Trip Points either voltage or current levels to trip a comparator

[Billy Mayo]

There is Two different kinds of comparators or op amps used as comparators:

COMPARATOR TYPE#1
1.) A Positive voltage that crosses the threshold limit on the input will make the Output of the comparator or op amp go HIGH
A Negative voltage that crosses the threshold limit on the input will make the Output of the comparator or op amp go LOW

I'm weird how the Positive voltage threshold value trip point is 5 volts , but the negative voltage threshold is like 2.5 volts
Not sure how they have two different threshold limits depending on the Polarity of the voltage

COMPARATOR TYPE#2
2.) Either Polarity, a Positive or Negative voltage will make the output go HIGH on the comparator or op amp

How can Either Polarity can cross the threshold voltage on the input and get a HIGH on the output?

Comparator type#1 , Positive polarity on input made output HIGH and Negative polarity on input made output LOW
Comparator type#2, Either positive or negative polarity will make the output HIGH

What I ran across was that the Positive and Negative polaritys threshold points were different

Positive Polarity at 5 volts made the comparator or op amp SWITCH to a HIGH state on the output
Negative Polarity at 3 volts made the comparator or op amp SWITCH to a HIGH state on the output

It's weird how the Positive polarity was 5 volts and the negative polarity was 3 volts

It seems like the comparator or op amp is off balanced when comparing

 

[audioguru]

The parallel capacitor is the stray capacitance or the IC input internal capacitance?

The series resistor feeds the IC input that has some stray capacitance to ground that slow the switching and causes a delay (a lagging network).
A capacitor parallel to the series resistor boosts high frequencies and speeds up the switching (a leading network).
If you select correct values for the resistor and capacitor then they cancel each other.

if you apply a positive voltage it will switch the comparator or op amp at 4.9 volts
if you reverse the voltage to negative , it will switch the comparator or op amp at 3.2 volts
i don't understand why the comparator switches earlier when applying a negative voltage or current to the threshold input of a comparator or opamp
why does the inputs polarity change the threshold switching?

Please learn about comparators.
The threshold voltage changes when the input polarity changes because of hysteresis in the circuit.

Why do i have to test the comparator input threshold at both Polaritys?

To see if the hysteresis works and is correct.

 

[audioguru]

Your millions of questions about basics indicates that you learned NOTHING about electronics.
I like to talk about complicated circuits with my peers but I do not like to teach the basics to you. Good-bye!

 

[Billy Mayo]

The threshold voltage changes when the input polarity changes because of hysteresis in the circuit.

I don't get how the input polarity can test and check the hysteresis

Both Polarity have a different threshold because of the hysteresis , how and why?

So the Positive Polarity has NO hysteresis?
the Negative polarity has the hysteresis?

Why do i have to test the comparator input threshold at both Polaritys?To see if the hysteresis works and is correct.

How do you check or test the hysteresis of a comparator or op amp ?

 

[Billy Mayo]

How do you know which Polarity has the Hysteresis? it is always the negative polarity that has the hysteresis?

If a comparator or op amp has hysteresis, how would a tech know the hysteresis is not working? when testing the inputs and outputs?

 

[ChrisP58]

Billy, Can you please post schematics of the circuits you are describing? Some (most) of your descriptions don't make enough sense to answer your questions accurately. An answer to a badly framed question will get you the a bad answer. And that will just lead to more confusion for you.

 

[Billy Mayo]

Bench Check Tests I do when testing comparators in the circuits , I'm guessing there is different types of Hysteresis

Hysteresis TYPE#1
1.) Raise the Positive voltage up until you FIND the threshold switching point, the output switches states to a HIGH
2.) Lower the Positive voltage down until you FIND the threshold switching point, the output switches state to a LOW

The Positive voltage going up will have a different threshold switching point voltage and current than the positive voltage going down
You turn the trim pot up until you find the threshold switching point, you turn the trim pot down until you find the threshold switching point
This will change the output state from HIGH to LOW
But the Positive voltage going up will have a different threshold hold switching point value than than the positive voltage going down

The positive voltage going UP will switch at +5 volts
But the Positive voltage going Down will switch at 1 volt

How does the Hysteresis do that?

I would think it would switch at +5 volts going UP and going DOWN , not at two different voltages

Why is that and how does the hysteresis do this?

It seems that the Positive voltage going UP doesn't have the Hysteresis ONLY the Positive voltage going Down?

What Hysteresis is this type called ? is this the positive feedback one?

Hysteresis TYPE#2
1.) Raise the Positive voltage up until you FIND the threshold switching point, the output switches states to a HIGH
2.) Flip the Polarity to a Negative voltage ( - ) and raise the voltage until you FIND the threshold switching point, the output switches states to a HIGH

It seems like there is Two different Hysteresis , one for each Polarity?

Because the Positive voltage will switch at + 5 volts
and the Negative voltage ( - ) will switch at -3 volts , I would think it would be at - 5 volts

But this output only goes to a HIGH state not a low state , either polarity positive or negative

What Hysteresis is this type called ? is this the positive feedback one?

 

[ChrisP58]

Please post some drawings of your circuits. Your descriptions aren't clear enough to understand. We don't need the whole system, just the circuit fragments that you are asking about.

Hysteresis just means that the rising trip point and the falling trip point are different. I have never heard of different types of hysteresis having names.

Hysteresis in a v-com or an op-amp is most simply accomplished with positive feedback with a resistor from the output to the non-inverting input. Basically you are changing the reference voltage at the non-inverting input so that the voltage at the inverting input needs to be at a different level to trip. It is possible to do positive feedback to the inverting input if you have some other circuit element that does a state inversion.

