Thermistor Comparator Circuit

[shachar85]

Hi

I want to build a small circuit which will alert me once the temperature in my fridge goes up. (its an old fridge and it stops working once in 2 or 3 months).

I thought to use a thermistor that will be in the fridge. as shown in the image attached.

I think it should work,but as i'm a beginner in electronics, I thought it would be wiser to be sure before I buy the components.

The main idea of my circuit is that V- of the comparator(LM311N) gets constant 3.33v (5v divided by 50Kohm and 100Kohm).
When the temperature is right, the thermistor value is 100Kohm and V+ of the comparator gets 2.5V = lower than 3.33V. then the output is VS- which is 0.

When the temperature rises above the alert temperature, the value of the thermistor is above 50Kohm and V+ of the comparator is above 3.33V. then the output is VS+ which is 5V.

This 5V triggers the relay. and the relay activate the alarm, which will include a buzzer and a led.

will it work?

and for another thing - once the temperature will rise, there will be a few minutes that the 2 inputs will be balanced. The output will probably flip from 5V to 0 all this time. will it be wise to connect a capacitor with a resistor in the output in a way that only if the output will give 5V for 2-3 seconds,it will trigger the relay?

thank you for any answer

 

[MikeMl]

Your circuit needs so positive feedback for hysteresis. Look at the schematic I posted in this thread.

 

[shachar85]

I see you used a PTC thermistor. I'm gonna use a NTC. should I connect a positive or negative feedback then?

Will it make a different if I use a relay instead of the mosfet you used?

 

[MikeMl]

The issue of PTC vs NTC is independent of the positive feedback to the non-inverting input of the comparator. The LM311 will sink sufficient current to pull-in a relay; the LM358 wont. Dont forget a snubber diode around the relay coil.

 

[shachar85]

all right. great

one last thing-
whats the importance of the 100ohm resistor and the 47uf capacitor?

 

[MikeMl]

Decoupling from a noisy power source. If yours is clean, leave out the 100 Ohm, but leave some sort of 5 to 50uF bypass cap.

 

[shachar85]

Great

Thanks a lot for the help