comparator's with hysterisis

[CameronHanson20]

Hello everybody. I am playing around with comparatorsband hysteresis. I have a circuit built up and have calculated the thresholds and verified them. My question is why do you use a proportion to figure out the feedback voltage and not use the voltage divider. I know the answer is probably simple but I just don't understand why. I've attached my circuit.

[IanB]

Hello everybody. I am playing around with comparatorsband hysteresis. I have a circuit built up and have calculated the thresholds and verified them. My question is why do you use a proportion to figure out the feedback voltage and not use the voltage divider. I know the answer is probably simple but I just don't understand why. I've attached my circuit.

When asking a question it's a good idea to lay out and describe everything you are doing and each step in your calculations, and then ask your question about which step is puzzling you.

Otherwise there are probably many things in your mind and many details written down that you have not shown us, and we can only guess at the missing information before attempting to come up with an answer.

Here you have said "why do you use a proportion to figure out...?". Well maybe you use a proportion, but I do not know what I would use. Maybe I would do things a different way. Do you see the problem?

[CameronHanson20]

Yes, I figured this would happen. So I was watching wae2w circuit fun about building a analog control for a rc servo. Link is http://m.youtube.com/watch?v=iaGZOO33Z-o. When he goes to the calculations towards the end. He has vth= vref + vref(R1\R2) and Vlh = vref - vref(R1\R2). That's the confusion. Why does he don't he proportion of R1 to R2, instead of the voltage divider rule?

[IanB]

Yes, I figured this would happen. So I was watching wae2w circuit fun about building a analog control for a rc servo. When he goes to the calculations towards the end. He has vth= vref + vref(R1\R2) and Vlh = vref - vref(R1\R2). That's the confusion. Why does he don't he proportion of R1 to R2, instead of the voltage divider rule?

But he does use the voltage divider rule!

He has:

    V_th = V_ref + V_ref (R₁/R₂)

Let's factorize out V_ref

    V_th = V_ref (1 + R₁/R₂)

Now put over a common denominator:

    V_th = V_ref [(R₂ + R₁) / R₂]

Finally, rearrange:

    V_ref = V_th [ R₂ / (R₁ + R₂) ]

And there is your voltage divider rule! It was just in disguise.
   

[T3sl4co1l]

https://www.eevblog.com/forum/beginners/comparator's-with-hysteresis/msg632402/#msg632402

Merge threads?