Op-amp comparator

Discussion in 'The Projects Forum' started by jimmychacha, Jan 28, 2011.

  1. jimmychacha

    Thread Starter New Member

    Dec 1, 2010
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    Hello all, I would like to construct a simple comparator circuit using only DC, no AC signal. My circuit design uses a single DC supply which varies between 12 and 13.5 volts, and my op-amp is an LM741. I'm using a 6.2v zener diode for reference into pin 2, and a voltage divider into pin 3, so that when the supply rises above 12.5, pin 3 will have a greater voltage, thus turning the output high. However I am getting an output of only .6v no matter what pin 3 does. The op-amp's Vcc varies with the supply, is this causing a malfunction, or am I missing something else in comparator function? Thanks
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I don't think the supply voltage explains your stuck output. You might not get more than about 6V swing out of a 741 with only 12V supply though.

    The best thing would be for you to post a schematic so that people can see exactly what you are doing.

    You should also check your circuit carefully for mistakes, such as missing connections, wrong connections, wrong resistor values, short circuits, chip wired in upside down...
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    You have lots of problems. 741's are very old op amp designs and are only characterized for dual rails, +/- 15 VDC. They have low input impedance and are not capable of operating like a voltage comparator. You would be much better off with an almost equally old design of a comparator, the LM311. It's a lot faster, can handle more than 10 X the current out, etc.
     
  4. jimmychacha

    Thread Starter New Member

    Dec 1, 2010
    10
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    Further Testing has revealed that when my load (a 5v relay) is disconnected, the op amp seems to be working fine, the output is very close to Vcc. As soon as I connect the load, the output drops below 2 volts. Any ideas?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Get a data sheet for a 741. Look at the output current it can handle. Then look at the current your relay's coil needs. There is bound to be a difference.

    It's just so much easier to use a comparator.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Not only does the 741 have only modest output current capacity, but its output cannot swing to near the supply voltage. Asking it to drive a relay tied to a 12V single supply sounds like a bad idea.

    The open-collector comparator is the right way to go unless you are really not able to buy parts. Remember to include a diode or other protection against back-emf when the coil is turning off.

    If you are desperate, you could probably drive a transistor from your existing 741, and drive the relay from that, but it would be a bodge. You would drive the transistor via a potential divider to make sure it switches off, since the 741 output won't go near the rails.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Come to think of it, did you say a 5V coil, but you have a 12V supply? You will need to drop some volts if you do get an effective relay drive, or the coil will get too hot!
     
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