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Automatic Refrigerator Temperature Controller

This circuit idea was requested to me by one of the keen readers of this blog Mr.Gustavo. I had published one similar circuit of an automatic refrigerator thermostat, however the circuit was intended to sense higher temperature level available at the rear side grid of refrigerators.

Introduction

The idea was not quite appreciated by Mr Gustavo and he asked me to design a refrigerator thermostat circuit which could sense the cold temperatures inside the fridge, rather than the hot temperatures at the rear of the fridge.

So with some effort I could discover the present CIRCUIT DIAGRAM of a refrigerator temperature controller, let's learn the idea with the following points:

How the Circuit Functions


The concept is not very new, neither unique, it's the usual comparator concept that has been incorporated here.

The IC 741 has been rigged in its standard comparator mode and also as a non inverting amplifier circuit.

The NTC thermistor becomes the main sensing component and is specifically responsible for sensing cold temperatures.

NTC means negative temperature coefficient, meaning the resistance of the thermistor will rise as the temperature around it falls.

It must be noted that the NTC must be rated as per the given specs otherwise the system will not functions as intended.

The preset P1 is used for setting the tripping point of the IC.

When the temperature inside the fridge falls below the threshold level, the thermistor resistance becomes high enough and reduces the voltage at the inverting pin below the non-inverting pin voltage level.

This instantly makes the output of the IC go high, activating the relay and switching OFF the fridge compressor.

P1 must be set such that the opamp output becomes high at around zero degree Celsius.

A little hysteresis introduced by the circuit comes as a boon or rather a blessing in disguise, because due to this the circuit does not switch rapidly at the threshold levels rather responds only after the temperature has risen to about a couple degrees above the tripping level.

For example suppose if the tripping level is set at zero degrees, the IC will trip the relay at this point and the fridge compressor will be also switched OFF, the temperature inside the fridge now starts rising, but the IC does not switch back immediately but retains its position until the temperature has risen at least upto 3 degrees Celsius above zero.

Automatic Refrigerator Temperature Controller


If you have any further queries regarding this automatic fridge temperature controller circuit, you can express the same through your comments



Need Help? Please leave a comment, I'll get back soon with a reply!




Comments

  1. why is the 2k2 and 47k7 (2 and 7 at the end?
    Is 47k7 a variable resistor

    ReplyDelete
  2. Why is the 2 and 7 at the end of 47k7 and 2k2 (is this 2k or why 2k2?) is 47k7 variable resistor?

    ReplyDelete
  3. i am running this circuit with 13.8V
    0.35A and transistor (BC___)is getting hot . P1 i used potenciometer 5K. Am i doing something wrong? i used relay and on the other side high power cooling power fan . System is not switching off at 5-0 °C
    Thanks,

    ReplyDelete
  4. your BC_ will never become hot if it's connected with a relay rated above 200 ohms coil and with correct pin configurations....there could be something wrong with your connections....moreover check the voltage of the opamp which is connected with the sensor....it should change with the temperature, if it's not changing means your sensor is not responding or is faulty.

    ReplyDelete
  5. Hello sir Greetings.

    I want to make this circuit. but unfortunately, i couldnt find the NTC thermister of 2k2.

    But i found 10k NTC in local market. can i use that thermister instead 2k2?

    if so, what parts of the circuit is need to be changed?

    please help me with this issue.

    Thanks.

    ReplyDelete
  6. Hello, you can use a 10K thermistor, make R1 = 47K as a modification for this NTC. Rest everything can be used as given.

