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Problem to set Simple Thermostat circuit

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aljamri

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Hi,

I'm trying to do simple incubator using thermostat found Here , but faild to find the correct values of Rs that make the window of ( 35C to 39C ) required for good hatching chance.

my NTC (R1) is about 9k in room temp and around 7k under the body temp ( around 37 C), R2,3,4,6,7=10k and R5 is a pot 10k. I've added a capacitor of 220uF parallel to the relay k1.

With the above values, I've turned R5 both sides fully, yet no response to the temperature change.

What do you think that I need to correct ?

EDIT: the working link is: **broken link removed**
 
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That link is broken.
 
Hi,

I'm trying to do simple incubator using thermostat found Here , but faild to find the correct values of Rs that make the window of ( 35C to 39C ) required for good hatching chance.

my NTC (R1) is about 9k in room temp and around 7k under the body temp ( around 37 C), R2,3,4,6,7=10k and R5 is a pot 10k. I've added a capacitor of 220uF parallel to the relay k1.

With the above values, I've turned R5 both sides fully, yet no response to the temperature change.

What do you think that I need to correct ?

Not sure why you changed the resistor values from the original schematic. But with your component values the circuit will never operate correctly. Comparators are all about proper biasing.

Anyway, try this: (note OpAmp comparator +input voltage level [Reference] and -input voltage level [variable set by R1 temp] and corresponding output levels)

Circuit with Temp at 33C:
TStat at 33C.JPG

Circuit with Temp at 39C:
TStat at 39C.JPG

The output levels are completely adequate for operating an on/off circuit (transistor driver/switch, relay, etc.) for your heater.

You may have to adjust some values but I think the 10K pot should be able to compensate for any oddities of the thermistor.

Note also the hyterisis (6 degrees C) is a little greater than your stated desire swing. Just try it and see.

<EDIT> The SIM thermistor I used is a 10K NTC type with a B25/85 thermal constant of 3.522K. Your's may be different. If so, let me know and I can adjust component values to compensate. Or you could play with circuit values.
 
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Wrong resistor values in post #1. Cowboybob beat me to it :)
Increase R3 in his circuit if you want less hysteresis.
 
:DSoory for the broken link, trying to be smart :) .

KeepItSimpleStupid, thanks for the proper link to the circuit.

cowboybob and alec_t, I'll try it in the weekend and let you know.

Thanks for all
 
Hi again,
I would like to know the Math behind those values to save you Recalculating for me for every change I do. That's if you don't mind, otherwise I would like to tell you that I decided to replace the thermistor with variable resistor to jump from a value to another fast, to avoid the annoying slow-temp-response change.
For that I did little experiment with thermometer and an Ohmmeter and found the following outputs from my thermistor:
at room temp ( around 26 C ) thermistor = 9.12 k Ohm
at around 36 C, thermistor = 6.7 k Ohm
 
...I decided to replace the thermistor with variable resistor to jump from a value to another fast, to avoid the annoying slow-temp-response change. ...
First, do you know the spec of the thermistor for its resistive range (the R25/85 value)?

What sort of variable resistor (which, of course, is what a thermistor is) are you considering?

My experience with thermistors has been a pracically simultaneous change of value with an ambient air (or water [generally a little faster], whatever) change.

Actually, some lag in change is desirable to keep the overall system from "jittering", that is, toggling between heat/cool too rapidly (too little hysterisis).

Don't forget to take into account the response time of the heating element(s).
 
Hi cowboybob,

I do not know the value of the R25/85, I've just picked it thinking that 10k value is enough, attached is a photo of my NTC.

Of course, lag in change will do no harm to my project and that's expect and accepted.

What is happening know, I've connected the 741 output, through a 10k resistor to a 2N2222A transistor which drives an LED.

The LED stays ON regardless the change in thermistor Resistance ( I simulate it with 7k and 9k Ohms).

Thanks
 

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I googled for a datasheet and found one. The attached screen shoots shows its features and Electrical Characteristics.

My NTC is the last in the table.
 

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Your simulation resistance values are way too high.

Try a 2k resistor (approx. 43C) and a 2.7K resistor (approx. 33C) for your simulations. If you followed the circuit design and values I posted, the circuit will work.

Or better yet, stick the thermister in the circuit (body temp [37C] won't kick it ON, try a hair dryer on LOW).
 
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