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I'm no expert, but from playing with Peltier modules I understand that, for a constant temperature difference between the two sides, they behave almost as a resistive load, with an almost linear relationship between input voltage and consumed current.

So, assume I have power supply flexibility to run the things at any voltage I like. That means I can make them consume any amount of current I like, below the max. So a TEC1-12706 is rated for max 6 amps, and a TEC1-12703 is rated for max 3 amps. But if I run a 12706 and 12703 at different low voltages so as to make them each consume the same total watts, will they perform differently?

From what I understand, when you run an individual Peltier module at any lower voltage than its maximum voltage, two differences are:

  1. It is more efficient, in that it produces a lower ratio of waste heat compared to the useful heat pumped thru it.
  2. It can only sustain a smaller temperature difference between the two sides.

Do I understand those two points correctly?

If so, still I can't understand what current rating of Peltier I should choose. There must be a difference, otherwise they wouldn't bother to make all the different ones.

Basically, in practice, assuming I have complete choice of input voltage, when are higher-current modules better, and when are lower-current modules better?

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    \$\begingroup\$ I imagine it should be derivable from the datasheets. Can you provide links and then help us understand what you've already looked at and cannot understand well enough to answer your own question? (It's possible the datasheets don't provide enough info. But if so, you should be able to point out the tables/curves that you've looked at and have determined are insufficient, and the reasons why.) \$\endgroup\$ – jonk Jul 29 '20 at 2:07
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    \$\begingroup\$ Presumably the devices rated for higher maximum current have a lower resistance, and therefore probably more total junction area. The device with a lower resistance will be more efficient, since resistive losses will be lower. \$\endgroup\$ – user1850479 Jul 29 '20 at 2:48
  • \$\begingroup\$ @jonk China eBay devices don't have specific datasheets; they are all generic. So I am wondering in general about the purpose of the common variants. \$\endgroup\$ – Boann Jul 29 '20 at 2:55
  • \$\begingroup\$ @Boann I might tend to agree with user1850479's comment. But I also know I'm too ignorant on the topic to trust a hunch. You should buy one of each kind and test/verify. Let us know what you find. \$\endgroup\$ – jonk Jul 29 '20 at 2:58
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I am wrestling with the same question. From comparing data sheets for different current ratings for the same line of elements I can't find much difference in behaviour, except that you can push higher rated devices to higher currents. On lower currents they appear to behave approximately the same. So number and types of junctions is important for currents well below max, and maximum current is not important in that region. My guess so far...

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