Inverting Op Amp Trouble

Discussion in 'Analog & Mixed-Signal Design' started by Jem2200, May 15, 2018.

  1. Jem2200

    Thread Starter New Member

    May 15, 2018
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    Atferrari, yes that is where I would input the voltage I want to change signs.
     
  2. ebp

    Active Member

    Feb 8, 2018
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    With the resistor between the noni-inverting input and ground, is the voltage at the NII exactly zero or a few millivolts off?

    Does the resistor from the output to ground cause any change? I the output voltage exactly equal to the input voltage?

    What are the voltages at the offset trim pins?

    One more experiment: Using the same gain setting resistors, try configuring the amp as non-inverting (gain of 2).
     
  3. Jem2200

    Thread Starter New Member

    May 15, 2018
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    with a resistor between the NII and ground the voltage at the NII is exactly 0V.

    With a resistor between the output and ground the output voltage decreases.

    The voltages at the offset pins are different for different op amps. I tried with a few. Some were 10mV and -10mV. Some were about 1.7V and -10mV. One was 12V and -10mV. I may have gotten the polarity wrong on those values.

    Setting it up as an inverting amp the output is 0V. The - pin is also 0V.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    So I see only two choices.
    You either have a bad op amp or a bad connection.
     
  5. Jem2200

    Thread Starter New Member

    May 15, 2018
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    I have tried 13 different op amps so I doubt they’re all bad. Must be my connections then. I don’t know what would be wrong. All the wires are tight in the breadboard, and I’ve moved the op amp around on it so I’ve tried multiple rows. I’ll take a closer look at the connections tonight.
     
  6. ebp

    Active Member

    Feb 8, 2018
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    I agree with crutschow.

    The offset adjust pins should be quite close to the negative supply voltage.
     
  7. Jem2200

    Thread Starter New Member

    May 15, 2018
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    Ok I’ll check connections. Fingers crossed it’s just a poor connection!
     
  8. Jem2200

    Thread Starter New Member

    May 15, 2018
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    Just checked all the connections. They’re all good. I guess I have 13 bad op amps. The odds of that seem insane to me but I guess that’s what I’m looking at.
     
  9. RamaD

    Senior Member

    Dec 4, 2009
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    Can you measure the voltage at Pin 4 w.r.t. Gnd? Also, Pin 7 w.r.t. Pin 4?
    I am still suspecting that the negative supply is inverted. That means, Pin 4 is at +12, Pin 7 w.r.t. Pin 4 is almost zero. That means the opamp is not powered, and the output voltage is connected to the input voltage through 2 resistors! That possibly explains all your results, viz., o/p voltage same as input, output voltage drops when you connect a resistor to the output to ground.
     
  10. BobTPH

    Well-Known Member

    Jun 5, 2013
    1,089
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    How about this possibility:

    You do not have LM741 op amps, which are 1 per 8-pin DIP, but actually have one of the many dual op-amps in 8-pin DIP?

    You could find out by connection V+ to pin 8 of one you are willing to sacrifice. If no smoke comes out, try connecting the rest of the circuit with:

    pin 1 - output
    pin 2 - inverting input
    pin 3 - non-inverting input

    Bob
     
  11. Jem2200

    Thread Starter New Member

    May 15, 2018
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    Pin 4 wrt ground is -12V and pin 7 wrt pin 4 is 24V :(
     
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  12. Jem2200

    Thread Starter New Member

    May 15, 2018
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    They all say UA741 on them. I suppose I could see if they somehow got mislabeled haha
     
  13. BobTPH

    Well-Known Member

    Jun 5, 2013
    1,089
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    If you got them from China, it would not be surprising.

    Bob
     
  14. BobTPH

    Well-Known Member

    Jun 5, 2013
    1,089
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    On a real 741 pin 8 is not connected, so it should do no harm to connect v+ there. Pins 2 and 3 are the inputs on both chips. So just change the circuit use pin1 as the output. It should do not damage, and if it works, the mystery is solved.

    Bob
     
  15. recklessrog

    Well-Known Member

    May 23, 2013
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    Something else springs to mind. Are you sure you have correctly identified the pins and orientation of the 741? viewed from the top and notch facing away from you (top) pin 1 is top left, 4 is bottom left, 5 is bottom right, pin 8 is top right. I say this because one of the members of my club had built a circuit that worked in simulation, but when he breadboarded it, he had it upside down. (ie pin 5 top left) if you see what I mean.
     
  16. Jem2200

    Thread Starter New Member

    May 15, 2018
    19
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    Thank you for your replies. The op amp is in the correct orientation. I just tried your idea bob, but no luck. Good idea though!
     
  17. ElectricSpidey

    Member

    Dec 2, 2017
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    In all my years working with the 741 I have never been able to get an inverting follower to function with just two resistors, in fact I am working with a circuit right now where I have to place a third resistor in the form of a VR…it is placed between the negative input pin and ground.

    I’m using a single supply, and 510 ohm resistors, the pot is 1 meg set to aprox 500k before inserting, then adjusted to bring the output to where I want it.

    Now I really don’t know how this relates to a dual supply, but maybe you can try something like I have explained here.
     
  18. Jem2200

    Thread Starter New Member

    May 15, 2018
    19
    4
    Eureka! Appearantly I bought 10 bad op amps off of amazon and the three op amps I got from my classroom were busted too. Just ran down to a local electronics store to buy another and the circuit works! Thank you guys for all your help! I really appriciate it!
     
    atferrari, RamaD and ebeowulf17 like this.
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