LM35 temperature control circuit

Thread Starter

Shakespeare

Joined Sep 12, 2022
16
I am building a temperature control circuit to control water temperature. The temperature has to be regulated between 80-85°C. I'm using LM35 template sensor. And ua741 op amp.


My question is should I use two op amps to created a window comparator or use resistors to create schmitt trigger? What is the difference between the two?
 

Alec_t

Joined Sep 17, 2013
13,153
Welcome from me, too!
What supply voltage(s) will you be using? The 741 op-amp is from way back in the dark ages, is intended for a dual-polarity supply, and doesn't like inputs within a couple of volts of either supply rail. The LM35 at 85C will only be putting out 0.85V.
A cheap and cheerful improvement would be e.g. an LM358 (dual op-amp) or LM324 (quad op-amp), or an LM339 (quad comparator).
 

crutschow

Joined Mar 14, 2008
30,804
As Alec noted, a 741 is not a good device for your requirements unless you have dual supplies.
Since you want a comparator function, I recommend an LM339 or LM393 comparator.

Edit: Removed comment about using a window comparator since I realized you need a comparator with hysteresis.
 
Last edited:

Thread Starter

Shakespeare

Joined Sep 12, 2022
16
Welcome from me, too!
What supply voltage(s) will you be using? The 741 op-amp is from way back in the dark ages, is intended for a dual-polarity supply, and doesn't like inputs within a couple of volts of either supply rail. The LM35 at 85C will only be putting out 0.85V.
A cheap and cheerful improvement would be e.g. an LM358 (dual op-amp) or LM324 (quad op-amp), or an LM339 (quad comparator).
I will be using 12V. Also a 12V relay to control the heating element
 

Thread Starter

Shakespeare

Joined Sep 12, 2022
16
As Alec noted, a 741 is not a good device for your requirements unless you have dual supplies.
Since you want a comparator function, I recommend an LM339 or LM393 comparator.
If you do use one of those, their outputs can be easily AND'd together with one pull-up resistor to provide the window comparator function, which is generally more accurate than using hysteresis.
Thank you. With LM339 how would I get a hysterisis of 810mv-890mv and switch on volatge of 850mV. What's resistors to use if you know of any formulas. Supply voltage is 12V
 

Ian0

Joined Aug 7, 2020
6,313
I am building a temperature control circuit to control water temperature. The temperature has to be regulated between 80-85°C. I'm using LM35 template sensor. And ua741 op amp.


My question is should I use two op amps to created a window comparator or use resistors to create schmitt trigger? What is the difference between the two?
Don't use an op-amp as a comparator. Use a comparator.
 
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