Digital thermostat for fridge

[Hayden Watson]

I have a icebox conversion with a Danfoss BD50 compressor and really hate the mechanical thermostat. I have several digital thermostats on my heating system and would like to use the same for my fridge. Any suggestions on how to go about that?
In looking at the wiring diagrams, it says that I need a 270 ohm resistor for 2500 rpm, 680 ohm for 3000 rpm and 1500 ohm for 3500 rpm. The system is on a small ice box conversion on a Catalina 30. Which resistor should I use and what type of a rating does it need?

 

[senang]

I bought the STC-1000 digital thermostat but have not yet installed it. You can find these anywhere on the net. AFAIK it is a simple on/off.

 

[Hayden Watson]

I bought the STC-1000 digital thermostat but have not yet installed it. You can find these anywhere on the net. AFAIK it is a simple on/off.

The challenge is that the control line needs to be between 270ohm resistance for minimum 2,500-rpm and 1500ohm for max 3,500-rpm. I do not have any first-hand knowledge but suspect that if the control wire does not have at least the minimum resistance of 270ohms, the pump will be running too slow and may even stall which would burn it up in a hurry. I am planning to install it with an A-B switch that will provide 270ohm for use when on battery power and 1,500ohm for when on shore power or motoring. That way I get quick cooling when I have plenty of power and efficient operation when on the batteries. I will also bump the set temp down when on charge and up a few degrees when using that batteries. That will be a fully manual solution to the functionality and efficiency of the Isotherm Smart Energy Controller

 

[Hayden Watson]

I did some additional digging for Danfoss info and found this. It appears that if you directly connect the compressor to the thermostat control wire without any resistor, it will still run but at the very lowest speed (2000 rpm) which will product the least amount of cooling and the longest runtimes. Depending on your box and insulation, that may be perfect. If you do not get enough cooling, then you will need to add a resistor to the line to speed things up. Do not go about the 1.5k-ohms.Danfoss BD50 specs

 

[planoron]

I kept the mechanical but added a wireless readout...works great

 

[Hayden Watson]

I just do not like the way that the mechanical t-stat is designed to work. It is bolted to the condenser and measures its temp which is gets cold in a few minutes and shuts off. I have done some testing and when starting from a warm box, it will run for 5-10 minutes and shut off even though the air temperature inside the box may have only dropped a few degrees. It then is off for 5 minutes or so and then repeats the cycle (over and over until it finally gets down to temp you want. I also found that once it has been running for a full day, the temperature only varies a couple of degrees over a 24-hour period. during this time, it runs 5-min and rests 10-min. By sampling the air, the runtimes and rest times should be much longer, especially given that I can set the temp difference. The FDA recommends that a refrigerator temp stay between 34º - 40º. When motoring or at dock, I will set it to shut off at 33º and restart at 36º. When on battery power I will use 36º - 40º which makes use of the extra 4º of latent heat storage to further minimize battery amps consumed.

 

[RoyS]

Hayden, you may want to consider using a generic programmable logic controller (PLC) in this application. These controllers accept multiple inputs and provide multiple outputs. The controller uses simple logic gates (and, or, nor, Etc.) and internal timers, counters, Etc. which you arrange to achieve the desired results. The better ones may be programmed and tested on your computer using a downloaded simulator program. The controller itself usually has a built in screen and simple keyboard for field program adjustments. You can design and test your system on your computer before buying the controller. Once you have a satisfactory program you may download it to your controller. Fun project. Prices for a, say 12 input, 12 VDC system may run from about $170. Look at IDEC FT1A-H12RA, for one example. There may be cheaper ones out there.

 

[Hayden Watson]

Hayden, you may want to consider using a generic programmable logic controller (PLC) in this application...

Thanks for the info. That looks to be more complication than I want to deal with. My current plan is just 1.5k resistor and a SPST switch wired in parallel on the control wire to the compressor. when the switch is off, the signal flows through the 1.5k resistor and I get max rpm and cooling. with the switch on, I get 0 resistance and slowest rpm and best efficiency for battery usage. The thermostat is the same as our heating system and very easy to bump up and down as needed. If it seams like I do not have enough cooling capacity on battery I will add some resistance to the switch line.