Question about non-computer Peltiers

[JKeefe]

At work I repair and calibrate pharmaceutical instruments. I'm having a problem with one of these instruments - it uses a peltier TEC to cool samples, but the sample compartment will not cool.

The TEC is controlled by a circuit board inside the instrument. While I have access to this board, I don't really know what it does. There is a thermometer inside the sample compartment to provide feedback to the cooling logic.

What's odd is that the sample compartment will heat; even though it is usually used to cool, it has the ability to heat above room temperature, and will do so.

Any feedback on why this TEC will heat and not cool would be welcome. Do you think it might be the controlling circuit board, the TEC itself, or something else?

TIA

[matttheniceguy]

When you put current through a pelt, it acts like a heat pump transfering heat energy from one side of the pelt to the other. This makes one side get hot and the other get cold. There is no difference between the two sides though, so if you switch the direction the current is flowing, you will switch which side gets hot and which side gets cold.

My guess is that the control board has somehow been messed up, and is either sending the current in the wrong direction, or more likely sending the pelt voltage with such a high amount of ripple it is nearly AC. This would cause both sides of the pelt to heat up.

You can test the pelt by connecting it to a different powersupply. Try to find the specifications of the pelt. You should know its maximum voltage and it's current draw at that voltage, or it's power. Even if you can't find the specks you can still test it, you just have to be carefull. If it is a fairly good sized pelt (say 40x40mm) it can probably take at least 12V, but it will work well enough that you will notice it even at 5V. Connect the pelt to the 5V line of a computer power supply. These lines can handle a lot of current even on cheap supplies so you don't heve to worry about frying the supply. If one side of the pelt starts to get cold, then the pelt is working fine and the problem is elsewhere.

You can test the supply by connecting it's output to an oscillicope (spelling) if you have one. With the pelt connected to the powersupply, measure the voltage accross the pelt. It should be a fairly steady DC voltage. You could also try to get some info about the voltage from a voltmeter, but it won't be able to tell you much about the amount of ripple in the voltage.

Good luck.