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Say I've got a peltier block that is rated at 15.2V and 6A, providing 90W of cooling power to one side.

Will I be able to operate it at 5V, and will it draw 2A, providing 30W of cooling power?

EDIT: sorry, error above. :P brain was not working. 5V and 2A would indeed be 10W instead of 30W above.

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No, because power is a product of the voltage and current:
15.2V * 6A = 91.2W
5V * 2A = 10W

You would need approximately 9V @ 3.6A, which would give you ~32W

Remember, peltier devices are not very efficient, so 30W of input will not give you 30W of cooling power.

Wikipedia cites Peltier devices at an efficiency of approximately 5-10%, which means that for a best-case application cooling a 10W heat load, you will need a 100W peltier with a heatsink that can handle the aggregate power, or 110W of dissipation to maintain the 10W heat load at ambient temperature (this is assuming a 0° C/W heatsink on the hot side of the peltier).

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  • \$\begingroup\$ Looks like the 90W numbers are wrong, since device is not efficient. Could it be that 90W is the total consumption figure when effective cooling is about 50% of this ? (by some fundamental physics law, I forgot which) \$\endgroup\$ – user924 Nov 11 '10 at 5:34
  • \$\begingroup\$ Peltier devices are normally rated by the power they draw, I believe. Your device is probably good for chilling a <5W load, or cooling a <10-20w load more efficiently than a heatsink alone. \$\endgroup\$ – Connor Wolf Nov 11 '10 at 5:36
  • \$\begingroup\$ @Rocket Surgeon, 5 or 10% of it, not 50%. \$\endgroup\$ – Kortuk Nov 11 '10 at 22:28
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Peltiers are almost always a disappointment. You will find that the fantastic headline figures they quote are always for when there is no temperature gradient across the device.

In reality that's rarely what you want (particularly if when you're trying to get something cold, rather than merely stop it getting hot). The device itself acts as a thermal short-circuit, busily conducting heat back from the hot site to the cool side.

I'm not saying they're useless, but ultimately your question is unlikely to be 'how much can I under-drive this device' and much more likely to be 'how much can I squeeze out of this (and the poor PSU) by running it at 120%'.

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