need help with the schmitt trigger

easily

It's

Thanks for the slap upside the head (mine, that is). I was _that close_ to launching into an analogy about flexible levers with detents...

Mark L. Fergerson

My copy of Abraham I. Pressman's "Switching and Linear Power Supply, Power Converter Design" is a first edition, I believe. (There are quite probably later versions.) However, in my copy there is a great explanation of much of what you want on pages 166 and forward. I wasn't able to grasp things so easily from the Art of Electronics on this subject, but found this one much better. Also, working out the rough details on paper isn't so hard.

How about describing what you think you know and what you just cannot get? It's a lot easier to answer specific questions that to write a treatise on the subject, you know.

Jon

A comparator is essentially a 1 bit analog to digital converter. It compares one voltage to another, and its output tells which is more positive (the output is high if the + input is more positive). The output state is indeterminate for inputs that are equal.

A schmitt trigger is similar in many ways, except that it has only one input that is compared to an internal reference voltage. And that reference is altered by the current state of the output so that when the last comparison indicated that the input was more positive than the reference, the reference charges to a more negative value (makes the decision stronger). Likewise, when the comparison indicates that the input is more negative than the reference, the reference changes to a more positive value. The effect is that once a decision has been made that the input has crossed through the reference voltage, the input has to reverse direction a bit before the decision can be reversed. It eliminates the possibility that a steady input can sit at equality with the reference with the output being in an indeterminate state for an arbitrary amount of time. It forces hard, clean output state transitions.

Any comparator can be configured to act as a schmitt trigger by adding some positive feedback components.

--
John Popelish

There is a Schmitt trigger oscillator made from descrete parts (resistors, caps, 3 transistors) and a short description of how it works at this address:

-Bill

"andrew james" wrote > im a sophomore electrical engineering student and i need some help in

The comparator has one threshold. When the input is higher than the threshold the output is in one state. When the input is lower than the threshold the output is in the opposite state.

The Schmitt trigger has two thresholds. When the input is higher than the upper threshold the output is in one state. In order for the output to change to the other state, the input must then go below the lower threshold.

You should, by now, have guessed that for the output to go to the other state again the input must go higher than the upper threshold again.

Your good health.

in short:

- a comparator compares one voltage level to another, and indicates which is higher by switching it's output digitally as a function of which is higher, using ALWAYS the comparision of both inputs

- a schmitt trigger will do identically, BUT the switching of the output is only done if and only if a specific difference between the inputs exist (called hysteresis)

this means a comparator is more sensitive to very small disturbances (like noise) in input (thus sometimes switching needlessly), while a schmitt trigger will ignore these very small differences and only switch at a relevant (or even predefined) difference

--- A comparator is like a box with two inputs and an output switch which can be either only on or off:

+-----+ +IN>----|+ | | OUT|---->OUT

-IN>----|- | +-----+

Its job is to make the output turn off when +IN is more positive than

-IN and turn on when -IN is more positive than +in.

Usually, the output switch is a transistor with its emitter connected to ground, and when +IN is more positive than -IN the transistor is turned off and its collector will be floating, but when -IN is more positive than +IN the transistor will be turned on and the collector will be connected to ground through the emitter:

Vcc | | +-----+----+ | |

+IN>----|+ C------->OUT | >--B |

-IN>----|-/ E | | | | +-------|--+ | | GND

By connecting a resistor to the output of the comparator and making

+IN more positive than -IN, like this:

Vcc | +--------+ +V | | | | | [R2] | +-----+----+ | | | | |

+IN>-+--|+ C--|---+---->OUT (+V) | >--B |

-IN>-+--|-/ E | | | | | | +-------+--+ | | +----------+ | GND

The transistor will be turned off and OUT will be pulled up to +V through R2.

However, by making -IN more positive than +IN, like this:

Vcc | +--------+ +V | | | | | [R2] | +-----+----+ | | | | |

+IN>--+-|--|+ C--|---+---->OUT (GND) | | | >--B |

-IN>--|-+--|-/ E | | | | | | +-------+--+ | | +------------+ | GND

The transistor will be turned on and the output will be pulled down to ground through the collector-to-emitter junction of the transistor.

Notice that by using this convention, when +IN is more positive (higher) than -IN the output will be high, (at +V) and when +IN is less positive (lower) than -IN the output will be low (at GND)

That's how a comparator works. Hysteresis tomorrow... :-)

BTW, Do you know how a voltage divider works?

-- John Fields

Great overview. But the OP claims to be a "sophomore electrical engineering student." At the university where I taught computer science, I also had a number of electrical engineering students in every class I taught (usually 4-6, out of say 55-65 students, total.) Bright folks, and the classes they were pulling down in their sophomore (2nd) year were substantial and "difficult." I desperately hope the OP isn't at the very basic level of understanding where your answers were targeted. If so, that school isn't pacing their students anywhere near fast enough. In my reply, I had assumed the OP was interested in analyzing the analog details of a basic schmitt trigger, for example. But if the OP was asking only about the essential idea, your answer is excellent.

Jon

Thanks. You are thinking about the student's educational plan much more than I do. I just try to figure out where their head is at the moment and punt it from that point.

--
John Popelish

--
Hysteresis...

Let's say that we have a see-saw which we can't see connected to a
switch, like this:


                                                
             =================O================
                              |                  
                     +V--[R]--+---->OUT
                              |
                              |OUT
                               |
                               |----|+     C------->OUT
        |  >--B    |
-IN>----|-/     E  |        
        |       |  |
        +-------|--+ 
                |
                |
               GND 


and simplifying it to:


              +V
              |
             [R] 
              |
              |
+IN>----|+   | 
        |  >--+--->OUT
-IN>----|-/      
        
we can liken +IN to the left end of the see-saw and -IN to the right
end.


Last installment tomorrow.

Maybe...