12V Fridge Description
These are screen shots of the original Danfoss controller datasheet and they are very similar to others for the BD35F compressor. The small circuit diagram in the top left hand corner is how most fridge manufacturers implement their fridges so it's isolated here with my own description.
The controller unit itself contains a microprocessor primarily used to run a small invertor to convert the DC current supply to AC to drive the compressor. It has an overheating and undervoltage cut-off mechanism. If the compressor motor or electronics overheats, power is cut off. Similarly, if the battery voltage goes to low and the battery might be damaged, the power is also cut off. This function can be controlled by the value of resistor (9) in the diagram, the cut-off voltages values shown in the tables below.
This voltage cut-off value can cause problems with long cable runs to a fridge where the voltage drop on the cable is too much. The table below shows typical cable lengths for reliable operation. The lowest it can go is 9.6 volts measured at the fridge, this is built-in to the controller.
As with many fridges, the power to the compressor is applied when a thermostat switch closes when the temperature in the fridge rises too high. However, in this case, the closing of the thermostat doesn't switch the actual power, that's done by the controller, it's only an indicator to the controller that the temperature has risen too high and to start running the fridge.
The speed of the compressor, and the resultant cooling and power consumption are set each time the thermostat closes and this is set by resistor (8) in the diagram. The speed can vary between 2000rpm and 3500rpm, and the cooling effect increases with motor speed. This also has an impact on power consumption and noise, with minimum noise and power consumption at 2000rpm. This has to be traded off against the run time of the fridge compressor, so high power for a short time, low power but longer running time.
This can be especially useful overnight when noise is unwanted, but during periods of warm weather, the fridge may never switch off if not cooling enough.
Because of the controller operation, reliable power to the fridge shouldn't affect the operation, and the battery switch (4) is only for isolation. Obviously cutting power will stop the fridge, but applying power won't start it!
The thermostat in the Waeco fridge installed is buried in the rear wall of the fridge with the electrical switch and temperature control mounted in the light unit at the front of the fridge. The thermostat also incorporates a power switch controlling the battery power input. In practice this can be a pain as the fridge settings are lost every time the fridge is switched off.
Different models of controller suit different fridge implementations and this controller is intended for use with a fridge in an enclosed space. The fridge must have some ventilation to remove heat and this is assisted by a built-in fan (5) which runs when the compressor is running. These fridges cannot be run efficiently in a fully enclosed space, there has to be somewhere for the heat to go, so some ventilation to to remove the heat is necessary. The fan only runs when the compressor is running and when the compressor stops, so does the fan, meaning that heat still in the system is free to dissipate in the fridge, reducing efficiency.
The controller has a basic error indication via a small flashing light (6) which will indicate the errors shown in the table.
This is the operation which the replacement fridge controller has to emulate or improve on.