About the principle of Schmitt trigger

[foxjuly]

Hi, I find it hard to understand the principle of Schmitt Trigger, especially the hysteresis part.
I watched Dave's explanation on Schmitt Trigger, But still cannot figure out how a positive feed back create the hysteresis, i.e. the upper threshold and lower threshhold. So, Could anyone give some suggestion on how to understand this part?
https://www.eevblog.com/forum/blog/eevblog-941-the-amazing-schmitt-trigger/

[IanB]

You can build one yourself to understand this.

Get a plastic water bottle and half fill it with water. Now balance the bottle horizontally on your hand. Tilt it slightly one way, and what happens? Now try to tilt it the other way--what happens this time? Play with this a bit and you will understand the principle of operation.

[foxjuly]

Thanks a lot! But according to the 2nd rule of Schmitt Trigger, the IC itself will always try to keep the two input at similar level. So, in this case how to use this rule to explain the phenomenon?

[IanB]

This "rule" is not really a rule. The actual rule is that the output of the comparator responds to the difference between the inputs. If input 1 is greater than input 2 then the output is maximum; if less than then the output is minimum. We can relate this difference to the "force" that is trying to tip the water bottle one way or another. If the weight on the left hand side is greater than the weight on the right then the bottle tips to the left; if less than then the bottle tips to the right. It is quite hard to keep the bottle balanced level, it always tends to tip one way or the other, especially if given a slight nudge.

The positive feedback is given by the water in the bottle. Once the bottle starts to tip one way the water rushes to that end of the bottle. This dramatically increases the tipping force and the bottle rapidly tips all the way.

[macboy]

The state of the output (either high or low) affects the reference point against which the input is compared. The amount of this effect could be small or large, but the key point to understand is: after the output switches state, the reference point moves.

This is analogous to the water bottle, where the state of the bottle (tilted left or right) affects the center of gravity or balance point of the bottle.
(p.s. excellent analogy, IanB!).

[w2aew]

Maybe my video on the Schmitt Trigger and the description of the Hysteresis will help:

[Dubbie]

I really like the water bottle analogy. Hadn't thought of that one before.

[foxjuly]

Hi, Thank all you guys for replying this topic. Ian's water bottle metaphor is great. Now I've already understand how Schmitt Trigger works. However, when we try to analyze the negative feedback OpAmp, I find it hard to understand how the OpAmp works in a "fidelity way", not a ON/OFF way, and in the OpAmp case why two rules "no current flow in/out of the IC", and "the OpAmp try to keep both input the same" apply. I find it hard to reconcile Schmitt Trigger with the general OpAmp rule, even though I can easily understand the rationale without the OpAmp rule. Yesterday I tried to use "comparator rule of OpAmp" to understand Negative feed back OpAmp but failed. So, does anyone know why we have to use the two rule to analyze an negative feedback OpAmp rather than use the "comparator rule"? Thanks a lot!

[Dubbie]

negative feedback is for removing distortion/setting gain etc,
positive feedback is a rail to rail comparator/schmitt trigger effect.