What is going wrong in this circuit which supposedly should step down AC to arduino friendly voltage?












5












$begingroup$


So I'm using this circuit
enter image description here



but when adapter output is disconnected and only midpoint terminal of the transformer is connected the Arduino input pin reads some mV. mid point reads perfect 2.5V. What am I doing wrong?










share|improve this question









$endgroup$








  • 2




    $begingroup$
    Reads some mV relative to what? Arduino ground? If so, you either have a short to ground somewhere, you are accidentally driving the pin low, or you have killed the I/O pin.
    $endgroup$
    – jms
    2 days ago






  • 1




    $begingroup$
    Double-check your wiring. If you built this on a breadboard socket, it's possible you have a wire in a hole next to the one it actually belongs in. It happens more often than you might think.
    $endgroup$
    – Dave Tweed
    2 days ago








  • 2




    $begingroup$
    What's the winding ratio of your "AC-AC adapter" (which I'd prefer to call a "transformer").
    $endgroup$
    – Marcus Müller
    2 days ago






  • 1




    $begingroup$
    Please re-write you question using separate sentences to describe which conditions work and which conditions do not work. Does the circuit work correctly when the AC adapter is connected and powered up? Does the circuit work correctly when the AC adapter is connected but not powered? What is the input impedance of the Arduino analog input pin?
    $endgroup$
    – AnalogKid
    2 days ago








  • 1




    $begingroup$
    Disconnect the Arduino pin, leave everything else, and make a measurement at the voltage divider output. Divide and conquer.
    $endgroup$
    – Spehro Pefhany
    2 days ago
















5












$begingroup$


So I'm using this circuit
enter image description here



but when adapter output is disconnected and only midpoint terminal of the transformer is connected the Arduino input pin reads some mV. mid point reads perfect 2.5V. What am I doing wrong?










share|improve this question









$endgroup$








  • 2




    $begingroup$
    Reads some mV relative to what? Arduino ground? If so, you either have a short to ground somewhere, you are accidentally driving the pin low, or you have killed the I/O pin.
    $endgroup$
    – jms
    2 days ago






  • 1




    $begingroup$
    Double-check your wiring. If you built this on a breadboard socket, it's possible you have a wire in a hole next to the one it actually belongs in. It happens more often than you might think.
    $endgroup$
    – Dave Tweed
    2 days ago








  • 2




    $begingroup$
    What's the winding ratio of your "AC-AC adapter" (which I'd prefer to call a "transformer").
    $endgroup$
    – Marcus Müller
    2 days ago






  • 1




    $begingroup$
    Please re-write you question using separate sentences to describe which conditions work and which conditions do not work. Does the circuit work correctly when the AC adapter is connected and powered up? Does the circuit work correctly when the AC adapter is connected but not powered? What is the input impedance of the Arduino analog input pin?
    $endgroup$
    – AnalogKid
    2 days ago








  • 1




    $begingroup$
    Disconnect the Arduino pin, leave everything else, and make a measurement at the voltage divider output. Divide and conquer.
    $endgroup$
    – Spehro Pefhany
    2 days ago














5












5








5





$begingroup$


So I'm using this circuit
enter image description here



but when adapter output is disconnected and only midpoint terminal of the transformer is connected the Arduino input pin reads some mV. mid point reads perfect 2.5V. What am I doing wrong?










share|improve this question









$endgroup$




So I'm using this circuit
enter image description here



but when adapter output is disconnected and only midpoint terminal of the transformer is connected the Arduino input pin reads some mV. mid point reads perfect 2.5V. What am I doing wrong?







arduino transformer ac






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 2 days ago









Vibhore JainVibhore Jain

8817




8817








  • 2




    $begingroup$
    Reads some mV relative to what? Arduino ground? If so, you either have a short to ground somewhere, you are accidentally driving the pin low, or you have killed the I/O pin.
    $endgroup$
    – jms
    2 days ago






  • 1




    $begingroup$
    Double-check your wiring. If you built this on a breadboard socket, it's possible you have a wire in a hole next to the one it actually belongs in. It happens more often than you might think.
    $endgroup$
    – Dave Tweed
    2 days ago








  • 2




    $begingroup$
    What's the winding ratio of your "AC-AC adapter" (which I'd prefer to call a "transformer").
    $endgroup$
    – Marcus Müller
    2 days ago






  • 1




    $begingroup$
    Please re-write you question using separate sentences to describe which conditions work and which conditions do not work. Does the circuit work correctly when the AC adapter is connected and powered up? Does the circuit work correctly when the AC adapter is connected but not powered? What is the input impedance of the Arduino analog input pin?
    $endgroup$
    – AnalogKid
    2 days ago








  • 1




    $begingroup$
    Disconnect the Arduino pin, leave everything else, and make a measurement at the voltage divider output. Divide and conquer.
    $endgroup$
    – Spehro Pefhany
    2 days ago














  • 2




    $begingroup$
    Reads some mV relative to what? Arduino ground? If so, you either have a short to ground somewhere, you are accidentally driving the pin low, or you have killed the I/O pin.
    $endgroup$
    – jms
    2 days ago






  • 1




    $begingroup$
    Double-check your wiring. If you built this on a breadboard socket, it's possible you have a wire in a hole next to the one it actually belongs in. It happens more often than you might think.
    $endgroup$
    – Dave Tweed
    2 days ago








  • 2




    $begingroup$
    What's the winding ratio of your "AC-AC adapter" (which I'd prefer to call a "transformer").
    $endgroup$
    – Marcus Müller
    2 days ago






