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Home >> Resources | Support >> How to Choose? >> 

How to Choose: TEC / Peltiers

How to Choose: TEC / Peltiers


by Cile Gray

You are encouraged to make links to this article from your website and tell your friends

The following article is based on years of experience. It is provided as a free service to our customers and visitors. However, Directron.com is not responsible for any damage as a result of following any of this advice.

Copying the contents for commercial purposes is strictly prohibited without Directron.com's written consent. However, you are welcome to distribute these computer support tips free to your friends and associates as long as it's not for commercial purposes and you acknowledge the source. You are permitted and encouraged to create links to this page from your own web site.


Have you every wanted to use a peltier to cool your system, to impress Lan-party buddies with your sub-zero cooling, but you have never had the courage to venture into the dangerous and unknown world of peltiers?

Since the beginning of my excursion into water-cooling and peltier super-cooling, I have seen many people lift an eyebrow when you tell them about putting water inside your computer. And they literally fall to the floor when you tell them that your system is hitting sub-zero temperatures. People seem frightened and traumatized by hearing the word "peltier". They take a peltier in their hands and look like they're touching space age electronic devices. This is not the case! This article will try to prove that peltiers are very simple to use, and do not require a degree in thermodynamics to safely utilize, but first, a brief description of the TEC, commonly known as the peltier.

TECs, standing for "Thermo Electric Coolers" are based on the Peltier Effect (hence the name) , discovered in 1834, by which DC current applied across two dissimilar materials causes a temperature differential.

The Cold and Hot side effect : The TEC is generally built by having Negative and Positive type semi-conductors made of bismuth-telluride, sandwiched between two ceramic plates. When current passes through the TEC, electrons jumping from the P type to the N type semi-conductors leap to an outer level of electrons, they absorb energy, thus "absorbing" heat. On the other hand, when they jump from the N type to the P type, they drop one level, thus releasing energy and heat. This is what creates the Cold and Hot sides of the Peltier.

Wattage: TECs measure their potential heat-dissipation in watts. Be careful though, a 156 Watt TEC will NOT dissipate 156 Watts of outside energy. To calculate outside heat dissipation implies complicated calculations having to deal with different Temperatures, Voltages, Amperages, insulation, etc, which is described in the advanced peltier studying article.

Delta T (Temperature difference between hot and cold side) : TECs work by pumping heat from the hot side to the cold side, but there IS a limit to this. Regular TECs have a Delta T (meaning the difference between hot and cold side) of about 68 to 70 degrees Celsius. This basically means that the coldest you can keep the hot side, the cooler the cold side will be. In order for this to work, your TEC must be able to dissipate all the heat produced by the CPU, or else both sides of the peltier will heat up and make your system run hotter than a CPU without a heatsink!

Condensation : If using a TEC that will drop the temperature below ambient temperature, there is extra care to be taken. Any air contacting a surface cooler than itself will condense, thereby creating small droplets of water that can kill your system. Before using high Wattage devices such as 120 or 156 Watt peltiers, be sure to have read and understood articles about preventing such condensation, and are aware of the risks.

If you feel that you understand the whole article, and are ready to learn more about TECs, and you are not scared by Math, read on. | Go to Top |

Some people tend to think that the higher Wattage your peltier, the better it is. They are right, but to a certain extent. How much exactly do you need? What if you want to cool only to around room temperature to avoid condensation? How much can you handle? I will explain how to choose the TEC that suits YOUR needs below...

Three factors influence the temperature you will be running at. These parameters are the hot surface temperature (Th), the cold surface temperature (Tc), and the heat load to be absorbed at the cold surface (Qc).

As a basic rule of thumb, you can estimate the temperature of the hot side by using one of the following three statements :

Natural Convection : 20C to 40C [Natural Convection refers to a heatsink not coupled to a fan, this will probably not apply to your system.]
Forced Convection : 10C to 15C [Forced Convection refers to aircooling your peltier with a heatsink and fan, like the Swiftech 360 for example.]
Liquid Cooling : 2C to 5C [Liquid Cooling is of course, in reference to water-cooling your system with a radiator and waterblock.]

