This method has been used for years at my camp. Refrigerator only operates daylight hours and uses mass
liquid storage to keep cold throught the night. As battery is not used for storage ample panels are required.
This can make it risky for perishable food if undersized. Ideal for a minimal system for drinks.
I use a microprocessor for a refrigerator controller off grid. This hardware version approximates that.
The same control could also be used for a pump and force it to only operate in a battery full condition
with sun. This would prevent operation at night or when battery was low. This is a set/reset voltage relay.
The first TL431 controls the turn on voltage and the second sets the turn off. Relay is latched to the on
state once the upper voltage is reached. The second TL431 controls the turn off. The W1209 temp controller
also provides time delay to prevent hot starts. The dry contacts of the temp board are used to turn your
inverter on and off, just parallel the on/off switch. This provides
a very low standby current of 1ma. Temp display only operates when enabled. A push button or switch enables
temporary temperature reading for up to several minutes. Longer than the preset delay will force the
refrigerator on, but it will not latch on. Once switch opens fridge will stop. Two schematics are provided.
One uses a PNP transistor to invert the signal. The other uses an opto isolator. The opto may be more common
than a PNP in old electronics.
All parts are fairly common and can be scrounged from old electronics. The 12V relay should be at least 375
ohms preferably higher.TL431 variations can be found in most old wall wart supplies. Anything with a 431 on
the case is likely one. Construction cost is less than $5 with the W1209 ($2 shipped). Pots have a wide
tolerance, usually 20%. This is the reason a resistor is placed in parallel to mitigate those
variations. The resistor across the LED shunts low currents and prevents ghosting. The LM431 always draws
some current and this prevents a dull glow. One diode prevents powering the board till the relay closes. The
other allows manual turn on of the board without latching the relay. There are many youtube videos on W1209
board. Try to find a relay with less than 30ma current.
Parts List
(1) W1209 temp controller board
(2) LM431 TL431 adjustable zener
(1) any PNP silicon 100ma transistor, many numbers start with B
(1) 12V relay 375 ohms or greater coil
(1) red LED
(2) 1-4.7uF, electrolytics to prevent false trips
(3) any general purpose diode, 1N4001 type
(1) 470 ohm resistor
(2) 1.2K resistor
(1) 4.7K resistor can be any value from 3.9K to 6.2K
(2) 20K resistor, two in series make 40K resistor which is an odd value
(1) 10K pot, 20K could be used with slightly different range
(1) push button or switch)