    ReplyDelete
  7. sir i want a circuit to keep a refrigerator at 16 Celsius, please help me

    ReplyDelete
  8. Srishan, you can probably achieve it by connecting the 220V input to the refrigerator though a programmable timer circuit, such as a 4060 timer circuit...this timer could be adjusted (with some experimentation) to ensure the right ON/OFF ratio for the refrigerator which would yield the required 16 degree temp

    ReplyDelete
  9. Hoping to experiment with the 4060 soon, if you come up a similar circuit as my requirement please let me know sir :D thank you

    ReplyDelete
  10. you can try the following circuit, this might perfectly suit your application

    https://homemade-circuits.com/2012/04/how-to-make-simple-programmable-timer.html

    ReplyDelete
  11. Hello,

    Thanks for comprehensive post. I have used almost exact same circuit , but for the purpose of building a water boiler. I used PTC instead Of NTC.
    I got confused here in one part, I didn't understand AC mains/fridge to the circuit. is it just taking part of the plug out of the refrigeratior and connect it in parallel with the ac mains ... just using simple wires?!

    ReplyDelete
  12. hello, thanks,

    the "fridge" section is actually one of the mains wires of the fridge cut and joined across these points....meaning any one of the plug wires of the load needs to be in series with the relay contacts and the mains input as indicated.

    ReplyDelete
  13. ...instead of cutting the fridge (load) chord in the middle, it would be better to use the end of the wire which terminates inside the fridge (load) body and connects with the associated circuitry

    ReplyDelete
  14. sir i want to make a circuit at hot side of refrigerator such a way when temperature rises the above 50 degree the circuit should get OFF can you help me for this

    ReplyDelete
  15. Ali, you can use the same circuit that's explained in the above article....

    ReplyDelete
  16. the above circuit works on 12v dc i want for 48v dc supply so how to do it

    ReplyDelete
  17. you will need to step down the voltage to 12V as shown in the circuit from this article, using BC546 transistor and the associated zener diode.

    https://homemade-circuits.com/2014/11/48-v-inverter-circuit.html

    ReplyDelete
  18. sir from link only i should take step down voltage BC546 And zener diode and assembled this in upper circuit given for refrigerator is it correct????

    ReplyDelete
  19. yes that's right, the collector side will need to be supplied from your 48V, and the emitter side connected to the above circuit's positive line.

    ReplyDelete
  20. thanks sir if any further inquiry i will ask you

    ReplyDelete
  21. sir which wire to be place on hot side of engine from the component for temperature sensing

    ReplyDelete
  22. Ali, it's not any wire, it's the NTC (2k2)thermistor which needs to be placed over the heat source for the sensing

    ReplyDelete
  23. Sir One question why buck converter get burns when connected what is the reason behind it.

    ReplyDelete
  24. Hello swagatam, i need an automatic temperature controlled cooling fan circuit, which when power on, the speed of the fan will depend on the increase in heat

    ReplyDelete
  25. Hello Solomon, you can try this circuit

    https://homemade-circuits.com/fan-speed-controller-for-heatsink/

    ReplyDelete
  26. Hello sir,
    I am a college student and trying to build a temp sensor and controller circuitry for an window AC limiting the temp in the range of 24 deg C to 28 deg C (+/- 2 deg C). Kindly guide me on how to achieve this at cheaper price i.e., around 500 INR.

    ReplyDelete
  27. Hi Harish, you can try the concepts explained in the following article:

    https://www.homemade-circuits.com/how-to-build-simple-electronic/

    I would recommend the second last one using LM35, it looks much easier and accurate

    ReplyDelete
  28. I need the circuitry for 230 V ac and for compressor having 10amp or 3amp min. Also how and where do i connect the compressor so it gets turned on at 24+3(threshold) i.e., 27 or 28 deg C and turn off below 24 deg C. Please help me sir with sincere request.

    ReplyDelete
  29. You can try the above explained concept. You will have to cut of the mains wires and connect it through the relay contacts, meaning remove one of the plug wires and connect it to the indicated relay contact and connect the other wire from the relay contact to the plug.

    ReplyDelete
  30. Thank you so much for helping out also how/where to include/ connect an led for notification such as red led notification for temp below 24 deg C and green led notification for temp above 24+3(threshold) deg C.

    ReplyDelete
  31. It's my pleasure! One LED can be seen connected with the base of the transistor, another LED can be connected across collector/emitter of the same transistor, as shown in the first image of this example article:

    https://www.homemade-circuits.com/how-to-make-simple-programmable-timer/

    ReplyDelete

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