Hi, I have a little experience with Arduino and electronic in general but I'm not too sure how to achieve this. My air conditioner thermostat can only go down to 17 degree Celsius. I'd like to reach 7 degree Celsius.
To do so, I'm planning to build a controller to take the ambient temperature of a room and to attach something (possibly a resistor) to the air conditioner temperature sensor. This "something" should generate enough heat (let say 20 degree Celsius) so the air conditioner will keep on running until the new temperature sensor reach the desired temperature and shut off the heat. Then the air conditioner will stop by itself.
Doing it this way will allow me to put that controller to any air conditioner without having to do any modification to the unit. I've already build something else dealing with one wire temperature sensor so the only problem I have right now is how can I generate that heat.
I thought of using a small bulb light, but I'm sure somebody can suggest me a more elegant way...
They make power resistors that can be mounted to a metal panel or heatsink which would in-turn heat that. You would need power to heat the resistor P = v**/R, so it might take a reasonably healthy power supply to get the heat you want (and keep in mind, you really, really don't want to start a fire). The current to the resistor could be switched with a power MOSFET connected directly to an output pin (this might take some research on your part to come up with the exact circuit (hint: resistor to V+ and FET drain; fet source to GND shared by you microcontroller; fet gate to control pin). It might be wise to attach a small heatsink to the FET although it should be fully on or off so won't dissipate much heat. Use a logic level Power MOSFET rated for the current you wish to pass through the resistor heating element, and rated for more than the voltage applied to it. Something like the International Rectifier AUIRLU2905.
Check Mouser or DigiKey for the power resistors (ad the MOSFET).
I agree with pwillard in his comment. The capacity of the air conditioner may limit the bottom temperature (and you might put an undo load on the air conditioner system by forcing it to the lower temperature), and I hope this isn't for a living space - that's pretty cold for most people.
Or just build your own thermostat or find one that's made for a refrigerator.
If it's always set atthe same temperature it should be pretty simple to build & program.... You may not even need a microcontroller if you don't need a digital display or if it doesn't have to be programmable... For many years thermostats were simple mechanical devices, and although you may not want to build a mechanical one, you might be able to get-away with a simple-electronic one.
And yeah... You might want to replace the thermostat with a switch or move the thermostat out of the cooled-space to temporarily fake it out to see how cold it can get (or how much temperature drop you can get).
Be very careful doing this.
Forcing the Air Con to cool well below its designed limits puts a heavy load on the compressor.
If its a modern Air Con , the compressor will simply shut itself down to protect itself from damage long before you get to 7 C.
An old one may sieze up from liquid refrigerant entering the compressors input from the evap coil.
I have to double the warning that others have mentioned. I also have to wonder if you spend a lot of time in hotels and the thermostat won't set low enough for you, I did and had the same thoughts.
I would think of using something like a small heat mat for animals inside an enclosure put over the thermostat, they are pretty safe from fire themselves. I would think that using 2 sensors and trying to regulate the inside of the compartment to 20 higher than the outside might be a safer way to try and regulate the temperature.
mauried:
Be very careful doing this.
Forcing the Air Con to cool well below its designed limits puts a heavy load on the compressor.
If its a modern Air Con , the compressor will simply shut itself down to protect itself from damage long before you get to 7 C.
An old one may sieze up from liquid refrigerant entering the compressors input from the evap coil.
I have to agree, you would be best researching how your AC works.
You could end up with components at below 0DegC, freezing.
Freezing causes stresses on the pump and plumbing, to the extent of burst pipes and condenser.
A good AC will not let this happen, due to programmed in safeguards, so your attempt will be futile.
If you could attain that level of cold, almost coolroom cold, it will cost you a fortune in energy.
You need a refrigeration unit.
Tom....
I doubt that this would work. Just lower the setpoint by 10 degrees and see that it takes hours to achieve this. Going to 7C would take days, if the air conditioner was even able to do this. How big is the space that you are trying to cool? If small, buy a stand alone freezer, and cut 2 holes in it for air-in and air-out. Put a fan and a movable damper of the holes for automatic closing when you hit 7C.
To achieve 7C in a room, you probably need -5C as a source of air.
Hi, I have a little experience with Arduino and electronic in general but I'm not too sure how to achieve this. My air conditioner thermostat can only go down to 17 degree Celsius. I'd like to reach 7 degree Celsius.
I think you have overlooked the fact that all air conditioners have a physical limit to how much heat they can remove from a room. It is pointless to have a thermostat that allows you to set a temperature that is beyond the air conditioner's capacity to remove heat. It is , after all, only a heat remover , (NOT a cold injector).
Heat pumps also have diminishing efficiency as the temperature difference between heat source and heat
sink increases. Air conditioners are basically heat pumps possibly with some condensation/moisture
management.
Your going to insulate a small room then cool it down to below 0 or close to it. These work if you get a big window AC 22,000 btu or bigger R22 unit are better then the 134A . You could keep meat in a small 5 x 8 room but there's more to this then set the thing to run longer.
The coil is only going to get so cold as is on most of these but you can make one that get's cold because of how a window unit is made it draws in heated air and dumps out colder air. But tricking it to go colder is more then changing the thermostat. At some point I'm sure you can guess the coil will ice over and then no more colder air.
The real key to making these things work colder is INSULATION.
Icing is really only a problem if the coil is below 0 deg and the door is opened regularly.
Getting to 7 deg should not be a problem.
azzabcd:
Doing it this way will allow me to put that controller to any air conditioner without having to do any modification to the unit.
Depends on the conditioner controls.
The link you provided gives details.
If there is no digital display it likely uses an electromechanical temperature sensor welded to the coil.
If you cannot remove it you are on a hiding to nothing.
The evaporator will remove the heat faster than you can put it in.
Only option then is to hack the existing thermostat or wire it out.
If you can remove it then just insulating it may be sufficient.
The room is around 8 X 10. It's already being used as a "cold room" (but it's not that cold in the summer). The insulation is pretty good. My wife is doing the flowers for 1-2 wedding during the summer so she needs the room to be cold only for a few days each time. This is why we don't want to invest in a real flower refrigerator...
Like in the link I've provided, I was also planning to put a temperature sensor on the coil to make sure it will not get close to freezing.
I have an old mug heater, you know to keep your coffee hot at your desk. I might try to use it to trick the existing temperature sensor of the unit, but I'm still wondering what they were using on that device to do it...
As my unit is having a mechanical sensor, I might as well completely remove it a and use a relay to control the AC...
azzabcd:
temperature sensor of the unit, but I'm still wondering what they were using on that device to do it...
That device depends on the AC being digital, i think it bypasses the existing controller entirely.
The old controller set to permanantly on with the coolbot controlling the power supply.
The instructions are not clear however.
