Goal: control Peltier plates from Ardruino using MOSFETs on a power shield with AC/DC adapters. In initial tests with the pictured wall adapter the plates get very hot but then start to cool back to room temp and then don't function.
I thought I had a good match in terms of voltage and amps to power these plates. Would I be better off to go to a DC power supply with adjustable volts and amps?
Have you measured the power supply output voltage? It may be significantly over 12V with a light load.
groundfungus:
Have you measured the power supply output voltage? It may be significantly over 12V with a light load.
His peltier has 5.8A current draw at 12V. I wouldn't call peltiers "light loads" 
Goat, are you keeping the hot side heatsinked? Because you can certainly fry a peltier if it gets too hot.
groundfungus:
Have you measured the power supply output voltage? It may be significantly over 12V with a light load.
It looks like a regulated supply to me so it should be OK.
frostygoat:
In initial tests with the pictured wall adapter the plates get very hot but then start to cool back to room temp and then don't function.
How hot, exactly?
The datasheet says 50-60 degrees is the operating temperature: http://www.sos.sk/a_info/resource/i/TEC1-12706.pdf
Have you tried measuring the temperature?
You are probably overheating the Peltier module, which melts the connections inside. You MUST cool the hot side with an effective heat sink (e.g. finned radiator or by water cooling).
Most readily available Peltier modules are destroyed if any part exceeds about 80 C, but you can buy high temperature versions (often used as thermoelectric generators) that are intended to function at over 200 C. Marlow Industries is a good source.
The first Peltier element I ever played with back in the 80s was a disaster. It was rated as 12vdc so that is what I applied and I watched that in less then 1 min it completely unsolder itself into goo. Lesson learned is that peltier usage requires you know and understand the heat management needed for a specific application.
There is a maximum temperature difference you can have across the two sides of a Peltier. If you exceed this then it starts heating both sides and burning up. So you need to make sure you have not got too greater thermal gradient.
And you MUST have a heatsink on the hot side. Period.
They are rather sensitive to voltage, too. A little too much voltage can mean so much excess heat in the peltier that it overheats rather than cooling.
polymorph:
And you MUST have a heatsink on the hot side. Period.
No. As long as the thermal gradient is below the limit there is no need. But you need to limit the current or turn it on and off to maintain this.
A 5A supply on a 5.8A peltier seems a recipe for disaster.
polymorph:
A 5A supply on a 5.8A peltier seems a recipe for disaster.
Only if you leave it on all the time.
No. As long as the thermal gradient is below the limit there is no need.
Not true. If ANY part of the Peltier module exceeds the maximum temperature, the connections will melt. Even if the thermal gradient is zero.
jremington:
No. As long as the thermal gradient is below the limit there is no need.
Not true. If ANY part of the Peltier module exceeds the maximum temperature, the connections will melt. Even if the thermal gradient is zero.
What do you think a thermal gradient is? It is the difference between one side of the device and the other. If you have hot spots on one side then that hot spot those hot spots will define the gradient.
To do it properly you need a thermal sensor and use PWM to control the temperature.
Since he's experimenting, I stand by what I said: Never run it without a heat sink on the hot side.
Since he's experimenting, I stand by what I said: Never run it without a heat sink on the hot side.
I agree completely. And don't overheat the less expensive modules using them as thermoelectric generators -- there are custom TEG Peltiers for that purpose.
While I'm on the subject, same goes for the power supply. Maybe after it is done and confirmed that the software will never allow more than 75% duty cycle, you can use a 5A power supply.
I'd not even run a 5.8A peltier on a 6A power supply. Too close to the limit. I have no idea how robust that power supply is, or how it responds to overloading.
Frosty as others pointed out... one side needs to be heatsinked otherwise the hot side overpowers the coolside the thing cooks...
Slap it to a big juicy block of metal bigger the better.
I'm intending to sink the heat from the Peltier into an aluminum bar and cool the other end of the bar with a reversed Peltier to create a thermogradient for biological application. Two realizations have come out of this thread:
I'm burning those plates out by leaving them on. I assume a resistance measurement on the voltmeter will likely confirm this but I haven't done this yet. I had the bar in place above the elements and aluminum heat sink plates below during the testing. A big part of my problem was that I didn't realize that the power rating of the Peltiers was the limit, I assumed it was more of an optimal power guideline. .
I need lower power source as I'm planning for the thermostat feedback to come from the bar itself and not the plate. The plates were burning out before they could heat the bar to a temperature that would be in my target range. 30 C to 10 C would be ideal for the thermogradient along the bar. So with the temperature controls in place, the plates would fry before the temp sensor could reach the threshold required to switch off the Peltier. I think an adjustable power source might be advantageous here as it might take some trial and error testing to get the power settings right. It will take perhaps 1/2 hour or more to get the plate up to temp. I estimate I was losing my plates within 2 minutes with power source pictured..