Thermostat question

Apologies if this is not the correct forum.
Thank you in advance for any advice / ideas.

I am working on an off-grid refrigerator (portable). I have built a unit that includes a working compressor, evaporator, and condenser. The only item I lack is a way to regulate the temperature (say between 32f and 50f).

I am thinking that the most direct way to regulate the temp is to use a thermostat or thermistor system that will start and stop the compressor.

Specs on the compressor: 24v 50hz (yes 50hz) 3 amps max.
Compressor made by Sawafuji.

On eBay there is a guy selling a 'universal thermostat'. Seller says it consumes no electricity and is basically a temperature sensitive potentiometer. The t-stat is rated at 120 v 6 amps which is way above my requirements.

I am thinking this should work....what am I missing?

Thanks Again


  • petertearaipetertearai Solar Expert Posts: 471 ✭✭✭✭
    Re: Thermostat question

    24 volts 50 hz ??? posibly more likley 240 volts 50 hz?
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
  • BB.BB. Super Moderators, Administrators Posts: 33,319 admin
    Re: Thermostat question

    An AC thermostat usually comes in several major types... There are mechanical types (metal strip, fluid filled bulb, etc.) that opens and closes a set of contacts. Usually with a 1-2C minimum temperature spread.

    Or you can have an electronic sensor plus some sort of relay or electronic switch for the AC power. These require a battery or AC power to operate (home thermostat is a common type).

    The mechanical are simple and require no external power, but are not that accurate (you have a few degree spread between on and off--usually OK for a refrigerator or freezer).

    The electronic type can be much more accurate, but require an power source to operate so may not work in an Off-Grid system unless they have batteries (this type of electronic thermostat (electronic remote sensor with a pair of AA batteries to control AC or DC power) seems to be a bit difficult to find for using in a freezer to refrigerator conversion project).

    Here is one electronic (no battery):
    Item # 3ZP77
    Electronic Temperature Control, Input Voltage 120/208/240 Volts, Switch Action SPDT, Sensor Thermistor, 2 Inch Long x 1/4 Inch Diameter, 8 Feet Cable

    And here is a typical mechanical type thermal switch:
    Thermostat,Remote Bulb
    Item # 2E834
    Remote Bulb Thermostat, Inductive Rating @ 120 VAC 8 Amps, Inductive Rating @ 240 VAC 5 Amps, Differential 3 1/2 to 16 Degrees Fahrenheit

    Depending on where you live, you can find switches already setup with a stacking AC power cord (plug into AC outlet, plug cooler into plug for beer brewing and such...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  •[email protected] Registered Users Posts: 7
    Re: Thermostat question

    Thank you for the insight:
    I have looked around at various kinds of snap disc thermostats (bi-metal as you mentioned). These seem to be designed for heat applications not refrigeration.
    I have only found NC or NO snap disc's that activate above 90F. Most snap disc thermostats are designed to operate a fan (such as a spill over fan).

    I have found a few bi-metal t-stats that are used for defrosting but seems that they have Open and Close temperature ratings that are the opposite of what I am trying to do. I would like to find a bi metal t-stat that opens around 35F and closes around 55F.

    I'll keep looking.

    By the way, the compressor is in fact 24v AC at 50hz.

  • BB.BB. Super Moderators, Administrators Posts: 33,319 admin
    Re: Thermostat question


    Are you looking for a particular form factor? Is the mechanical (bulb type) too big?

    There where some mechanical that had the open/close set points adjustable to 12F+ differential. Is there a reason you want such a wide temperature swing? If you are storing food--those wide swings tend to draw moisture from food quickly (aka freezer burn for frozen food).

    24 VAC is better for mechanical contacts vs 24 VDC (DC is much worse for arcing).

    Also, 24 VAC 60Hz (should work at 50Hz)--you should find some low voltage 24 VAC based electrical thermostats for HVAC (heating/ventilation/air conditioning) systems.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  •[email protected] Registered Users Posts: 7
    Re: Thermostat question

    Good point about the temp swings. Seems best to target a temp around 45F for a fridge. The size of the bulb is not an issue (unless we are talking a football). I searched bi-metal thermostats on eBay and could only find the defroster type which operate the NO NC opposite of what I am thinking.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Thermostat question

    If you can find a thermostat of any type that does what you want it doesnot matter if the contacts do the opposite to what you want .. Simply add another relay and have the thermostat contacts opperate it. If you use a DP DT or a SP DT relay you can have any combinatio you want
  •[email protected] Registered Users Posts: 7
    Re: Thermostat question

    Thought I'd share the results of my project.
    I converted a 150 quart Igloo cooler into portable refrigerator. The evaporator uses R134 refrigerant and is driven by a Sawafuji compressor.
    Nice to have when ice is not available. I got the evaporator down to 6 degrees F after the unit ran for about 1.5 hours. Used a Thermal gun and old fashion outdoor thermometer to measure the temp.

    Thanks for all your help and input.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Thermostat question

    Re thermostat question: Been there, done that. Bimetal type lasted a couple of years, but had to keep readjusting it as one of the metals rusted away faster than the other, then finally became inoperable. Had been thinking for some time re a house thermostat that also controlled air conditioning. Finally found something that works VERY well. White Rogers. Don't find a model # on it, but this is what it looks like:
    (on the link: WHITE RODGERS Item #5570651 Model #750)
    Now the full story as I recorded it at the time back in early October.
    Home Hardware has a White Rodgers thermostat, their SKU # 5570-651, will do heat, or cool, and has a mini relay rated 0 to 30 volts, AC or DC, up to 1.5 amps, which could be used to control a 12 volt relay, which in turn could control power to the freezer compressor, turning it into a fridge. Problem is, the thermostat only controls down to 45F, but that's no problem - - Remove the sensor thermistor from the board and add a few feet of small (speaker type) wire between it and the board in the thermostat, and ADD a 470K resistor ACROSS the sensor terminals on the board, which fools the thermostat into thinking that an actual sensor temp of 35F is instead, 50F. This puts it within range to control a fridge. Now all you have to do is use a thermometer in the "fridge" to check actual temp and adjust thermostat until you get the temp you want. Then note and remember the setting of the thermostat. It will be showing somewhere around 50F, while the actual temp inside the fridge will be more or less 35F. Only the sensor on the end of it's wire will be inside the fridge, the thermostat itself will be outside where it's dry, comfortable, and you can see and adjust it.
    NOTE: The 470K resistor will definitely NOT work with all thermostat makes and models, but with this one, it does work. Also, the sensor and it's wire connections will need to be protected from moisture. I did that with "Marine Goop", also a HH product :)
    NOTE! The thermister is VERY sensitive, so to prevent too rapid cycling, you may have to surround it with something to absorb and release heat slowly. I glued mine to a small block of metal.
  •[email protected] Registered Users Posts: 7
    Re: Thermostat question

    Thanks Wayne.
    My target temp is 44F. I have been testing the 'snap disk' thermostat for a couple of weeks. Seems to be in the ball park at around 46F. The 'snap disk' cycles the compressor off-and-on without any hysteresis (rapid off-on looping). Your suggestion about the thermostat is appreciated.
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