schmitt trigger comparator

[robismyname]

I built up a schmitt trigger circuit (attached). I placed an o-scope on the output pin (#3) and saw that my high transitions are clipped whereas my low transitions are not clipped (see 2nd attachment).

My resistor values suggest that a negative transition occurs when a voltage of 4.921V or greater is at +Vin (pin #2).

My resistor values suggests that a positive transition occurs when a voltage of 2.98V or less is at +Vin (pin #2).

Since Im using a sine wave input going into pin 2 (via a function generator) why am I not seeing a clipped sine for both positive and negative transitions?

Please help.......

 

[nashwan]

hi
i think u use here open collector comparatore bcz use R 3k between output pin and vcc.

i do simple simullation for ur circuit with lm339 its work ok but positive transition
occur at 3.9v .

function generatore here is sin wave 50hz swing from 0....to 5v i add a d.c components to shift sin wave.
if u apply sin wave swing from positive to negative ur circuit will not work
just chek that

 

[robismyname]

hi
i think u use here open collector comparatore bcz use R 3k between output pin and vcc.

i do simple simullation for ur circuit with lm339 its work ok but positive transition
occur at 3.9v .

function generatore here is sin wave 50hz swing from 0....to 5v i add a d.c components to shift sin wave.
if u apply sin wave swing from positive to negative ur circuit will not work
just chek that

3K is just a pullup resistor........

I didn't calculate the threshold voltage myself. I used an online voltage threshold calculator. Its telling me triggering voltage occurs at 4.9V and 2.9V.....

http://hyperphysics.phy-astr.gsu.edu/HBASE/electronic/schmitt.html

my sine wave from the function generator was 1KHz swing from 0.....to 5V with no dc component. I will add dc offset and see if that changes anything......But why will it not work with positive and negative swing sine wave?

Added after 50 minutes:

hi
i think u use here open collector comparatore bcz use R 3k between output pin and vcc.

i do simple simullation for ur circuit with lm339 its work ok but positive transition
occur at 3.9v .

function generatore here is sin wave 50hz swing from 0....to 5v i add a d.c components to shift sin wave.
if u apply sin wave swing from positive to negative ur circuit will not work
just chek that

-2.5V DC Offset provides what I believe to be the correct ouptut I would expect to see from a schmitt trigger comparator. But why does -2.5V seem to work but 1.5V and/or 1.8V dc offset doesn't provide a clipped sine wave output?

Please help.....

 

[nashwan]

hi
the circuit in that link
http://hyperphysics.phy-astr.gsu.edu/HBASE/electronic/schmitt.html

use dual power supply and output of that comparatore is push-pull
with this circuit u can apply ac signal swing from positive to negative

but ur circuit seems like single power supply with open collector output.
check that pls

if u work with single power supply and ur input signal swing from 0..to vdd
u can use digital ics as CD4093,CD40106,CD4584 as schmitt trig

 

[FvM]

why am I not seeing a clipped sine for both positive and negative transitions

Yes, that's a plausible question.
Actually a square wave could be expected with a correct working comparator.

I think, your comparator is not working correctly, because you the input voltage, also the threshold at the +ve input is exceeding
the common mode voltage of Vcc - 1.5 V. You may want to correct the problem by changing the resistor ratio, e.g. 2k ínstead of 110k.

Input voltages below zero aren't a problem as long you limit the comparator input current. A series resistor of e.g. 1 k would do.

As another comment, changing the oscilloscope input coupling to DC would considerably improve the informational value of
the waveforms. It's a bit annoying to see this fine instrument used improperly.

 

[robismyname]

hi
i think u use here open collector comparatore bcz use R 3k between output pin and vcc.

i do simple simullation for ur circuit with lm339 its work ok but positive transition
occur at 3.9v .

function generatore here is sin wave 50hz swing from 0....to 5v i add a d.c components to shift sin wave.
if u apply sin wave swing from positive to negative ur circuit will not work
just chek that

what software are you using for this simulation. Is it free?

 

[nashwan]

hi robis
i use circuit maker its not free but there is CircuitMaker Student Edition is a free download
**broken link removed**

and see that
**broken link removed**

and
www.tina.com