Automatic defrost circuit

[Acuario]

Well winter is just around the corner again in Northern Spain as I write this on 29th September so it's time to revise the heating system.

Last year the home made heat pump was a fantastic success so I'm obviously going to continue using it. The only problem I had was a lack of an automatic defrost for those extra cold nights. The result was a block of ice hung on the wall - not too efficient.

So here is my attempt at solving this minor problem - a PIC processor controlled defrosting circuit.

The circuit itself is pretty basic and uses a Dallas DS18B20 digital thermometer to measure the temperature of the evaporator. I've started with a temperature of -2 degrees celcius as my trigger for the defrost cycle - this is a bit of a guess, anyone have any better figure?

I could have used a thermistor and the pic ADC circuit but I wanted to have a go at coding for the 1-wire protocol and I also use the same code in other projects so why complicate life?

When the temperature reaches (currently) -2 then the defrost cycle kicks in. Again, the cycle itself is a best guess but it can be easily modified if necessary. The cycle is:

• Turn off compressor
• wait 1 minute
• Switch to cooling mode
• wait 1 minute
• Turn on compressor for 5 minutes (defrost)
• Turn off compressor
• wait 1 minute
• Switch to heating mode
• wait 1 minute
• Turn on compressor

The pic code was compiled using the Microlab X IDE (free) and the schematic using KiKad (free).

I decided to use a relay module bought from ebay for the valve control and a 40A solid state relay (also from ebay) for the compressor control. You could design/build your own bits for this but I had them available and they hardly cost any money.

I've attached the circuit, some photos and the code I wrote in case anyone else needs a defrost circuit. I'll update with how it performs when the temperature drops!

Acuario