A simple yet effective electronic fridge thermostat circuit has been explained in the following article. The post is based on the request sent to me by Mr.Andy. The proposed idea incorporates just a single IC LM 324 as the main active component. Let's learn more.
The email which I received from Mr.Andy:
I am Andy from Caracas. I have seen that you have experience with thermostats and other electronic designs, so i hope you can help me. I need to replace the mechanical fridge thermostat which is not working anymore. I'm sorry i didn't write directly on the blog. I think it's too much text.
I decided to build a different schematic.
It is working well, but only for positive temperatures. I need the schematic to operate from -5 Celsius to +4 Celsius (to use VR1 to set the temperature inside the fridge in the range of -5 Celsius +4 Celsius as the old thermostat knob used to do).
The schematic is using LM35DZ (0 Celsius to 100 Celsius). I’m using LM35CZ (-55 Celsius to +150 Celsius). To make the LM35CZ send negative voltage, I put an 18k resistor between pin2 of LM35 and the negative from the power supply (pin4 of LM358). (as in page 1 or 7(figure7) in the datasheet).
Because i’m using a 5,2v stabilized power supply, i operated the following modifications:
1.ZD1, R6 are out. R5 is 550 ohm.
2.VR1 is 5K instead of 2,2K (i couldn’t find a 2,2K pot)
The design is not working at temperatures below 0 Celsius. What else should i modifiy?
I did some measuring.
At 24 Celsius, LM35CZ is giving 244mV
At -2 Celsius, LM35CZ is giving -112mV (at -3 Celsius is -113mV)
At -2 Celsius the voltage between TP1 and GND cand be set from VR1 from between 0 to 2,07v
The solution is probably much simpler than it may appear to be.
Basically the circuit is responding only to positive temperatures because it incorporates a single supply. For making it respond to negative temperatures. the circuit or rather the opamps needs to be fed with dual supply voltages.
That will most certainly solve the issue without the need of modifying anything in the circuit.
Though the above circuit looks superb, new hobbyists may find the ICs LM35 and TL431 quite unfamiliar and difficult to configure.
A similar type of circuit of an electronic fridge thermostat can be built using just a single IC LM324 and by an ordinary 1N4148 diode as the sensor.
The figure below shows the simple wiring done around the IC LM324.
A1 produces a virtual ground to the sensing circuit opamps, thus creates a dual voltage supply very simply avoiding complicated and bulky wiring.
A2 forms the sensing stage which utilizes the "garden diode" 1N4148 for doing all the temperature sensing.
A2 amplifies the differences generated across the diode and feeds it to the next stage where A3 is configured as a comparator.
The final result obtained from the output of A4 is finally fed to another comparator stage consisting of A4, and the subsequent relay driver stage. The relay controls the fridge compressor On/OFF switching as per the settings of the preset P1.
P1 should be set such that the green LED just shuts off at -5 degrees or any other lower temperatures, as per the users demands.Next P2 should be adjusted so that the relay just triggers at the above condition.
R13 should be actually replaced with a 1M preset. This preset should be adjusted such that the relay just deactivates at around 4 degrees Celsius or any other closer values again depending upon the users preferences.
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