Control Freezer/Fridge that has no thermostat (microcontroller)

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Alderley, Brisbane
Hi everyone,

I'd like to use something simple like 2x STC-1000 to control the compressor and damper/fans in my side-by-side freezer/fridge. The goal is to have the freezer at ~3deg C, and the fridge at 15-18 deg C.

I was hoping to replace the existing thermostats to achieve this. My problem is that my freezer/fridge does not have thermostats. It has thermistors that supply information to a microcontroller, which in turn controls the compressor and the damper/fans. More info down the bottom of the post.

Does anyone have any experience wiring up an STC-1000 or any form of control to a freezer/fridge with microcontrol like this?

If possible I'd like to avoid turning the whole freezer/fridge on and off as the method of temperature control. I'd prefer the built-in method that keeps the fridge on all the time (along with all it's lights, bells and whistles), but takes control of the compressor and damper/fans.

One way I thought might work is to add the STC-1000 cooling relay in line with the compressor power wire (effectively adding another switch for the compressor). For the compressor to run, the internal microcontrolled relay and the stc-1000 relay would both have to be set to ON. I figured since the existing freezer thermistor will be reading +3 deg C constantly, it will be trying to switch the compressor on all the time anyway. Then when the STC-1000 decides it is time to cool, it will complete the compressor circuit. Similarly for the fridge thermistor and the damper/fans.

I have a few questions that I was hoping someone smarter than me could answer:

1) Is there a better way to do it?

2) Is there a problem with having the microcontroller always wanting to turn the compressor on? (ie. is there a problem with constantly supplying the compressor "on circuit" on the control PCB with 5V control voltage).

3) Is there usually feedback from the compressor to tell the microcontroller whether it has been successful in turning the compressor on? My worry would be that the fridge wants the compressor to be on, the STC-1000 wants the compressor off, and the fridge decides there is a compressor error and shits itself.

4) Is there anything else that normally comes on with the compressor? Since most components look to be individually controlled by the microprocessor, cutting power to the compressor with the STC-1000 relay may not turn off the other bits and pieces that would want to come on with the compressor.

More info:
Fridge is a Samsung SRS584DP.
Wiring diagram photo attached.
Service manual for a "similar" model attached, to explain some of the circuit logic. Not the same circuit, but some of the same bits and pieces.
There is some more information for a similarly controlled fridge if you search for the American model number RS21DANS1/XAP.

I truly appreciate any help that anyone can give. I've spent days trying to find other examples of people controlling a fridge that does not have thermostats, and haven't found much useful information at all.


Samsung SRS584DP RS21DANS1XAP .jpg

View attachment Samsung RS21DANS1 XAP (pp15-25).pdf


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1) there probably is a better way, but what you are planning is simple and should work fine.
2)No, not likley.
3)Depending on how smart the fridge is. There may be something in the logic that signals a problem if the compressor is running (or supposed to be running)for x amount of time and the fridge isn't at temp. In saying that I think it is unlikely. Generally speaking manufactures will keep things simple as over complicating it just leads to unwanted problems.
4)Most of the other stuff looks to be to do with the ice maker, and since your not interested in that I don't think it would be a problem.

If I were you, I'd disconnect the compressor and run it for a while and see if the relay stays in, ANC what else comes on... That will answer 3 and 4 for sure.

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