741 annoying problem

Discussion in 'General Electronics Chat' started by hazim, Jun 20, 2009.

  1. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    What is the problem here:
    A 741 op amp used as a voltage comparator. 12V is connected to pin 7, ground at pin 4, a constant voltage 1.9V at pin 2. Pin 6, the output, is free for now. I varies the voltage at pin 3 from 0 to 12V, and the output does not vary, it's always about 10.7V...
    Actually it works fines sometimes, but when I test the voltages at the input, the problem happens, and the output of the 741 becomes always 12V. And it works fine again if I cease trying it for few minutes, and this is being equipped..
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Can you post a schematic?
    It may sound simple but a drawing tells more.

    Greetings,
    Bertus
     
  3. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    It's very simple and basic....anyway I think I found the problem... it seems because I am comparing low voltages at input (1.9V) this problem happens. what I am doing is the second attached file, a heat sensor circuit... in this circuit I may change R1 from 47k to 10k and it should work fine.... I'll try it now
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    When you want to "detect" lower voltages, you could try a LM324.
    That is an quad opamp.

    Greetings,
    Bertus
     
  5. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Yes I know about LM324... I have one here...
    Thank you anyway
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A lousy old 741 opamp does not work when its inputs are closer than 4V from a supply voltage.

    an LM358 is a dual opamp made the same as an LM324 quad opamp.

    An MC34071 is a single opamp with the same pins as a 741 and works fine when its input is as low as 0V in your circuit.
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    You know, I don't think lousy is part of the LM741 name... :D
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The 741 has been a lousy opamp for 41 years.
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Ever try to use a 709?
     
  10. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287

    Hi Hazim:

    Your annoying problem is caused by the 741 being an annoying little chip. What you are experiencing is a phenomenon called "latch up" for which the 741 is notorious. It is NOT a suitable chip for a comparator for exactly this reason.

    What happens is that, if your input voltage exceeds the supply voltage it goes into an INTERNAL POSITIVE feedback state...the only cure is to power cycle the thing.


    The real solution is to use an op-amp designed for comparator usage.

    Eric
     
  11. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287

    Dear Guru:

    Though you are absolutely correct, we don't need to be too harsh. Remember, it was the 741 that put the op-amp in the common man's hands. For the first time, a novice could build an op amp circuit that sort of worked without having to contend with custom compensation and all that. That left the rest of us to play with 709's, aka, "The 741 for consenting adults." :D

    Eric
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    In the '70s Motorola made an opamp that did not have any of the problems of the lousy old 741 opamp and the datasheet showed the comparisons.
    It worked from a supply voltage that was much lower.
    It worked from a single polarity supply voltage.
    It had a much better frequency response and a much higher slew rate.
    It had a lower amount of noise.
    It costed the same.
    But I can't remember which one because there are many opamps that are better than the lousy old .....
     
  13. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    A good choice for comparator is the LM339, it is 4 in 1 package. They make bad op amps, op amps and comparators are cousins, but separate components.

    Eric, I don't think Audioguru can ever accept the 741 as a good teaching tool. Took me a while to realize it, but all the problems it has are perfect to show students where various limitations are. It is as if every thing that can go wrong has been exaggerated in the 741, and yet it works for a lot of applications just fine. The fact they are cheaper than dirt doesn't hurt either.
     
    Last edited: Jun 22, 2009
Loading...