PowerChill Thermostat

Author: Dave
Date: 07.21.10 - 7:26pm

Coleman has a 40qt thermoelectric cooler that is designed without a thermostat. The Peltier element basically just runs all the time and will bring the cooler down to about 40 deg below room temperature.

I have been looking for a way to keep my cigars at 70deg this summer, and keeping the AC on my bedroom is to much. Sooo..I figured if i could control one of these coolers with a thermostat I would be set.

The retrofit was kinda fun. I dont work with electronics much. First i measured the heatloss of the cooler. With a thermoter inside and it off (vertical orientation) the cooler warmed by about 12deg an hour. When plugged in, the cooler cooled at 12deg an hour. So the cooler must be able to suck out 24deg of heat from the inside per hour. This led me to estimate that once on a thermostat, the unit would be running about 50% of the time to maintain a set temp.

So the next is how. I picked up a cheap thermostat at homedepot for $11. It says on the pack, can not be used to control a cooling system. For heating system use only..hummm ok. So i check out the circuit with my contunity tester and find that it completes a circuit when it is too cold. I need it to connect a circuit when it is to hot. With a little fussing I was able to solder up a low power relay board to invert the signal.

The low power thermostat circuit is driven by two AA batteries internal to the thermostat. I didnt want them in the humid cooler and where they would be a pain to get to, so i desoldered the battery carriers and ran wires out of the cooler with the temp wires and remounted them on a control panel. This 3v circuit operates the thermostat only and sadly wasnt enough to run the exernal relay board. For that i dug out an old 5v laptop supply which did the trick. So the low power relay board feeds into the inputs of a high voltage 120v AC relay board i had on hand from cnc4pc.com. This board is wired to turn a standard AC plug on and off which the cooler draws its power from.

Not the best solution, but except for the thermostat it was all scrounged from surplus parts i had laying around. I have to also say I am not an electronics guy so can not comment on the safety or efficiency of this design. Tinker at your own risk.

So in the vertical position, the cooler ran for 8 minutes and was off for 10..which is pretty close to my estimates generated by cooling/heating calcs. In the horizontal position this is increased to on for 8 off for 16. (lid seal must leak that much when vertical.) In the horizontal position, the cooler is maintaining temp within about 0.3 degrees. Vertical it was worse because the thermostat was at the top and there was no internal fan to circulate.

For observation, i put a wireless thermometer in there that is part of a Weather Channel WeatherStation. (designed to give you an outdoor temp reading). So i can remotely see the temp of my cigars from my living room.

The cooler does generate condensation, however so far (9hrs) there has been 0 impact on the cigars which are still at a perfect 70% humidity. (Cigars are in a small humidor with about 4oz of beads)

Anyway, it was a fun little project and its satisfying to see it working so well. Will see how the battery life is, they said 11mos for conventional system. And will see how the cooler deals with all teh on/off cycles. Wish i had taken a pict of the bottom of the relay board i made, it was puuurrdy.

Update: While i did this with all just stuff i had on hand, its kind of a shame it so bulky and bit of a kluge. I did a little more playing and think i am going to switch over the design.

The cooler actually runs off of 12v dc with a cigaratte lighter adpator plugged into a wall converter. I was going to use this for travel in the RV so current design works but wont do any good while driving. So i think I am going to switch over to turning it on and off on the 12v circuit instead of the AC circuit.

I will remove the 3v battery circuit and the 5v logic circuit, and instead put in a 12v to 3v voltage convertor ($14) Then all i have to do is find a relay that can take 12v 5amp on its contacts and ideally a coil which can be triggered with 3v then i can run both the relay and thermostat off of the converted voltage, and run the heavy 12v cooler load only through the relay.

Much smaller profile, more utility, probably safer, no batteries or redundant power sources and only have to order about 20 bucks in parts. This version of the mod should be able to be wired by anyone (no soldering required) and come to a grand total of about 30 bucks.

Comments: (5)

On 08.03.10 - 6:47am Dave wrote:
So, a real simple design would be to just use an auto relay and run the full 12v straight through the thermostat. It does work and only takes a couple crimp connectors and a cigrette lighter extender to pull off, but the power supply puts out 5 amps, and the cooler is fused for 7amps. The traces on the thermostat board are not that hefty and its internal relay is only rated for 1amp. I didnt measure actual current draw, but pretty safe to say its going to be over 1amp. Looks like no matter what (at least with this thermostat) its going to need low power and a high power circuits. CNC4PC.com does have a high power DC relay board controlled by low power inputs. And I can probably rip apart an old cellphone charger to get the 12v to 3-5v converter.

On 08.05.10 - 5:49am Dave wrote:
So I ran a test today using just the thermostat and an auto relay, running the full 12v through the thermostat relay. While this is the simplest wiring you can get, its just not up to the task. I measured the ampage and the cooler draw 3.8 amps. The thermostat relay is rated at 2amps and the traces unknown rating. But after about 10 minutes of running, the thermostat switches itself off. This could either be the microprocessor in it detecting an overload, or it could just be the heat it is generating skewing the temp sensor. Either way, looks like it is going to need a low power switching circuit for the thermostat.

On 08.05.10 - 7:44am Dave wrote:
ok one more..Its stupid to have a second relay board. This can be eliminated if you modify the operation of the thermostat relay. One input pin was connected to the relays common terminal. The other to its Normally Open (NO) connector. To invert it, cut the trace to the pin switched on by the relay, and then solder a wire from the Normally Closed relay contact to the switched pin stud. Way better than making a second relay board.

On 02.08.12 - 7:14am Rick Starr wrote:
If you want an internal fan, RV stores have a small batty powered job (2 D cells) which run about 4-6 weeks before a change is needed. Cost about $15, about the size of two cigarette packs. (half that size if you eliminate the batteries and just run another small wire into the unit.) I has one in the regular RV fridge and it makes a significant difference evening out the variations. I also have one in the Coleman cooler and it helps there as well.

On 03.10.13 - 1:34am Dave wrote:
instructions on how to remove the ColeMan PowerChill Peltier element can be found at the bottom of this post

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