Fridge thermostat. Does it require power to operate? Or purely mechanical?

Discussion in 'Analog & Mixed-Signal Design' started by boydage, Dec 25, 2018.

  1. boydage

    Thread Starter Member

    Oct 7, 2016
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    Hi all. Merry Xmas and all that. So I have a solar setup for my camping. Am running a small fridge. I have a wee problem, one I am going to resolve. But this is it, and hey, any better ideas from the community would be cool.

    Current setup is a 96AH deep cycle battery, 5 panels each giving on average 3-4A at 12VDC. My 120w fridge, when plugged in used approx 0.202kwh measured on the 230vac side over the space of approx 6 hours. I was curious how efficient it would be. Anyway. This is not my problem.

    My problem is, my inverter. Pure sinewave inverter is 2500W. Chinese. Cheap. Too large infact as well. It is drawing a full 1.6A full time. Over the space of night time, even without the fridge plugged in the inverter actually drains a reasonable amount of power. Now, unless the battery is at full charge and full capacity when the sun goes down, if I leave the fridge plugged in, when the fridge starts, for the split second of inrush start current, it is enough to beep the inverters alarm. Wakes everyone up. I just dont like it.

    Out of all the obvious solutions, replace the inverter, turn it off at night, etc etc.

    I am thinking I might like to control the inverter, on/off function by plugging into the fridge thermostat circuit. Ya know? Fridge drops below temp, click, turns on the inverter which in turn supplies power to the compressor, magic evaporation thing happens, cold fridge. Less of valuable power lost. No beep waking everyone up in the tent. Happy children. Happy wife....

    But before I do this. Anyone know if the thermostat in a modern fridge needs live constant 230vac to operate or is it a mechanical type which will switch on/off by mechanical means only. I can determine this by checking it out but if someone knows, please let me know.

    The rest I can sort. I have a bunch of different voltage SSrs. Although, I dont know if I should install a short delay between firing up the inverter and the compressor. Or just simultaneous. I dont think my fridge likes it even with a small voltage drop, and the inverter seems to start at 220vac momentarily before settling at 230vac..... Although I guess thats not too bad.
     
  2. geekoftheweek

    Active Member

    Oct 6, 2013
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    I'm no appliance expert, but the idea made me curious enough to poke around online a few minutes.

    From what I can tell from a little time on google looking at various schematics the only way to know for sure about the thermostat is find a schematic for your fridge. It should be on the fridge somewhere... if not a little digging should turn one up. Some thermostats seem to go directly to the compressor, some get power from a controller then to the compressor, some are wired only to the controller which then powers the compressor, and other variations.

    Is this inverter dedicated to the fridge only? If so would it be possible to add your own thermostat to turn the inverter on and off and keep the fridge thermostat at it's lowest setting and leave the wiring as it is? If it isn't dedicated and you decide to turn it on for something else keep your fridge in mind... I'm guessing you are planning on hooking in to a remote feature of the inverter.
     
  3. boydage

    Thread Starter Member

    Oct 7, 2016
    64
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    Awesome thanks for the reply. The inverter is 90% for the fridge. The rest I use power for is charging a laptop (which I could actually probably do with the 12VDC) and phones, actually I think my solar controller has 5v USB outputs.

    So the fridge is relatively 99% for the fridge. Ok. So I took a quick look at the fridge. The wiring diagram doesn't show more of the thermostat rather just as a component. But it is purely mechanical. Operates without any input power so I can just utilise it into an isolated 12vdc control circuit, which in turn can control the start/stop function of the inverter with a little SSR.

    I will put in an override function so the inverter can be utilised for any other appliance just by hitting a switch.

    The big question is, do I, may I, is it cool too just control the compressor on the fridge by switching the inverter on and off??? To me, it doesnt feel right. The inverter trying to start up with a 120w motor trying to start simultaneously connected to its output. Although, I think its only 0.8A start current. We are not trying to run a wood planner....