And voltages are not always positive. If a v-comp is set up with both positive and negative supply rails, then the input thresholds can also be positive and negative. And the output will also switch between the two rails. Which means the output will not be at ground when low, but negative.

 

[large_ghostman]

when dad made the first electronic home built incubator we had, he used a velleman kit which from memory (it was 3-4 years ago now) used a comparator, one input was set with a resistor divider and the other was a divider formed with a fixed resistor and a thermistor, there was also hysteresis and you could set the amount by changing a resistor so you could have the heat source trigger on at say 100f and off at 101 or any other number you like.
it was that circuit that made him first join here and ask a question about the circuit because at the time he could barely wire a household plug lol, i cant remember the part number now but it was a old quad comparator and when it went high it drove a relay so you had to be careful and set the hysteresis so you didnt get relay chatter, i mention it because the reason you get a different trip point depending on polarity will probably be due to how they have set the hysteresis , so if you look at the datasheet or the part you should be able to work out what it is by the value of the resistor in the circuit used

 

[Billy Mayo]

a different trip point depending on polarity will probably be due to how they have set the hysteresis

Yes, that is what is happening, but how do they do this?

When you switch the Polarity switch to a ( - ) , it will cause the output to go LOW , until you raise the (-) polarity until you cross the threshold trip point which will switch the output to a HIGH

When you switch the Polarity switch to a ( + ) , it will cause the output to go LOW , until you raise the (+) polarity until you cross the threshold trip point which will switch the output to a HIGH

The output starts off in the LOW state , until you cross the threshold

Hysteresis Type#1
the Hysteresis for the Positive and Negative Polarity have different Crossing thresholds

The Positive rising has a different rising trip point than the Negative Rising trip point

( When you Flip the Polarity switch it causes the output to switch to a LOW state)

This Hysteresis type#1 we never do a falling trip point , we only switch the polarity and so Rising Trip Point

Hysteresis just means that the rising trip point and the falling trip point are different

YES , That is what I am getting, But how do they set or what sets the rising trip point and what sets the falling trip point? they both have different voltages

Hysteresis Type#2
The Hysteresis for the Positive Rising has a different voltage than the Positive Falling trip point

 

[audioguru]

Hysteresis is made by adding a little positive feedback. Then the input must swing a little more than the reference voltage.
I copied a lot of this diagram from the datasheet of the LM339 quad comparator IC:

 

[large_ghostman]

LM339 i think that was the chip! i will ask him later! only mentioned it because it was that got then me into electronics!

 

[Billy Mayo]

The Positive-Going Threshold and the Negative-Going Threshold have different values
At work they reverse the Polarity on the Vin input
1.) Positive-Going threshold for the Positive Polarity, cause output to go HIGH
Postive voltage on Vin input, Move the trim pot CW until the output is HIGH
2.) SWITCH the polarity to Vin input to a Negative voltage
3.) Negative-Going Threshold for the Negative Polarity, causes output to go HIGH also
Negative voltage on Vin input, Move the trim pot CW ( not CCW ) until the output is HIGH

They other way they do it at work is
The Positive-Going Threshold and the Negative-Going Threshold have different values
They keep the Polarity Positive on the Vin Input
1.) Move the trim pot CW until the Positive-Going Threshold switches the output to a HIGH
2.) Move the trim pot CCW until the Negative-Going Threshold switches the output to a LOW

Can't you see the difference between the Two examples?

The Ones in the datasheets are different also cause they Flip the non-inverting input or the inverting input to be Vin input

Why do they use Hysteresis?

I have seen transistors used as a switch with Hysteresis, but what for? so the input doesn't cause False switching from noise?

 

[audioguru]

I answered this long reply on the other website forum one minute ago.

 

[Reloadron]

In addition to all the fine information and help provided by audioguru **broken link removed**. It explains comparator circuits fairly well with a focus on the LM339 family. There is a good explenation of comparator with hysteresis as well as window comparator circuits.

Ron

 

[large_ghostman]

good info ron thanks!

 

[Reloadron]

Most welcome LG, one of my favorite inks.

Ron

 

[large_ghostman]

if i could make pdf's i would turn alot of that info into one and print out for my folder, i will have a hunt round later see what you have to do to make pdf's, i am not ripping the site off but useful info for the folder and takes ages otherwise to copy the drawings and write it all out

 

[Reloadron]

if i could make pdf's i would turn alot of that info into one and print out for my folder, i will have a hunt round later see what you have to do to make pdf's, i am not ripping the site off but useful info for the folder and takes ages otherwise to copy the drawings and write it all out

Morning LG over across the pond.

Here is something else you can try. Often really good websites or web pages simply go away. Now how to save them will vary from browser to browser but all you need to do is Left click File at the top of page and choose Save Page As. Leave the File Name as it is or create your own in the dialog box. For Save As Type choose text or HTML. If you choose HTML you can save the web page to anywhere on your computer (Documents or even create a sub folder in Documents) and open it anytime on your computer. The entire voltage comparator web page is only about 19.5 KB.

There is also Web2PDFConverter for use with the Google Chrome browser which will save a web page (most anyway) in the pdf format. Also other stuff out there but given the options I just save as a web page (.html or .htm and use any browser to open the saved file.

Ronhttps://chrome.google.com/webstore/detail/web2pdfconverter/bkanhckocooacphbnclgcndnpfpoppdk

 

[large_ghostman]

thanks for that, i will save it and print out, its not my computer so i dont get a say when it's suddenly formatted i keep most my stuff on usb stick but i have had one fail so i print the best stuff for my folder