Peltiers revealed

In this post I will try to explain how Peltier elements work.

A Peltier element is a small device which basically transfers heat or cold from one side to other side.

In the industry they are widely used to cool laser modules because the wavelength depends of the temperature.
And since they are becoming cheaper and easier to get also computer enthusiasts are using them to cool their pc in the hope to get better performance.

How do they work?

A Peltier element, also called a thermoelectric module or cooler (TEC), is a semiconductor-based electronic component that functions as a small heat pump.
By applying a low voltage DC power source to a Peltier element, heat will be moved through the module from one side to the other.
One module face, therefore, will be cooled while the opposite face simultaneously is heated.

It is important to note that this phenomenon may be reversed whereby a change in the polarity (plus and minus) of the applied DC voltage will cause heat to be moved in the opposite direction. Consequently, a thermoelectric module may be used for both heating and cooling thereby making it highly suitable for precise temperature control applications.

Controlling a Peltier element

Thermoelectric coolers are DC semiconductor devices. All their standard specifications (performanceparameters) are given by manufacturers for DC current and voltage.
In many applications customers prefer to use PWM control. It means simple electronics and simple logical control.
At high frequency PWM the averaged current delivered to controlled object is estimated as equivalent of DC current of the same value.
It is true for many applications, but is a great mistake in a case of TECs.
TEC controlled by PWM operates less effectively than at DC current.
The PWM control is always less effective than TEC operation at the same average DC current and power consumption.

Neither are relays suitable to control Peltiers, these will cause a reduction of the TEC’s life time and you can’t exactly control the temperature of them, it will fluctuate too much.

DC Closed loop control is the best solution for TEC operation. It provides most effective use of TECs.In practice two control methods are in use: “Proportional-Integral” (PI) Control and“Proportional-Integral-Derivative” (PID) ControlFor most applications PI Control is enough.These PI(D) controllers can be implemented in software or done by discrete components (see my previous post)

Another important design note is that Peltier elements must always be cooled on their “hot”-side.
Otherwise you run the risk that you damage the Peltier by melting the internal solder.

Sources:
http://www.tec-microsystems.com/EN/Intro_Thermoelectric_Coolers.html
http://www.memmert.com/tekhnologii/koncepcija-temperaturnogo-kontrolja/sistema-nagrevaokhlazhdenija/
http://www.rmtltd.ru/tec_app_tips.htm
http://www.silram-cor.co.il/gp.asp?gpid=56