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13 June 2009
Author: Giorgos Lazaridis
The Peltier Thermo-Element

History

The discovery began in the middle of 1821, where J. T. Seebeck discovered that two not similar metals, if they are connected in 2 different points and those points are held in different temperatures, there will be a micro-voltage developed. This effect is called the "Seebeck effect" as of it's discoverer.

Some years later, a scientist discovered the opposite of the Seebeck effect. He discovered that if someone applies voltage to a thermo-couple, one junction shall be heated and the other shall be cooled. The scientist was called Peltier and the effect called the "Peltier effect".

What is the Peltier thermo-element?

A Peltier thermo-element compared to a AA battery

A Peltier thermo-element is a device that utilizes the peltier effect to implement a heat pump. A Peltier has two plates, the cold and the hot plate. Between those plates there are several thermo couples. All those thermo couples are connected together and two wires comes out. If voltage is applied to those wires, the cold plate will be cold and the hot plate... hot.

The device is called a heat pump because it does not generate heat nor cold, it just transfers heat from one plate to another, and thus the other plate is cooled. It is also called a thermo-electric cooler or TEC for short.

Because TECs have several thermocouples, a lot of heat is transfered between the plates. Sometimes it can reach a temperature difference of 80 degrees Celsius or more!

What are Peltier elements made of?

Peltier thermo-elements are mainly made of semiconductive material. This means that they have P-N contacts within. Actually, they have a lot of P-N contacts connected in series. They are also heavily doped, meaning that they have special additives that will increase the excess or lack of electrons.

The following drawing shows how the P-N contacts are connected internally within a Peltier TEC:

Now, imagine tens or hundreds of those P-N material between two plates. The following drawing shows how can many P-N contacts exist in a rectangular area like a Peltier TEC.

You can see how the P and N material are connected in series together to implement a long strip of P-N junctions. The top plate is the hot plate and the bottom is the cold plate. When power is applied to the two wires, the heat will be transfered from the cold plate to the hot plate and thus the cold plate shall cold.

Peltier Markings

Sometimes, the TECs have identification markings on their face, just like the following picture:

In this picture you see the ID: TEC1-12709

• The first two digits shall be always "TE"
• The next digit shall be "C" or "S". "C" stands for standard size and "S" for small sized.
• The following digit is a number and indicates the number of stages that the TEC has. In our example (and the vast majority of TECs) is a one-stage TEC
• Right next comes a dash. After the dash, the 3 first digits indicates the number of couples that the TEC has inside. In our case it has 127 couples. If the couples are 2-digit, then the number has a leading zero, for example 062 for 62 couples.
• Next comes two more numbers that indicate the rating current of operation for the Peltier. In our case this is 9 Amperes
• Some times follows a "T" and three numbers. This indicates the maximum operating temperature for the TEC. For example, "T125" is 125°C rated.

• Peltier characteristic curves and operation

Peltier elements can give more than to 80°C temperature difference between their plates. But this is not a standard value. Actually, this would only be achieved in ideal conditions. The actual temperature difference (ΔT) is usually smaller. The specifications of a TEC usually shows the achieved temperature difference in conjunction to the power transfered in watts. The diagram should look like the following:

Looking the above diagram, we can calculate the temperature difference that will be achieved according to the power that the TEC will have to move across the plates. The power is measured in watts, but we actually talk about the thermal power. You can use our temperature unit converter to convert watts to your desired units.

You should not confuse the power of Peltier operation with the power that it transfers. It is most common that TECs are sold with the electric power indicated. A 125 Watt peltier may NOT be able to transfer 125 Watts of thermal power across the plates. Instead, it is most possible that it will draw 125 Watts electric power at max conditions.

Peltiers comes usually with the datasheet that indicates the performance curves of the device. Those curves are essential if you want to make your theoretical calculations for the optimal device operation.

The first characteristic curve for a peltier is the Temperature difference vs Heat pump capacity. This curve indicates the temperature difference to be achieved in order to pump specific power of heat. It may be one or more curves for different current loads. An example of such a curve is shown in the following diagram:

The above curves comes from a real Peltier and are not imaginary. What we could conclude from the above is that if we need for example to transfer 30 Watts of heat, then - with appropriate voltage as we will see right next - there would be created a temperature difference of 20 degrees and the TEC would draw as much as 3.02 Amperes.

The next characteristic curve is the Temperature difference VS voltage. With this curve, we can calculate the voltage needed to be applied on the TEC in order to achieve the appropriate temperature difference. Here is one -also real- characteristic curve:

Let's continue the example we started before. We calculated that we need 20 degrees temperature difference and 3.02 amperes to achieve the 30 Watts power transfer. How much voltage must we apply to the Peltier to achieve this? From the above diagram, it's easy to find that we need about 6.5 Volts.

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