Help with Volt comparator

Discussion in 'Technical Repair' started by gerryr1gerryr, Mar 26, 2017.

  1. gerryr1gerryr

    Thread Starter Member

    May 24, 2011
    13
    0
    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
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    8,625
    1,964
    .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.
     
  3. StayatHomeElectronics

    AAC Fanatic!

    Sep 25, 2008
    1,021
    71
    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.
     
  4. crutschow

    Expert

    Mar 14, 2008
    19,539
    5,445
    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.
     
  5. gerryr1gerryr

    Thread Starter Member

    May 24, 2011
    13
    0
     
  6. gerryr1gerryr

    Thread Starter Member

    May 24, 2011
    13
    0
    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
     
  7. crutschow

    Expert

    Mar 14, 2008
    19,539
    5,445
    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.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    8,625
    1,964
    The LM393 is a dual comparator, which needs pull-up resistors from its outputs to the +ve supply rail.
     
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