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Amplify AC signal using dual rail op amp

  1. Aug 10, 2012 #1
    Hi !!

    I want to amplify the output of my hall effect current transducer ( CSLA2CD ). The output of my hall effect sensor is 15.6mVac. And I used a dual rail op - amp to amplify this small ac voltage which the gain is 101 ( as shown in the schematic below ). I tried to simulate using multisim, and it does show me the correct output. However, in practical, I can't get the correct output from op amp.

    I attached the photo of my hardware circuit as well.

    I built it exactly like the schematic. But I couldn't get the correct output as the simulation result.

    Is there anything wrong with my circuit?

    FYI, from what i measured, the output of hall sensor when the bulb is off, is around 0.5mVrms. and the output of hall sensor when the bulb is on, is around 15.6mVrms, which is very small.

    I should get the output with 101 gain from op amp. But it doesn't work.

    opamp.png

    waveform.png

    amplifier.jpg
     
  2. jcsd
  3. Aug 10, 2012 #2
    And here is how I coil the Live wire through hall effect sensor current transducer. bulb.jpg
     
  4. Aug 10, 2012 #3
    Forgot to mention. For the hardware part, the other pin of the 1uF at pin 6 of op amp is connected to multimeter to measure the output. ( it is not floating )
     
  5. Aug 10, 2012 #4

    uart

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    Put a resistor to ground (10k or more would work) on the output side of your capacitor (so the resistor is in parallel with the voltmeter).

    Also, double check that you don't have any unintended grounds. Make sure, for example, that the negative side of your negative rail supply isn't unintentionally grounded.
     
  6. Aug 10, 2012 #5

    berkeman

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    I don't see any power (+/-) connections going to that IC on the breadboard...
     
  7. Aug 10, 2012 #6

    berkeman

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    And when you do hook up +/- Vcc, be sure to decouple the rails to ground well with capacitors.
     
  8. Aug 10, 2012 #7

    Yes. I already connect a 10k resistor at the output.

    I think I built the wrong +/-15V circuit. This is how my connection.

    dual rail.jpg

    I am using a 24V regulated voltage adaptor to supply the voltage to LM7815 and LM7915.
    which means now the gnd from 7915 is supply to the input of 7815 and vice versa. This should be wrong right? I should have a common ground for both 7815 and 7915. So I assume that I have to use a Step down transformers to build Dual Regulated Power Supply +/-15v. Am I right?
     
    Last edited: Aug 10, 2012
  9. Aug 10, 2012 #8

    Averagesupernova

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    Do you have a connection to this 24 volt adapter in any way to ground?
     
  10. Aug 10, 2012 #9
    No. the ground of adaptor has been connected to input of 7915. and the ground pin of 7915 is connected to the input of 7815, which means actually I just try to get the negative value by changing the polarity. Which I just realized that I did a huge mistake. Is this the main cause that my op amp doesn't work?
     
  11. Aug 10, 2012 #10
    I did connect my circuit to GND and VCC exactly as shown in my schematic diagram.
     
  12. Aug 10, 2012 #11

    uart

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    Ok, without a diagram I'm not sure exactly what you've done there, but it doesn't sound very sensible. Not sure if I'm reading it correctly, but it sounds like you've made the two supplies in parallel (without any isolation) and then connected them in series. You definitely need to get the power supply sorted before any other aspect of the design can possibly work. Always measure that your DC levels are correct in any circuit as the first part of any fault finding exercise.
     
  13. Aug 10, 2012 #12
    I'm sorry. I posted the wrong image just now. Could you please check the post #7 again, that diagram shows my connection on building the dual-rail power supply. Thanks.
     
  14. Aug 10, 2012 #13

    Averagesupernova

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    Just had a look at post #7. There is no way that can work. That is the problem. If you want to make a split supply you need 3 leads coming from the actual power supply. You can cobble it and make your circuit work by forming a voltage divider out of the 24 volt supply using 2 470 ohm 1/2 watt resistors. The node formed between the 2 resistors will be your ground connection. This will work, but you cannot expect your circuit to deliver much current. It will amplify your signal and it will display nicely on the scope, but it is a compromise. The 470 ohm resistors will run a bit warm.
     
  15. Aug 10, 2012 #14

    uart

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    Ok yeah I can see the correct image now. Yes that circuit wont work as intended, you need to have the two grounds common.
     
  16. Aug 10, 2012 #15
    Thanks alot ! I will try to get a step down transformers and rebuild the dual rail power supply.
    Meanwhile, I'm trying to build a darlington pair to amplify the ac signal. Can Darlington pair amplify small Vac? From my understanding, the darlington transistor amplify current that pass through base right? Then how do I amplify the VAC from hall effect output, I don't even know what is the current of the output??
     
  17. Aug 10, 2012 #16

    Averagesupernova

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    Don't bother until you get a proper power supply. Otherwise it's like building a house on a foundation made of spaghetti.
     
  18. Aug 10, 2012 #17

    berkeman

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    Agreed. A better short-term goal would be to get the 741 opamp circuit working with two 9V batteries as the +/- voltage sources...

    And please do connect wires to the +/- power supply inputs of the opamp on the breadboard... :smile:
     
  19. Aug 11, 2012 #18
    Something like this? So you mean the node between the 2 resistors will be my ground connection right? which means i have to connect all my capacitors to the node between these resistors? Am i right?


    supply.jpg
     
  20. Aug 11, 2012 #19
    I did not read through all the posts. From reading your original post and the pictures. You use too high resistance. You want gain of 101, use 10K feedback and 100Ω instead of 1M and 10K. You are using a breadboard, with high resistance, you are going to see strange things because of wires in the air, no ground plane and parasitic capacitance.

    Also put a 0.1uF cap between pin 4 and 7 to give some filtering.

    Also, You have gain of 100, the frequency response of the op-amp is reduced by about 100. If you use an op-amp with 1MHz GBW, you only get 10KHz!!!! Make sure you opamp is fast enough.
     
    Last edited: Aug 11, 2012
  21. Aug 11, 2012 #20

    Integral

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    Think about it, your rails are 30v apart, while you input 24v. Not going to work. The input to your regulators is 12v so you will not be able to get 15v out. You need a lower voltage regulator.
     
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