# TEC 12706 not drawing more than 1.6A when powered by a 12V PC SMPS

I have recently bought TEC 12706 device from Sunrom.com; The specification says that the device can draw more than 6A current at 15.4V and internal resistance is ~2 ohms. But when I measured the internal resistance, the device shown ~6.5ohms and when I powered it using 12V PC SMPS, the device did not draw anything beyond 1.6A. I had tried connecting the device to my fully charged inverter battery (12v 120Ah) measuring a float voltage of ~13.5V. To my surprise, the device drew just over 2A max.

I was using a slightly larger heatsink with a cooling fan on the hot side and was keeping the cold side open.

1. The SMPS can easily supply 10A.
2. Heatsink was typically 3 times the area of the peltier device

What could be wrong in my configuration?

• A link to the datasheet would be useful – PlasmaHH Aug 13 '15 at 10:11

Thermo-Electric Cooler efficiency "is a function of ambient temperature, hot and cold side heat exchanger (heat sink) performance, thermal load, Peltier module (thermopile) geometry, and Peltier electrical parameters." And "A single-stage TEC will typically produce a maximum temperature difference of 70°C between its hot and cold sides. The more heat moved using a TEC, the less efficient it becomes, because the TEC needs to dissipate both the heat being moved, as well as the heat it generates itself from its own power consumption."

So a heat sink on the "hot side" alone isn't enough. The cold side gets cold quickly, having little thermal mass, and efficiency falls to a very low value. Also as the hot side warms up, each junction's internal resistance increases, drawing less current.

TECs are most efficient when both sides are the same temperature (and little heat is being moved.) The best overall working efficiency (in terms of pumping heat) may be up to about 13% of Imax. Beyond these currents, resistive heating losses increase greatly, and the device spends more power moving heat it generates itself.

• If you found these answers helpful, please "up-vote" the useful one(s) and select one as an answer. Thank you. – rdtsc Sep 13 '15 at 21:43

Neither are the specifications per se.

You say it can draw 6A at 15.4V. (Datasheet? Perhaps? Or at worst a link to exactly what, where how. But, really, preferably a datasheet.)

You may note that neither 12V nor 13.5V are 15.4V.

Even if a Peltier were a purely resistive device I = V/R very, very clearly shows you that it will then never draw the stated current, because you are not giving it the specified voltage. Period.

Now, to add, the Peltier is a semiconductor device, which has not got a nice flat line, like a perfect non-thermally-active resistor would have. This whole "resistance" thing is a pointless endeavour almost all 'simple'/discrete semiconductors, unless it is a stated differential resistance around its operating point. But I don't know that, because... datasheet?

Compare it to a LED or a diode, if you measure the resistance of a diode with a meter that forces 1mA you will get 650 "Ohm" in the forward direction and "infinite" in the reverse, because it just measures the voltage at a fixed current, and that current is super low.

(granted, a Peltier comes a lot closer to flat line than many, if not most, other semicon devices, so the comparison is a little bit off-base, but to make the point, I feel it is fair enough.)