Building my own fridge control board

[akis]

My fridge is broken, it was always bad bad bad and I have deciced to build a new control board and throw away the crap that they shipped it with.

I have examined the existing board, I do not have a schematic but I can tell the following:

1) it receives 240V AC
2) converts it to 24V in order to power the two fans and two relays
3) converts it to 5V (7805) in order to power an MCU
4) uses a standard 24V relay to power the compressor/motor
5) uses a standard 24V relay to power the defrost heater
6) uses a triac in series with a PTC EPCOS C810 to power the two 240V lamps inside the main compartment
7) it has three NTC sensors , the two are equivalent and sense freezer and fridge, the third is used for the defrost heater I have not measured it to make sure it's the same
it has a reed relay to sense when freezer or fridge door is open
9) communicates with a display unit / interface board which is on the door of the fridge and you can set temperatures etc through 3 wire connector.

The board is an abomination. Firstly it is made of a crappy material and it has flexed inside its enclosure and cracked ! It has 240AC lines running all over the place mixed with 24V, 5V anything, even exposed wire jumpers carrying 240V on the top.

So to make my own I will use an Arduino Nano so I can connect via long USB cable, and will use the same connectors and two relays and PTC and triac as the old board (I will hack them out). For 24V DC I will use one of these ebay modules.

The existing software on the board has always been bad and the fridge has never worked properly. In contrast I have two other 30 year old fridges and they work solidly - no MCUs and stupid software.

My first problem is I do not understand a bit around the two lamps inside the fridge. Why are they 240V and not 24V DC as it already uses 24V to power the two fans? Why does it not use a relay to power the lamps but instead uses a triac and a PTC ? Why the PTC, I do not get it. In case you stick your hands on the lamps? Will the PTC save you from electrocution?

Also, how do I use the Triac considering its main pins are connected to 240V mains? What do I do with the gate? I am thinking of using another relay for this to be on the safe side.

Any help appreaciated with ideas and suggestions!

[ez24]

Don't forget a time delay.  I ruined a frig once when I replaced a thermostat that had too short of a delay.  It cycled too often.  The frig had a "bug".  It turned out to be an ant caught in the points of the thermostat relay that prevented the relay from making contact.  Found out this after replacement and failure.  Good luck with this project.

[Seekonk]

I did a solar fridge control, a five minute hot start prevention delay is what i use and a minimum run time. A long delay/averaging is suggested for the sensor. Don't try to read too much into their design as it favors cheapest construction.