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When the NTC hits 80°C, the comparator will output LOW signal, but it will only be HIGH if only the NTC hit 75°C. So before the NTC hit 80°C, the circuit will work as intended. But the problem is when I start the simulation with NTC temperature starting point at 76-79°C, it works as intended.

But when I start the simulation with temperature below 76°C or above 79°C, it doesn't work as intended. When I start below 76°C, the comparator give the LOW signal at 80°C (which is working as intended) but it give a HIGH signal when it hit 79°C, it should give HIGH signal only if the NTC hit 75°C not 79°C. The reverse is also true if I start the simulation above 79°C, when I decrease the temperature to 75°C, it give HIGH signal, but if I increase it to 76°C, it immediately give LOW signal.

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The VCC is 5 V, and the VIN is 5 V. I don't know what makes it behave like that, is it the value of the resistor or what.

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  • \$\begingroup\$ Where is VIN? I don't see it anywhere. \$\endgroup\$ Commented Jan 28 at 9:23
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    \$\begingroup\$ @SimonFitch VIN would be the node labelled R2(1). Proteus names those kind of nodes by the component it is connected to by default. OP should really get in the habit of naming things clearly \$\endgroup\$ Commented Jan 28 at 9:39
  • \$\begingroup\$ Did you try to use a potentiometer resistor instead of the NTC? Maybe there is something wrong with the NTC simulation model? \$\endgroup\$ Commented Jan 28 at 16:36
  • \$\begingroup\$ Oh the potentiometer do work, thank you very much. But do you know what can I change to make the NTC work, maybe the simulation setting or the NTC? I was using 220k Ohm NTC with material constant of 4050. \$\endgroup\$ Commented Jan 29 at 5:52

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Please consider that the LM393 does not provide rail-to-rail inputs. It can only handle common mode input range of VCC - 2V.

This means that with your +5V supply, the common mode input range is limited to max. 3V.

Maybe this is related to your simulation problem.

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  • \$\begingroup\$ Oh I didnt know that, but i do have the voltage divider that makes the input around 2.5V, the high threshold is 2.5V at 80 C, and the low threshold is 2.3V at 75 C. But i have lowered the VIN to 3V and its still have the same problem. \$\endgroup\$ Commented Jan 28 at 13:08

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