LM339 problem

Discussion in 'The Projects Forum' started by rsfoto, Jul 27, 2013.

  1. rsfoto

    Thread Starter Member

    May 14, 2013
    134
    1
    Hi,

    I have breadboarded a test set up with a LM339N and my problem is I can not compare voltages lower then 1V. If I set the reference below 1V nothing happens.

    What am I doing wrong ?

    Thanks for any tips :D
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,120
    3,046
    You might try eliminating (or increasing) the hysteresis resistor to see if that has an effect. The common mode for the comparator should include ground, so that should not be the problem. A non-rail-to-rail op-amp would show this problem, but again this comparator should be OK.
     
    rsfoto likes this.
  3. rsfoto

    Thread Starter Member

    May 14, 2013
    134
    1
    :
    Hi,

    No matter what I do I am stuck at ~1V. I went uo to 3MΩ with the hysteris resistor and did not change anything. I took it out no change ...

    :confused:

    After writing this it came to my mind ¿ Where do I have to measure the voltage ... Between Input and Output PIN In this case PIN 4 and PIN 12 (GND)

    I have been measuring at the +12V and the slider of the POT.

    ¿?

    OK, now I know how to adjust my POT in order to get the Low REF and Input voltages. I was working the way around as I was measuring on the wrong points :(

    Shame on me. I really feel like a fool now having made the ridicule here ...
     
    Last edited: Jul 27, 2013
  4. wayneh

    Expert

    Sep 9, 2010
    12,120
    3,046
    So for example if the + input is held at say, 0.8V, and the - input passes from 0.7 to 0.9V, the comparator does not respond? But the same experiment at 1.8V would work fine? Try a different comparator on the same package. It sounds like a bad part.

    You haven't shown a 0.1µF decoupling capacitor across the power pins. Probably won't change the problem, but it's a good idea especially when you have a problem. How clean is the DC supply? Your reference voltages are tied to it, so it should be regulated.
     
    rsfoto likes this.
  5. rsfoto

    Thread Starter Member

    May 14, 2013
    134
    1
    Thanks Wayneh,

    I corrected my previous post after you wrote this last message.

    It was an User error not knowing where to correctly measure the voltages. Now that I played a bit and informed myself in the WWW how to judge a Voltage divider I now undestand what I was doing wrong.

    It is good to write down the ideas and so many times one tries to search the other way around. When I was confronted with the problem I was measuring on the wrong points and so the result was not what I expected.

    Sorry next time I do not edit my previuos posts I just add my ideas as a new post.

    Again I appreciate your great support.

    It is a good power supply, at least I think, like this ones http://www.agspecinfo.com/pdfs/H/HF100W-S.PDF

    :)
     
    Last edited: Jul 27, 2013
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Your feedback resistor needs to go to the +input for hysteresis. The way you have it connected, you have negative feedback. Comparators will generally oscillate with negative feedback.
    Also, LM339 is only guaranteed to sink about 6mA. You are sinking about 11mA. Most will probably work. Some may not.
     
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  7. rsfoto

    Thread Starter Member

    May 14, 2013
    134
    1
    Thanks Wayneh and Ron. It is great to get so much help.

    I am just reading the TI datasheet and if I understood correctly you refer to

    " Output Sink Current " and there says min 6.0mA and Typ 16mA

    http://www.ti.com/lit/ds/symlink/lm339-n.pdf

    I just made some tests and I can go down to 40mV and it switches On and Off.

    It was a total User error from my side measuring in the wrong way my Voltage dividers :(
     
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