How to Fit Thermostatically Controlled Fridge Booster Fans in Your Eriba.

If you have ever used your Electrolux/Dometic fridge in high ambient temperatures, you will have noticed that it becomes less efficient when the temperature outside is above 25°C or so. One way of improving fridge performance is to fit a fan (or fans) to draw hot air from the fins at the rear of the fridge out via the upper external vent.

Fitting a simple ‘on/off’ fan is relatively easy but I wanted a control system that will turn itself on when the ambient temperature rises above a pre-set level and then turn itself off when it falls below it. This is how I did it.

Parts list: (Maplin part numbers quoted for on-line ordering)
Velleman thermostat kit MK 138 (RR51F)
2 x ultra-quiet 120 mm computer case fans (A90CJ)
1 x multicolour LED 5mm (YH75)
LED clip 5mm (YY40)
Box and Base (YN36)
IP65 Centre off switch (N20CL)
2 x In-line fuse holders (PC78)
2 x miniature 1K resistors (M1K)
20mm 1A fuses (GJ90) 10 pack
20mm 500mA fuses (GJ87) 10 pack
Wire; Heat-shrinkable insulation; Screw terminal blocks (sometimes called ‘chocolate box’ connectors)
Spade connectors (female)
‘Piggyback’ spade connectors (if needed)
Cable ties
(Total cost approximately £50)

Step 1 Build the thermostat kit

You will need a miniature soldering iron, solder, small screwdriver and a pair of side-cutters.
The kit comes with comprehensive, step-by-step assembly instructions and if you can use a soldering iron its construction should present no problems provided you pay attention to polarities when soldering the diodes, zener diode, electrolytic capacitors and the LED. Be sure also to install the integrated circuit (IC) socket and the IC itself with the notches in their correct positions.

When completed the underside of the printed circuit board (PCB) should look like this:





Pre-installation bench-testing (it was at this stage that I realised the system was working ‘back-to-front’ and would need modification.)

Step 2 Modify the PCB

As this thermostat is normally used to control a heater in a fish tank, a chick incubator or similar application, it will operate your fans ‘the wrong way around’, i.e. the fans will come on when the temperature is below the set temperature and turn off when it is reached. To put this right you need to reverse the polarity at the part of the IC that operates the relay. (IC1D) To do this, cut two tracks on the PCB using a Stanley knife, remove small pieces of the copper strips and swap over the connections to pins 12 and 13 of the IC using small pieces of insulated wire (as shown below). Be sure to check that the circuit is working correctly at this stage before proceeding with the installation.




Step 3 Mount the components

How you mount your fan(s) will depend on what materials you have lying around and how much time and effort you want to put into the task. I simply used a 380mm x 146mm piece of 5mm plywood from which I cut two circular holes with a jigsaw. The fans are bolted to it and the ply is attached to the opening by four screws along its base and three cable ties at the top. The additional holes in the ply are to ensure air circulation by convection when the fans are not needed.



I mounted my switch and LED into the aluminium front fascia of the fridge. To remove the fascia, pull off the fridge control knobs and undo the two screws. The switch shown below needs a 20mm x 30mm rectangular hole to be cut and the LED clip snaps into a 6.5mm diameter hole. To cut the hole, drill a series of small holes around the switch rectangle, remove the centre and file the edges smooth.



Step 4 Wire up the circuit

I mounted my thermostat into a plastic box drilling a couple of holes for the adjusting knob and relay LED and cutting an access slot for the cables. The thermostat was fitted behind the lower fridge grille and secured to an existing 240v cable with a cable tie. Access to the system fuses is gained from the outside of the caravan via the lower grille.



I extended the leads on the thermistor and attached it to the outer edge of the lower grille with a cable tie so that it will respond to the external temperature and not to that at the rear of the fridge compartment.



My 12v supply comes from a 12v socket (top left in the previous picture) using the piggyback spade connectors but you can use any convenient 12v supply.

Step 5 Test the installation

1. Check all wiring carefully before powering up the system. If there is no power, check whether any of the fuses in the BIP unit have blown (mine blew the brown 7.5A fuse a couple of times) and rectify the problem.
2. Turn the thermostat knob fully clockwise (30°C approx.)
3. Switch in central ‘off’ position: – thermostat LED off; panel LED off; fans not running.
4. Switch ‘on’ (position 1): - thermostat LED off; red panel LED on; fans running continuously
5. Switch ‘on’ (position 2): - thermostat LED off; green panel LED on; fans not running
6. Hold thermistor between your finger and thumb until relay switches on: - thermostat LED on; yellow panel LED on; fans running
7. Allow thermistor to cool: - fans switch off; thermostat LED off; green panel LED on
If your LEDs show the wrong colour (green/red instead of red/green) you can either leave things as they are or swap the connections to the LED to reverse them.

Step 6 Enjoy COLD beers on a hot campsite!

Detailed instructions for this are not required.

Possible modifications

The temperature range of the thermistor can be altered by changing the value of R6 on the thermostat PCB so it should be possible to have the thermostat respond to the temperature either of the fridge fins themselves or to the air temperature to the rear of the fridge. This would require further calculation and experimentation to determine the desired operating temperature range, appropriate resistor value and optimum thermistor placement.

Acknowledgements

Sincere thanks to John England Neston Caravan Services for developing such a simple and elegant circuit.