THE HEATSINK GUIDE: Peltier Guide, part 2

Getting into the details

The second part of the Peltier Guide deals specifically with sizing peltiers and heatsinks, to fit a given application. Hopefully it will also show some of the problems in more detail, and help you judge about merits and tradeoffs when using peltiers.

The fact that you are still reading, and weren't scared off by the first part, shows that you have a keen interest in peltier cooling. If you are more interested in general information about Peltier elements, or in in-depth information about the theory behind them, you should definitely have a look at Melcor's excellent Thermoelectric Engineering Handbook.

This article here mainly focuses on how to apply peltier cooling to PC processors or graphics chips; it is not as general as the information you can find on the websites of Peltier manufacturers.

First, a word of warning: Read the disclaimer before proceeding. If you damage something while following the instructions here, I cannot be held responsible. Do not supply power to a peltier element without a heatsink, after a while it will overheat and the connectors will melt.

In the last part of the guide, you will find an Excel spreadsheet with VBA code that will help you with the necessary calculations for designing a peltier-based cooling system for your PC, but I ask you to please read the article first before you download the spreadsheet.

Peltier Performance

TEC graphOne thing we must consider is that a thermocouple will always be a thermocouple - and thus when you apply a voltage and get a temperature difference - you will also cause a back voltage created by the Seebeck effect. This is very similar to the back EMF created within an electric motor - and thus much like motors TECs show a negative linear load dependent output curve. The other thing that happens when a voltage is applied across the TEC unit is that current flows through the TEC. This causes internal heating through I2R losses. This is a very important fact because this imposes a lot more heat on the heatsink to cool - we will get to that later.

A performance curve from Tellurex is shown here at the left. This, by the way, is the same curve that Joe over at shows in one of his articles.

The curve shows heat pumped versus temperature difference achieved across the peltier for 3 different current inputs. I find this plot not particularly easy to read. The main problem I have is that the information is presented at constant current, whereas PC freaks are likely to have a constant voltage source available. The other thing that is not shown is the power generation from the TEC itself - you can however glean this information from the voltage and current. I have rearranged the same information into another chart I find more usable.

Qmax vs Tmax

This chart shows the same TEC as above - but only at 12 volts. The left-hand vertical axis is for both temperature difference (C) and also for total power to heatsink (watts). The right-hand vertical axis is for current(amps). The first thing you'll notice is that the independent axis is power transferred (CPU power). The next thing you might notice about this format is that you can immediately see the amount of power transferred to the heatsink as a function of the amount of power transmitted by the TEC. For example at 15 watts of heat transfer across the peltier element another almost 30 watts of heat is added by I2R losses to make nearly 45 watts transferred to the heatsink. This illustrates that TEC applications add a lot of "overhead" heat to the total system, as it was already pointed out in the first part of this guide.

Go to part 3 of the Peltier Guide (or return to part 1)

Note: This article was originally written by "Bo", a visitor of The Heatsink Guide, who wishes to remain anonymous, and only slightly modified by me. Thank you Bo for sharing this article with us!

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