different temperatures, the emf’s will not become zero and there will be a net current flowing
through the circuit. The total emf flowing through this circuit depends on the metals used within
the circuit as well as the temperature of the two junctions. The total emf or the current flowing
through the circuit can be measured easily by the suitable device.
The device for measuring the current or emf is connected within the circuit of the thermocouple.
It measures the amount of emf flowing through the circuit due to the two junctions of the two
dissimilar metals maintained at different temperatures. In figure 2 the two junctions of the
thermocouple and the device used for measurement of emf (potentiometer) are shown.
Now, the temperature of the reference junctions is already known, while the temperature of the
measuring junction is unknown. The output obtained from the thermocouple circuit is calibrated
directly against the unknown temperature. Thus the voltage or current output obtained from the
thermocouple circuit gives the value of unknown temperature directly.
The amount of emf developed within the thermocouple circuit is very small, usually in millivolts,
therefore highly sensitive instruments should be used for measuring the emf generated in the
thermocouple circuit. Two devices used commonly are the ordinary galvanometer and voltage
balancing potentiometer. Of those two, a manually or automatically balancing potentiometer is
used most often.
ii) Thermopile
A thermopile is an electronic device that converts thermal energy into electrical energy. It is
composed of several thermocouples connected usually in series or, less commonly, in parallel.
A thermopile is an array of several thermocouples connected in series. A thermopile with N
thermocouples will output a voltage N times bigger than the one produced by a single
thermocouple, increasing the sensitivity of the transducer. With enough elements in the
thermopile, a useful voltage can be generated in order to control another process. This type of
transducer is often used to measure heat flux.
Thermopiles do not respond to absolute temperature, but generate an output voltage proportional
to a temperature difference or temperature gradient.
A thermopile is an array of several thermocouples connected in series. A thermopile with N
thermocouples will output a voltage N times bigger than the one produced by a single
thermocouple, increasing the sensitivity of the transducer. With enough elements in the
thermopile, a useful voltage can be generated in order to control another process. This type of
transducer is often used to measure heat flux.
Thermopiles do not respond to absolute temperature, but generate an output voltage proportional
to a temperature difference or temperature gradient.
