About Operational Amplifier?

Hi, I connected UA741 IC to the typical non inverting output config.

The amplification seems to be fine. When I give an Input signal of 5 volts I get an out put of 14v.

But the problem comes when I bring the input signal to zero the output doesnt fall to zero it remains at about 7 volts.

Seems like the amplifier just adds another 7 volts to the input signal. But, according to the opamp equations ie., for the non-inverting configuration Vout=Vin(1+R1/R2) so theoretically when Vin =0 Vout should also become zero , but this doesnt happen.

I need to connect the microcontroller output amplified by UA741 to control a 12 v relay , but since I cant get 0 o/p I am not able to control the relay.

Please tell me why the Vout remains stuck to 7 volts instead of turning 0.

5 Answers

  • 1 decade ago
    Favorite Answer

    First of all, the last way you'd ever want to drive a relay that is being controlled from a microprocessor would be from an opamp. Only reason I could see is if you already had a dual or quad op amp package in your design with an extra amp that was unused. Even then, I would avoid using to drive a relay.

    Again, unless this is a specific classroom assignment to use the op amp as a relay driver just to learn from it, do not do it!..

    Use the op amp when you need to amplify some kind of ac signal or need to get gain or a high impedance input for a measurement or instrumentation circuit, like an input stage for one of the analog inputs that you might find on a microcontroller.

    What you might be seeing is related to the output that you are driving the op amp with. Most logic outputs never go to zero volts, and you might even see somethong around 1.4 or more volts at the output. This output is probably whats driving the opamp to a higher than expected output when you have the logic ouput in the off state.

    Also, the 741 is designed to operate with both a positve and negative power supply. Do you have the neg supply pin connected to ground, or to a neg voltage supply?...If its connected to ground, you are seeing what I would expect, zero Volts in should indeed cause you to see half the supply voltage on the op amps output!

    There are several much easier and less expensive ways to drive a relay.

    Why not just drive the relay with a single transistor like a 2N3904 or something similar.

    You could also use an fet, or an opto isolator.

    Go to this link and you will see avery simple way to do this: http://www.wam.umd.edu/~toh/ElectroSim/relay.html

    Also, is there a particular reason you are using a 12V relay? There are plenty of relays out there that will drive directly from a microcontroller like one of the Microchip Pics. Most of those can drive 25 ma of current at the Vdd voltage.

    Another tip, if you drive a relay from a uC output or logic output. place a small diode like a 1n914 across the relay in the reverse bias , in other words cathode to the postive voltage side. This keeps the high voltage spike that a relay or inductor can produce when it turns off from damaging your drive circuit.

    Good luck!..and feel free to write directly for any suggestions or help.

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  • 1 decade ago

    Another thing that hasn't been mentioned is DC offset and bias current.

    The op-amp isn't really ideal. What kind of gain do you have?

    You say you put 5V in and get 14 out. Is this because you have a gain of about 3 or is your gain way high and you are reaching saturation at 14V?

    If you have a really high gain (R2/R1 is biiiig) then the output voltage you see can come from imperfections in the op-amp.

    If this is the case there are a few things you can do to reduce the offset. I'd recommend reading more op-amp theory.

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  • 1 decade ago

    It depends. What are the voltages of your two voltage supply inputs (pin #'s 4 and 7)? What value resistors do you have for R1 and R2?

    You could try replacing the 741, in case the one you are using is fried.

    p.s. By the way, the 741 is considered old technology, as better op amps have come along in the 40-or-so years since the 741's inception. My own current preference for a general-purpose op amp is the TL081 / TL082 / TL084 series.

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  • Paul F
    Lv 6
    1 decade ago

    If properly connected you would have R2 from ground the opamp -, R1 from opamp out to opamp -, Input terminal to opamp +, +Vcc and -Vcc to the proper pins.

    Temporarily connect the input to ground, you should have no voltage dfference between opamp - and opamp +, if you do, something is hooked up wrong or the opamp is bad, and opamp out should be 0VDC.

    Now apply +5VDC between input terminal and ground, if you get +14VDC, then your gain is 14/5 or 2.8. Since the non-inv gain is 1+R1/R2, then the R1/R2 ratio you have is 1.8. is this the ratio you have? If it is, then I would guess that something is biased wrong.

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  • Anonymous
    1 decade ago

    It should work... please post a schematic, or tell us exactly what all your pins are hooked up to. Maybe there is something you overlooked.

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