Designing a TEC controller

Thread Starter

strantor

Joined Oct 3, 2010
5,694
Good evening,
I'm going to build a circuit to drive some TEC (Peltier) units in some used "climatic" (heated/cooled) Range Rover seats that I've purchased to put in my old pickup. This will be a "build thread" I guess, somewhere for me to document my ideas, ask questions, etc. and a historical record of any discoveries made that might help anyone else who chooses to follow the same path. These climatic seat modules are used in a wide array of vehicles and anything learned here should be applicable for most high-end vehicle seats anyone might want to retrofit into an older vehicle.

To start, ... I have too many questions, too many thoughts, and too many blanks spaces where thoughts should be. So I'll start by divulging what I've learned thus far, and I supposed I'll ask my first question at the end.

What I know so far:
  • The seat modules primarily consist of a brushless fan, a short duct, and a TEC with heat sinks mounted to both sides.
  • The duct from the fan splits just before the TEC and ~50%/50% split of the air flows over the top heat sink and the bottom heat sink. The air flowing through the top duct and over the upper heatsink flows up into passages in the seat (I call this the "climate controlled" side of the TEC). The bottom duct flows over the bottom heat sink and is exhausted right under the seat (I call this the "waste" side).
  • There is a NTC thermistor between the TEC and heatsink on the climate-controlled side, and no feedback at all on the waste side. By heating and cooling the module in my freezer and oven and taking measurements, I have deduced that it is a 1kΩ NTC thermistor with a B value of 3250 ± 3%.
  • Jaguar/Land Rover documentation I have found on the internet suggests that: [1] The max current for the TEC is 5.5A. [2] The climatic seat controller in a Range Rover (in the dash) uses PWM to power the TEC. [3] The Range Rover seat controller uses closed loop control via the thermistor only in the heating mode; In cooling mode it runs in open loop.
  • Through testing with a bench power supply, I have learned that the optimal current for seat cooling is well below 5.5A. With increasing current from 0A to about 3 or 4 amps, the readings from the thermistor indicate that the climate-controlled side is getting colder. Then as current is further increased, the thermistor indicates that it is getting warmer.
  • Testing of the opposite polarity with bench power supply indicates that the TEC is capable of getting very hot, even with the fan at max. Just a brief disconnection of the fan while sending 5.5A through the TEC is enough to send temperature well over 100C. This is a much more profound effect than the cooling.
Since the folks at Land Rover didn't see fit to put a temperature sensor on the waste side of the TEC, and since they chose to run the TEC in open loop for cooling mode, I assume that [1] they only care about not catching the seat on fire and the carpet under it be damned and/or [2] in cooling mode they are sending a fixed DC voltage of probably 5 or 6V (oops I mean 35-45% duty cycle PWM) to keep it in that "sweet spot" that I observed before the increasing current turns into decreasing cooling. And [3] this could only ever truly be a seat heater, with some barely perceptible cooling capacity if you reverse the polarity and barely feather its throttle.

I plan on running these TECs on fixed DC voltage, no more PWM. Except maybe for heating mode, since I don't think I have any reason to care about efficiency in that scenario.



So for my first question: is it actually safe (for the TEC) for me choose a fixed DC voltage for cooling mode, to run open loop, based on figures arrived at by testing in my air conditioned shop? From the white papers I've read (and barely understand) it seems that the greater the difference between hot side and cold side, the more heat will be generated by the TEC as it pumps heat. So my fear is that any design I contrive that works well in my shop, might fail catastrophically when I go to use it inside the sweltering interior of a truck that's been sitting in the Texas sun all day. ..except, since the differential exists between upper duct and lower duct, which are fed from the same fan, they should always be the same, so the ambient temperature should be moot. Or am I wrong about that?
 

crutschow

Joined Mar 14, 2008
28,177
is it actually safe (for the TEC) for me choose a fixed DC voltage for cooling mode, to run open loop, based on figures arrived at by testing in my air conditioned shop?
I see to problem doing that.
Note that a TEC cools more efficiently (transfers more heat) with a steady DC voltage as compared to using PWM, due to the greater I²lR loses the the TEC resistance with PWM.
since the differential exists between upper duct and lower duct, which are fed from the same fan, they should always be the same, so the ambient temperature should be moot. Or am I wrong about that?
I see no particular problem with that also.
It could develop frost on the cold side if the ambient is cold, but I assume you would not normally run it under those conditions.
 
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