Brew Fridge – Part VI: The Wiring
For the mechanically inclined, working in the electrical-sphere can seem a bit daunting. Though electrical engineers are little more than glorified electron plumbers, they still work in a space that demands total respect and attention. Don’t mess with electricity. I’ve worked in the 800 volt domain for over five years, but it’s imperative to retain caution around this magnitude of energy.
A fridge does not approach this level of danger, but it’s still a box filled with high pressure freon lines and 120 volt potential. The first rule of wiring work: Don’t attempt if you don’t know what you’re doing. The second rule: Never leave anything plugged in when you’re working on it. The third rule: Learn how a multimeter works and use it liberally. I’m using ITC-308 controllers for this project, so unlike many similar builds, I don’t actually have much wiring to do at all. Many of these projects integrate the controller (usually a Love TSS2) directly into their fridge circuits, at times leveraging the existing thermostat. I want mine to be simple and modular. This means ripping most of the wiring out of the fridge. Below is what I’m starting with.
The main harness comes through the back into the freezer and routes through the wall into the fridge. Various wires branch off to the coils, the ice maker, light bulbs, and door dispenser. The harness terminates into a bin on the fridge side that houses the thermostat, defrost timer, and fridge bulb.
The ITC-308 has an integrated temperature probe. I’m handling humidity with a simple moisture controller in the keg side, and the fermentation chamber is running at 60 degrees Fahrenheit most of the time. Additionally, I’ll be using LED strips for accent lighting down the road, and I’m building custom air exchangers between the two chambers. With all of this in mind, the major components described above must be gutted. By the end of this surgery, the fridge should see power from the wall equating to a constant compressor cycle. In other words, by plugging the fridge into the cold side of the ITC-308, it can then regulate the compressor using its own probe. I just have to dial in my range and compressor delay settings.
The thermostat is the most obvious place to start. It’s the main component I want to bin, and it’s essentially last in line in the circuit. The thermostat regulates air temperature by expansion or contraction of gas in the phial. This is the coil attached in the picture above. As the gas state changes, this operates a switch in the main body that closes the circuit to the compressor, turning it on until the temperature returns to set limits and creates another state change in the thermostat (off). The acceptable temperature range for these thermostat types can be fairly large, and not optimal for holding fermentation temperatures. It also follows that since this is essentially a switch, bypassing it completely should result in a constant duty cycle in the compressor.
There’s one more issue here: the defrost timer. When the freezer door opens or the seals begin to wear out, warm air is introduced to the cooling coils. This moisture in the air condenses and makes its way to the coils where it ices over. Over time, the efficiency of the internal air exchange degrades and the fridge side becomes warm despite thermostat control. Defrost is simply a system that heats the coils to prevent frosting at various intervals. The controller for this is an electro-mechanical relay that operates on a mechanical timer. This timer is a non-concentric gear that opens or closes these contacts to turn defrost on or off. I’m gutting this too, but it means I have to differentiate the five wires running into the defrost relay. The gray wire feeds from the thermostat into the defrost housing. After opening it, this contacts the blade to the black wire. The first test is to cut the gray and black wire and connect them together (pictured above). The thermostat is still attached, but if I’m correct, this will cycle the compressor when I turn the fridge on, effectively bypassing the defrost timer.
Fridge turns on! Next step is to remove the thermostat and work backwards.
It becomes quickly clear that most of the wiring mess can be reduced to a very simple circuit once the controls are binned. Bulb wires are cut, ground wires are simplified, and the thermostat circuit can be traced back significantly. I’m keeping one component, which is the circulation fan that presides over the false wall. Rather than install another PC fan for this, I decided to use the existing fan since it uses a purpose built housing for trimming this area out. The only implication is that this fan uses a common wire with the thermostat circuit, so rather than splice these together at the base of the fridge, this will happen in the freezer section. As wires are assigned to the trash can, a resistance check at the bottom connector tells me where to cut on the back side. These are then pulled through the wall and thrown away. This fridge is losing some weight! After an afternoon characterizing the harness and using a multimeter like I know what I’m doing, the mess is simplified!
Though this still looks like a mess of wires, it will exist behind the false wall when the final build takes place. I will of course route these and zip them cleanly along the back wall. After so much effort into the air exchange prints, it’s time to bench test them. This is a very simple test and must answer two questions, 1) do the housings assemble to design intent and 2) do the fans push air through the flaps?
The air exchangers work! These are not high CFM fans so the flaps don’t open very much, but the airflow is good enough for this application. I’ve included an awkward video below.
That concludes the bulk of wiring work for now. The cap on top of the fridge will aggregate all of the connections by the end of this project, but it’s surprising how simple this architecture really is.