Non-Inverting Open Collector Comparator

ericgibbs

Joined Jan 29, 2010
18,766
hi pp,
Do you want to try to derive an equation for the Comparator, assuming a single 10Vdc supply.?

Design for:
Vp=10V, Rin =10k, Rp=5k , Vref=+1V and the hysteresis threshold is 0.5V, using a LM393.
Rhyst = __________.?
E
OT: Hope you had a Good New Year.
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
351
Hi Ericgibbs,

Happy New Year to you, all Moderators and Members.

Thank you for the explanation.

But, I have derived the equation again, which is attached below. In which, this time, I have considered pull-up resister in this derivation. So, please have a look and tell me that is that derivation is correct or wrong?.
 
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Thread Starter

pinkyponky

Joined Nov 28, 2019
351
hi pp,
It looks over complex and wrong, but I will check more closely and post what I find.
[most likely posting tomorrow morning]

E
If you feel that this is the not the way to derive the equation, Then, please could you derive for me if possible?. Because I have put all my effort and I couldn't able to derive the correct derivation.
 

Jony130

Joined Feb 17, 2009
5,487
To find the threshold voltage assume that the "switch" inside a comparator is Closed (Vout = 0V).
And now try to find for what value of a Vin the voltage at V node is equal to Vref. For what Vin do we have V = Vref?
And to get a second threshold voltage we assume that the switch" inside a comparator is Open, and again we need to find Vin at which V = Vref.
 

ericgibbs

Joined Jan 29, 2010
18,766
hi pp,
I am still not sure if you understand the operation and purpose of a comparator, with hysteresis.
That's why you are having difficulty in deriving the equations.

We can try to work out the problem, if you want to try.?

E
In this circuit, assume initially Vi = 0v and Vref =+1v.
What will the voltage be at Va and Vo.??
Use simple resistive divider calaculations.

EG 1192.png
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
351
hi pp,
I am still not sure if you understand the operation and purpose of a comparator, with hysteresis.
That's why you are having difficulty in deriving the equations.

We can try to work out the problem, if you want to try.?

E
In this circuit, assume initially Vi = 0v and Vref =+1v.
What will the voltage be at Va and Vo.??
Use simple resistive divider calaculations.

View attachment 256712
When I run the circuit without compactor, I got the different results, than compare with the full circuit simulation results. Please can you have look my simulated circuits:
 
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ericgibbs

Joined Jan 29, 2010
18,766
When I run the circuit without compactor, I got the different results, than compare with the full circuit simulation results. Please can you have look my simulated circuits:
hi,
You will get slightly different results.

I asked in post #46 that you calculate the required results, not use a simulation.
You did say you wanted to derive equations.?
E
 

ericgibbs

Joined Jan 29, 2010
18,766
hi

I asked:
In this circuit, assume initially Vi = 0v and Vref =+1v.
What will the voltage be at Va and Vo.??


Your answer:
Va = (10k)/200k)*10V = 0.5V

Vo = (195k)/200k)*10V = 9.75V


Conclusion:
If the Inverting input of the Comp is at +1V, how can the Vo be High at 9.75.???

And if Vo is Low, at close to 0V , how can Va be 0.5V.??

E
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
351
hi

I asked:
In this circuit, assume initially Vi = 0v and Vref =+1v.
What will the voltage be at Va and Vo.??


Your answer:
Va = (10k)/200k)*10V = 0.5V

Vo = (195k)/200k)*10V = 9.75V


Conclusion:
If the Inverting input of the Comp is at +1V, how can the Vo be High at 9.75.???

And if Vo is Low, at close to 0V , how can Va be 0.5V.??

E
As I know this theoretical operation of Comp is:

At the initial stage, If the non-inverting input of Comp is 0V since 0V is applied to the Vi and at the same time we have a continuous +1V at the inverting input of Comp. So, the inverting input is higher than the non-inverting input, thus, the output of the Comp will be approximately 0V (Actually, it will be delivered negative saturation voltage as an output, but in this case, the negative supply of the Comp is connected to the GND). Therefore, the Comp output is Approx. 0V.

Do you think that, Am I understand the operation of Comp?.

But, I realize that somewhere I did the wrong calculation as shown on the post #49. Please correct me.
 

ericgibbs

Joined Jan 29, 2010
18,766
But, I realize that somewhere I did the wrong calculation as shown on the post #49. Please correct me.
hi,
No, you must do it
Perhaps some other member will offer any further help on this Thread.

E

.
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
351
Why did you use 10V if Vo = 0V and Vin is also at 0V?

Also, can you calculate what Vin is needed to get 1V at the Va node (non-inverting input ) when and the comparator output is at Vo = 0V?
I have calculated the Va, as same as the post #31. That's why I got 0.5V at the Va.
 
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Jony130

Joined Feb 17, 2009
5,487
I have calculated the Va, as same as the post #31. That's why I got 0.5V at the Va.
But I hope that you see your mistake?


Now for this circuit

untitled.PNG
Try to find Vin needed to get 1V (Vref) at Va node. And this will be the threshold voltage (Vth1). Can you do it?
 
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