    What do you think?
     
  4. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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  5. boydage

    Thread Starter Member

    Oct 7, 2016
    64
    6
    Hi Nsaspook. I actually have a few of those in my box but have a feeling they are quite large 5A or bigger. What are your thoughts? To protect the fridge or inverter? Its a 2500w inverter 120w fridge. I am keen to know what you were thinking.
     
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I have a 2000W 120vac inverter that runs a 5000btu aircon in summer from solar power with 400Ah of battery. The compressor surge puts the inverter in overload without some sort of current limiting unless the bank is full charged. I plan to mod the compressor with a series thermistor so I can remove the line stabilizer I'm currently using to run the shop AC during the hot months.
     
  7. boydage

    Thread Starter Member

    Oct 7, 2016
    64
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    Aw yep. What is the size of the motor running the aircon? Also, the thermistors I have, although I didnt read the tech sheet you showed me, I bought because they pop and are non re-settable. Be nice if there was a soft start or way to smooth out your loads on a 1ph motor huh? But I only know of soft starts for three phase.

    Hey so I discovered there are two control thermostats in fridges today. Well thats in my head as I walk out of my workshop. There is one that controls the temp of the fridge, sends a signal down to the compressor. Then the compressor seems to have its own control, deciding to just run when it feels like it. I guess it needs to work when it does, only running to chill the lines. When the lines are chilled to max, not much goes on. But, it definitely appears there are two controls in a fridge. You know what? I am going to download a proper schematic of a fridge. One with EVERYTHING. Compressor as well. Not just the silly little ones on the back where the repair dude just replaces components and leaves without troubleshooting anything......
     
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    The aircon.
    [​IMG]


    It's like horses and water, the information is there.
     
  9. boydage

    Thread Starter Member

    Oct 7, 2016
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    Aw yep. I watched about 30 seconds of that - sorry. Just saying it works is good for me. Ok so that little component may be your solution? I guess its all down to if its reaction is quick enough to beat the sensing circuits inside your inverter before they fault yer?

    Current draw on start can be huge. Everyone I have spoken to say to multiply your normal running current by 6. So your 2000w inverter just may need to be upgraded? I was looking at them a little while ago and honestly the 4000w ones are what the 2000w ones were last year in cost?

    On the subject of current draw on start, I had a 5hp single ph motor here, they sent me a 60hz model instead of 50hz. I found it popping 40A C curve breakers only controlling it with a D curve 40A until a replacement motor arrived. Oh, we are 230VAC here.

    As for your thermistor - a different machine I was supplied had a 230V inverter installed inside a 400V machine. This is what happens when they receive too many electrons at once.

    Anyway as for me I am going to build a little time delay/on circuit for my fridge. Job done.
     
  10. geekoftheweek

    Active Member

    Oct 6, 2013
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    It sounds like you pretty much have it down.

    My only thought was if you are using the thermostat to turn the inverter on and off for the fridge, then decide to turn it on to power a USB charger, your laptop, or something else what is going to turn off the compressor when the thermostat kicks out? I'm kind of imagining two circuits... One to control the function of the inverter, and the second to control the fridge. It wouldn't be that hard to do and maybe add in a delay to ensure the inverter is fully up to charge and all before loading it down with the compressor. It seems you may be concerned about that.
     
  11. geekoftheweek

    Active Member

    Oct 6, 2013
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    As for the second "thermostat" in the compressor it's most likely a pressure switch. Long story short the refrigerant has to be held in a specific pressure range in order for everything to work.
     
  12. boydage

    Thread Starter Member

    Oct 7, 2016
    64
    6
    Thanks for confirming that Mr Geek. Great. Ok well to update, it works! Took a little time to build a simple 12vdc time on delay circuit. But, the fridge thermostat clicks, on comes the inverter, 30sec later the fridge compressor starts!

    Have a nice day.
     
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