Off-Grid Solar Refrigerator

[mbr1989]

Hi,

would it be possible to write a firmware that uses the heating output to switch on a 230V inverter?

A delay element, which switches on the inverter a few seconds before the compressor, would be very nice.

best regards
Marcus

[matsstaff]

Hi!

Could? Possibly (without really knowing the actual requirements).
Should? Probably no. If all that is needed is for an inverter to be triggered before compressor starts, there must be an easier, more robust, cheaper, more power efficient solution. Personally, I'd try to make my own trigger and delay circuit (with a relay before the compressor) and use the existing fridge thermostat. I guess the implementation of that would depend on what fridge and inverter and stuff...

Cheers!

[mbr1989]

Hi,

we use a EPEVER IP Plus 2000 22 (24 Volt) pure sine wave inverter.
https://www.epsolarpv.com/product/72.html

The inverter has a potential-free contact to turn it on.

The fridge thermostat is already disassembled.

Power efficient solution is to not run the inverter all the time in standby ~ 15 W @ 230 V

Cheers!

[mbr1989]

https://assets.danfoss.com/documents/35412/AN223886433202en-000101.pdf

https://assets.danfoss.com/documents/35418/AN223886433209en-000101.pdf

https://assets.danfoss.com/documents/99785/AF143086415692en-000101.pdf

... this is to expensive

[matsstaff]

Ok.Well, if that is the case, I guess it might make sense. That is, it could be do-able.
You'd at least need to modify the stc for 12v operation. Not that hard.
You'd also need to do some firmware hacking. That might be a bit more challenging :)
Basically, I still think it would be easier to just use an existing thermostat and build the delay circuit.

Cheers!

[mbr1989]

You'd at least need to modify the stc for 12v operation. Not that hard.

https://www.amazon.de/gp/product/B07RD9DRXZ/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Bought the 24 Volt version.

Basically, I still think it would be easier to just use an existing thermostat and build the delay circuit.

How the hell is that supposed to work ?
I've already thought about it, but haven't come to a solution.
The thermostat built into the refrigerator has no potential-free auxiliary contact.

[matsstaff]

Ok, first off, not all STC-1000's are equal. There have been numerous different 'clones' spotted, all using varying microcontrollers (they look similar from the outside though), so it depends on what actual hardware you got if it can even be reflashed.

Secondly, that fridge thermostat you pictured can most likely be thought of as a temperature actuated mains switch, it probably just sits in series with the mains connection to the compressor, closing the switch (turning on the compressor) when temperature in the fridge rises. It would be potential free if you disconnect the spade connectors from it... Note that these thermostats are mechanical devices, the do not need mains power to function, they actuate by the change in pressure of a contained gas due to change in temperature.

Here's more info:
https://assets.danfoss.com/documents/163014/AI357347699370en-000101.pdf
https://store.danfoss.com/en/Climate-Solutions-for-cooling/Appliance-Controls/Standard-thermostat%2C-B62/p/077B6222

My suggestion is, that you disconnect mains in and mains out from the thermostat, short those two leads together (making the compressor always run) and connect the 'potential free contact' on the inverter to the thermostat instead.
Should this not work as is, then look for a relay (or contactor) delay module to delay the start of the fridge until the inverter has had enough time to start.

Cheers!

[mbr1989]

It would be potential free if you disconnect the spade connectors from it ...

yes of course

My suggestion is, that you disconnect mains in and mains out from the thermostat, short those two leads together (making the compressor always run) and connect the 'potential free contact' on the inverter to the thermostat instead.

The inverter is also for other electricity consumer ... if no one is there, the inverter is switched off ... and will be avtivated by the potential-free contact ... the other electricity consumer are disconnected then ...

Should this not work as is, then look for a relay (or contactor) delay module to delay the start of the fridge until the inverter has had enough time to start.

Well sounds not much easier to me.

[matsstaff]

The inverter is also for other electricity consumer

See, you didn't mention that before :)
Well, no biggie... Do it this way then... Disconnect the thermostat, feed it 12V instead and let it provide power to two relays. One for 'potential free contact' on the inverter and one for mains for the compressor.
You could use something like this:
https://www.ebay.com/itm/124367538472
for both the relays, to also get the option of adding a delay, and you'd still have a couple of spares.
See this for usage:
https://youtu.be/RjWW4nbUyio?t=264
You could do the same thing with STC-1000 instead of the internal fridge thermostat, if you really want to use an external thermostat (which honestly, the only real benefit would probably that wiring might be a bit easier externally).

Well sounds not much easier to me.

Easier than what? Modifying the firmware of a thermostat that may not even be modifiable?
Let's put it this way. If I wanted to do what you want to do now, I would do it the way I suggested (and I got some known "good" STC's and I'm pretty familiar with writing code for them) :)

Cheers!