Take the temperature from the list that applies to you, and ADD it to the current room temperature, and presto, you have instant hot-side temperature. For water-cooled rigs, the 2-5C rise is in comparison to water temperature, and not the room temp.

The cold-side temperature is easily obtainable, through the following operation. T = Th - Tc
This operation is then transformed to put the "known" variables together, and the Tc by itself. Th - T = Tc
[This formula is only applied when Vmax and Imax are present, with a room temperature of 25C]

Figure 1 : The Basic functionning of a TEC

Now you would like to know if your peltier will be able to pump out all your processor's heat don't you. Here is where it gets a little more complicated.

Estimating Qc, the heat load in watts absorbed from the cold side is difficult, because all thermal loads in the design must be considered. Among these thermal loads are :

The ambient air. Some of the watts of heat dissipated by the cold side will be used at dissipating the surrounding's heat.
· Conductive materials. There will be some sort of waste occurring from actually cooling the wires, connectors and screws.
· The amount of heat produced by the TEC getting current flowing through it.
· Lastly, if the TEC is not completely isolated from outside air, the dissipation characteristics will often drop.

Here is an example application of the formulaes to calculate if a TEC is right for you.

In this example we will be using the Melcor #CP 1.4-127-06L Module to cool a CPU that produces 30 Watts of heat, down to say 5C.We will water-cool that TEC, meaning the hotside will only gain about 5C from room temp (30C). Here are this module's characteristics. :

Imax (Amps) 6.0
Qmax2 (Watts @ 25C) 51.4
Vmax (Volts) 15.4
Tmax (C) 67

The parameter T follows directly from Th and Tc. Since the cold side of the thermoelectric is in direct contact with the object being cooled, Tc is estimated to be 5C. Assuming a 5C rise above ambient for the water-cooled system, Th is estimated to be 35C. Without knowing the power into the thermoelectric, an exact value of Th cannot be found. The following equation gives the temperature difference across the thermoelectric:

T = Th - Tc = 35C - 5C = 30C

Figures 2 and 3 show performance curves for the CP1.4-127-06L at a hot side temperature of 35C. Referring to Figure 3, the intersection of Qc and T show that this thermoelectric can pump 30 watts of heat at a T of 30C with an input current of 6 amps (Imax).

T = Th - Tc = 35C - 5C = 30C

Figures 2 and 3 show performance curves for the CP1.4-127-06L at a hot side temperature of 35C. Referring to Figure 3, the intersection of Qc and T show that this thermoelectric can pump 30 watts of heat at a T of 30C with an input current of 6 amps (Imax).


Figure 2: T vs. Voltage


Figure 3 : T vs. Qc


These values are based on the estimate Th = 35C. Once the power into the thermoelectric is determined, 2 equations can be used to solve for Th and to determine whether the original estimate of Th was appropriate.

The input power to the thermoelectric, Pin, is the product of the current and the voltage. Using the 6 amp line in Figure 2 for the current, the input voltage corresponding to T = 30C is approximately 15.2 volts.

Using these equations, Th can now be calculated.

Qh = Qc + Pin
Qc = 30Watts (Cpu heat)
Pin = 6Amps * 15.2 Volts = 91,2 Watts (electrical input for the TEC)
Qh = 30 Watts + 91,2 Watts = 121,2 Watts.

Th = Tamb + (O) (Qh)
Tamp = 30C (Ambient temperature)
O = 0.10C/watt (thermal resistance of heat exchanger, IE Coldplate and waterblock...value estimated here)
Qh = 121,2 Watts
Th = 30C + (0.10C/watt)*(121,2 Watts)
Th = 30C + 12,12C
Th = 42,12C

Now, since the Calculated Th is 7,12C higher than our anticipated Th of 35C, it means that the TEC used here will NOT be able to cool the CPU down to 5C. It will probably be more realistic to think that the Tc would be around 10C.

Conclusion
I hope that this article has helped you understand even more about peltiers, and about knowing WHICH one is right. I have learned alot while writing this article, and it is my goal to make you understand how to calculate a TEC's acceptable load, and how to estimate Th and Tc temperatures before having to even use the peltier.

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