I assume this is what is known as a "dorm room" refrigerator. And it has a compressor with an induction motor. I looked up what your power is in Chile, 50Hz, 220V for low volt single phase.
You can make your own thermostat, or you could purchase a simple one off eBay, like this
http://www.ebay.com/itm/All-Purpose-Temperature-Controller-STC-1000-Thermostat-Aquarium-NTC-sensor-220V-/251312628190?pt=LH_DefaultDomain_0&hash=item3a836661de
My search was ("STC1000", "STC 1000") (220v, 230v, 240v)
There are others that have PID control, but I purchased one, that was impossible to use, as the instructions were incomplete (not fully translated). The above one is very easy to use. It also has a built in delay timer for use with refrigeration so the power stays off for a programmable number of minutes so the pressure in the system can equalize and the motor can start with no load.
A refrigerator, does not need any sort of PID control, just a bang-bang (on or off) controller.
The only issue with the above thermostat, is it has a relay that most likely can not be used directly to switch power to your compressor motor. It does say 10amp@250V, but it does not have a motor rating power stated, it may work with out problems. If not, either a larger relay with contacts that support the motor current, or a solid state relay should be used.
You indicated that the PTC of your existing released gas when you opened it. Was there a copper tube connecting it to the electrical switch? I'm guessing this had a saturated refrigerant type working fluid.
Also did you mean "highest" temperature setting (versus lowest), as it never shut off, causing the temperature to drop below freezing of your cola. The release of this working fluid would result in the thermostat never shutting the compressor off.
Lots of nice lab work and measurements. Good luck.
The above one is very easy to use. It also has a built in delay timer for use with refrigeration so the power stays off for a programmable number of minutes so the pressure in the system can equalize and the motor can start with no load.
A refrigerator, does not need any sort of PID control, just a bang-bang (on or off) controller.
The only issue with the above thermostat, is it has a relay that most likely can not be used directly to switch power to your compressor motor. It does say 10amp@250V, but it does not have a motor rating power stated, it may work with out problems. If not, either a larger relay with contacts that support the motor current, or a solid state relay should be used.
Using a mains-class relay + and ATMega microcontroller + a DHT22 temperature sensor for intelligent temperature control. it's probably also possible to upgrade to the raspberry pi for Humidity control (add humidifier) and control over the Internet.
Hi, thanks. Yes you are right about the fluid/gas but that happened after I disassembled the refrigerator part (it was not working properly anyway). This is the thing that released the gas:
The STC1000 makes me question myself about if is really necessary to go for the AVR full homemade device solution or just trust these off the shelf solutions. The next step I am thinking to do is to extract 3 math relations (1: initial heat increase due cool down, 2: cool down, 3: motor off) using ambient temperature as a variable, to find a way to minimize the powered on time (in excel solver?).
1) How did you calibrated the delay timer? (it is needed for a refrigerator too?)
2) This motor is very small, back label says 70W, including the 15W lamp, so the motor should be very small. I like the SSR but they produce heat, so if I don't want to modify a lot the internals (I am thinking on reuse the same case that hosts the lamp bulb) it may probably affect the efficiency (and add more consumption?)
I buy them regularly. The genuine article is often $20 from the agents, who simply provide a $3 cheap thermostat in their own box ( or not, just put a sticker on the white box over the original part number) while a genuine Ranco is around $5, so I buy those. They often come with a universal kit, new spade terminals and covers along with a new knob and scales.
The simplest solution is what what hedgewallace and SeanB have indicated, just replace with same or equivalent thermostat. The last time I tried to fix a room window air conditioner, they wanted $130 for this. So I found a surplus thermostat I mounted externally. It should have been less then $25, but I could not find one anywhere for cheap.
Yeah hedgewallace and SeanB suggestions are the simplest solution, but I was really never happy with the stability of the original thermostat, also shipment time here is usually 45 days, so I will look for a replacement to order tonight but in any case I have time to try to fix it in a DIY way.
What is missing to develop the pid controller for the attiny (first simulated) is to understand the first phenomen, I can match the cooling process in my graph to this exponential solution:
But what is happening in the first slope? (when the temperature rises the first 30 mins?) should I consider that part of the cooling process? More data-collection is needed? (motor off time as one variable now)
No need for expensive very unreliable inverters, the regular rotary units have had impressive gains in efficiency over the years. Buy a good name unit and you will have both a lower cost of running and a lower power bill. Cheap units are $300 here for a 9000BTU One Hung Low Mystery manufacturer unit, and a good 12000BTU is $700 from Carrier. The 12000BTU has a lower power consumption than the 9000BTU one by about 15% despite the higher rating. As well needs piping added, while the cheapie comes with a premade 5m pipe and cable kit. The difference is in thicker copper, extra coils and a better compressor.
Here in the states, the local HVAC supply vendors sell for way more. Understand there's so much regulation, tax, lawyers, that all add to any business overhead. Don't worry, the whole world will soon be on the same page. O_o.
I always find it mildly amusing how Americans think that they have the most tax and red tape in the world.
Guys, yo do not. I absolutely appreciate how you are trying to cut back on the runaway regulation and taxes, and I'm sure this mindset allows you to, well, still be one of the best places in the world to do business.
Just check out Europe. You think 30% tax is too much? Try on 70% for size! Also, ever heard of ROHS?
Yeah hedgewallace and SeanB suggestions are the simplest solution, but I was really never happy with the stability of the original thermostat, also shipment time here is usually 45 days, so I will look for a replacement to order tonight but in any case I have time to try to fix it in a DIY way.
What is missing to develop the pid controller for the attiny (first simulated) is to understand the first phenomen, I can match the cooling process in my graph to this exponential solution:
But what is happening in the first slope? (when the temperature rises the first 30 mins?) should I consider that part of the cooling process? More data-collection is needed? (motor off time as one variable now)Are you making your temperature measurement directly to the evaporator surface, or to the air temp somewhere remote to the evaporator, or on a item like can of cola? I'm believing your measuring the evaporator temp. But really what is wanted is long term data from the warmest part of the refrigerator, when it's fully loaded with lot's of items, as these all stabilize the thermal mass long term, and provide the longest off time of the compressor. All a thermostat is doing is performing off on about a set point, and provide hysteresis, so you don't end up with short cycling of the compressor. Yes you could use a fancy control system, to overshoot the setpoint, if the intent is to cool the newly added warm items at the fastest rate. As items in or near the evaporator are cooled first, and the time it takes to remove heat from the further items from the evaporator, is dependent on how stuffed your frige is.But where a PID control is really used is where you can modulate some apsect of the control. Here you have a compressor that runs at a fixed speed from your 50Hz mains. You can't control the power into your system. And the real heat transfer rate is dependent on the temperature of the condenser. And this is a function of the room temp (and that's just a coil on the back of your unit, depending on mostly convective heat transfer, unless you put a fan on it), and the size of the condenser. You can only change the room temp. unless your going to make major changes to your system. I'm trying to get you to see that a PID controller is for systems where you can control or "throttle" some aspect of your system. In this case it would be heat removal rate. And that is "fixed" by your existing system. You can improve on the system control, but you need to have some sort of mass measuring algorithm, and it needs to know when you add items and how much specific heat needs to be removed. Your end goal was to reduce power consumption. But your stuck with your current system of too small a condenser, and non speed controllable compressor. And as I said before, more insulation is the most important system aspect to reduced power consumption so you make the off time very long, and the on time is to remove new heat load energy.The only easy solution is for a temperature switch, that you can control the cutin, and cutout temperature. This give you full control of the hysteresis, which is fixed in these mechanical thermostats.