brunnhilde's refrigerator control - part 3

 

The prospect to have a dedicated digital thermostat for the refrigerator and able to see the cold plate temperature (of the thermistor) got me excited with this little project. If done right it costs so little and I have fun along the way. I like little electronic projects like this that enhances the existing design, while being challenged to maintain the aesthetics and integrity of Westfalia's design spirit.

I use this digital thermometer to monitor the actually temperature of the refrigerator; I am looking forward being able to retire it and put it to use for monitoring the cabin and external ambient

It is all too easy to jump in and starting making modification. I wanted to ensure I will not jeopardize the Central Display Console nor the refrigerator. Doing damage to either can be very sad as well as expensive to correct. So I studied the Danfoss manual as well as the Westfalia's wiring diagram to understand how the interface works. Only when I feel that I am very certain that I understand it before proceeding to design my bench test.

The bench test is intended as a go/no-go test if this project were to be implemented. Only if it performs equally but brings additional benefits that it will be implemented. I also want to ensure the Central Display Console will behave gracefully when the control of the refrigerator is taken away from it.

this is the closest manual to the BD35F compressor used in the Dometic in Brunnhilde; what we don't have is the lamp power and the terminal contacts are a little different

I believe the rest is accurate within practical purposes

this are all the interface signals between the refrigerator and the Westfalia wiring; which includes the signals to the Central Display Console as well as the power and ground for the compressor

for the bench test I want to make it as simple as possible so I did not cut or splice any existing wires; I keep the connector connected so the compressor and the electronic module power and ground are maintained

I wanted to remove the CPT module as it has a relay inside which would become redundant as well as consuming a few watt of power when the controller calls for cold

I fabricated my equivalent of the CPT module san the relay

for now I only have the low battery voltage resistor and leaving the compressor to run at 2000 RPM

after I am very sure about my interface design that I set out to hook up the bench test

here the C, P, and T terminals are wired up

I place the Ink Bird's thermistor where the Westfalia is; I cannot reuse the Westfalia's as it is 0.5k while the one from Ink Bird is10k

bench test fires right up and the compressor is running

A few of the parameters will need to be established during this bench test. That includes  the dead band temperature spread and the compressor lock out duration. At this point I see no reason this project is not a go.

I also figured out how to keep the Central Display Console behaves gracefully that the fridge control is taken away from it. That means when someone try to use it to switch on the refrigerator, it will be apparent that the function is not supported by it. Naturally everything that controls the refrigerator will be done on the Ink Bird, including switch on or off the refrigerator without the need for an external master power switch.

Better modern camping through design with low cost China made DIY electronic modules. 😬