Why is this Comparator output is high?

[Pulasthi_Perera]

Can someone explain why Comparator U3's output is high in this simulation? LTspice file is attached.

 

[KlausST]

Hi,

you have a simulation tool --> then use it.

Start by measuring the individual comparator input voltages. --> Tell us.

Klaus

 

[Pulasthi_Perera]

I did. A voltage of 2.8V was appeared on non inverting input. Furthermore, the voltage at inverting input is 2.2V.Therefore,U3 output became HIGH.

I can't understand why there is a voltage present at non inverting input

 

[KlausST]

Hi,

Now it´s clear why the output is HIGH ... simply because the inputs tell to do so.

I can't understand why there is a voltage present at non inverting input

This is the second question.

But wait ... before you go on: please tell exactly what you expect ... and why.

******
After that..
... you can measure each single node .. to find out why the voltage is 2.8V.

Klaus

 

[Pulasthi_Perera]

But wait ... before you go on: please tell exactly what you expect ... and why.

My expectation was that U3 output will be LOW just because non inverting input is grounded.

 

[kd502]

This is bistable circuit, it has positive feedback. if the output is low it stays low, if it reaches high it stays high.

non inverting input is grounded.

It is not grounded, it is connected to divider.

 

[FvM]

Question: Why did you design the circuit as you did? Apparently positive feedback through D3 and R6 is stronger than it should be. What's the purpose of D2 and D3?

 

[KlausST]

Hi,

Yes, I agree, the non inverting input is not grounded.

I need to repeat myself: You have a simulation tool, so use it. The simulation (if set up properly) will show that the non inverting input is not grounded.
So it´s an easy way to verify if your "expectation" is vaid or not.
*****

additionally one needs to read the datasheet. ... on the first page it says:
"The output stage includes a class “B” pullup
current source, eliminating the need for an external
resistive pull-up and saving power."

This means: in any "undefined state" (like supply voltage below minimum) one could expect the output to be pulled HIGH.

***
As already stated: it is a bisstable circuit. And often bistable circuits have "no defined initial state". If you need a defined initial state .. you need the circuit modify to get this.

Klaus

 

[crutschow]

The circuit is operating as designed.
If you designed it, then determine what needs to be changed to get the output you want.

 

[danadakk]

Your original post file attached implied circuit was for 72V detection, where were you
going to sense the 72V in the circuit ?

Note the attached file, a RAR archive, my RAR utility says its corrupted....

 

[barry]

My expectation was that U3 output will be LOW just because non inverting input is grounded.

The non-inverting input is most certainly not grounded.

 

[D.A.(Tony)Stewart]

Your best advice will follow when you offer your design specs or functional intent for this LTSpice file.