Help with Volt comparator

Thread Starter

gerryr1gerryr

Joined May 24, 2011
20
As you can tell from my question I am a real newbie amateur but could someone please take the time to explain why the attached voltage comparator actually.
I have simulated it on LTSpice and built it and it does work.
I am confused as to why the LED lights up as it seems to have 2 Positive inputs one from the OpAmp and one from the power supply.
What am I missing and if possible in baby language please,

Thanks hoping some kind soul can explain.


PS tried to upload the LTSpice file but get an error" there was a problem uploading your file".

Is there some problem uploading .asc or .jpg files
 

Alec_t

Joined Sep 17, 2013
14,280
.asc and .jpg (unless excessive in size) are both allowable uploads, but you may not have permission unless you have already posted a certain number of times.
 
A comparator is only really looking at whether one input is larger than the other. The value of concern is (V+) - (V- ). If this number is positive then the comparator output goes to positive, and if negative the output goes to the negative side.

That is the general idea.
 

crutschow

Joined Mar 14, 2008
34,285
How did you try to download the .asc file.
You should just use the "Upload a file" at the bottom of the posting window.
For graphics infor you can just cut and paste a screen dump directly into the post.
 

Thread Starter

gerryr1gerryr

Joined May 24, 2011
20
As you can tell from my question I am a real newbie amateur but could someone please take the time to explain why the attached voltage comparator actually.
I have simulated it on LTSpice and built it and it does work.
I am confused as to why the LED lights up as it seems to have 2 Positive inputs one from the OpAmp and one from the power supply.
What am I missing and if possible in baby language please,

Thanks hoping some kind soul can explain.


PS tried to upload the LTSpice file but get an error" there was a problem uploading your file".

Is there some problem uploading .asc or .jpg files
 

Thread Starter

gerryr1gerryr

Joined May 24, 2011
20
Thanks all for your replies.
Tried again to upload files but still get the error message.
Overnight I sort of worked out the problem.
It seems I was assuming that the output was POSITIVE but in reality it was NEGATIVE.

Am I correct in assuming that (I am using a LM393P OpAmp) that
if V reference is pinned to the NEGATIVE on the Op Amp then the output will be POSITIVE (HIGH)
and
if the V reference is pinned to the POSITIVE then output will be NEGATIVE (LOW)
Any further basics would be appreciated

Thanks All

PS would also appreciate any help in overcoming the UPLOAD problems
 

crutschow

Joined Mar 14, 2008
34,285
For an op amp if the positive (+) input is more positive then the negative (-) input, the output will be high.
If the (+) input is more negative [(-) input more positive than the (+) input], then the output will be low.
 
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