  • 1




    $begingroup$
    Please re-write you question using separate sentences to describe which conditions work and which conditions do not work. Does the circuit work correctly when the AC adapter is connected and powered up? Does the circuit work correctly when the AC adapter is connected but not powered? What is the input impedance of the Arduino analog input pin?
    $endgroup$
    – AnalogKid
    2 days ago








  • 1




    $begingroup$
    Disconnect the Arduino pin, leave everything else, and make a measurement at the voltage divider output. Divide and conquer.
    $endgroup$
    – Spehro Pefhany
    2 days ago








2




2




$begingroup$
Reads some mV relative to what? Arduino ground? If so, you either have a short to ground somewhere, you are accidentally driving the pin low, or you have killed the I/O pin.
$endgroup$
– jms
2 days ago




$begingroup$
Reads some mV relative to what? Arduino ground? If so, you either have a short to ground somewhere, you are accidentally driving the pin low, or you have killed the I/O pin.
$endgroup$
– jms
2 days ago




1




1




$begingroup$
Double-check your wiring. If you built this on a breadboard socket, it's possible you have a wire in a hole next to the one it actually belongs in. It happens more often than you might think.
$endgroup$
– Dave Tweed
2 days ago






$begingroup$
Double-check your wiring. If you built this on a breadboard socket, it's possible you have a wire in a hole next to the one it actually belongs in. It happens more often than you might think.
$endgroup$
– Dave Tweed
2 days ago






2




2




$begingroup$
What's the winding ratio of your "AC-AC adapter" (which I'd prefer to call a "transformer").
$endgroup$
– Marcus Müller
2 days ago




$begingroup$
What's the winding ratio of your "AC-AC adapter" (which I'd prefer to call a "transformer").
$endgroup$
– Marcus Müller
2 days ago




1




1




$begingroup$
Please re-write you question using separate sentences to describe which conditions work and which conditions do not work. Does the circuit work correctly when the AC adapter is connected and powered up? Does the circuit work correctly when the AC adapter is connected but not powered? What is the input impedance of the Arduino analog input pin?
$endgroup$
– AnalogKid
2 days ago






$begingroup$
Please re-write you question using separate sentences to describe which conditions work and which conditions do not work. Does the circuit work correctly when the AC adapter is connected and powered up? Does the circuit work correctly when the AC adapter is connected but not powered? What is the input impedance of the Arduino analog input pin?
$endgroup$
– AnalogKid
2 days ago






1




1




$begingroup$
Disconnect the Arduino pin, leave everything else, and make a measurement at the voltage divider output. Divide and conquer.
$endgroup$
– Spehro Pefhany
2 days ago




$begingroup$
Disconnect the Arduino pin, leave everything else, and make a measurement at the voltage divider output. Divide and conquer.
$endgroup$
– Spehro Pefhany
2 days ago










6 Answers
6






active

oldest

votes


















3












$begingroup$

Trash will couple from primary to secondary, at frequencies the opamp cannot control.



How about this





schematic





simulate this circuit – Schematic created using CircuitLab






share|improve this answer











$endgroup$









  • 4




    $begingroup$
    Care to explain why this is better than the other one? and something about what it does?
    $endgroup$
    – laptop2d
    2 days ago








  • 1




    $begingroup$
    Its just a transformer followed by a high pass filter. Not sure why'd you use a high pass filter to try and eliminate HF noise though.
    $endgroup$
    – Stiddily
    2 days ago



















1












$begingroup$

That's the normal behavior for the circuit you posted. Your adc can read voltages from 0 to 5v but the ac signal (if the voltage divider is spot on) is -2.5v to 2.5v. So you mix a dc 2.5v with the ac signal and subtract the offset voltage the arduino measures to reference your measurement circuits ac 'ground' reference. So 0-127 will be negative and 128-255 will be positive. Openenergymonitor project has some great tutorials and sample code that may keep you from killing yourself literally or figuratively figuring this stuff out.



The buffer opamp isn't needed in your circuit as you are not using much current from the reference voltage. Just Capacitor c1 would give the 2.5v divider a clean voltage. A 3pin 2.5v Ldo chip would give a more accurate reference but is overkill for a toss off arduino circuit.



But I think the real answer to your question is that you should use ac voltage setting on your meter to test the circuit. if you use a meter in DC mode with the mains plugged the voltage will be 0 as the average of a sine wave and 2.5v with no mains signal as that is your 'ground' voltage not 0v.






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Michael Valliant is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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$endgroup$





















    0












    $begingroup$

    When you disconnect the adapter there is no current flowing in secondary coil, as there are two 12k resistors which make voltage divider by ratio of 1:2 so in the mid point of it there will be 2.5 v which is applied to the non inverting terminal of op amp which is set to unity gain so the output of op-amp will b 2.5v and as the is no current flowing in the resistor R1 the voltage drop will be 0v so the 2.5v is seen on the Arduino Input pin.






    share|improve this answer








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    Yash Jasnani is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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    $endgroup$





















      0












      $begingroup$

      The 'non-midpoint' ground of the secondary transformer should have the same potential as the ground of the Arduino just like analogsystemsrf has done. Otherwise, who knows what kind of voltages you could see at the ADC of the Arduino?



      Also, since this is AC, and the 'positive/top' side of the transformer would go negative with respect to the secondary ground, so you need a rectifier so the ADC pin does not go below ground. A series cap like analogsystemsrf would eliminate DC bias.



      Edit: Another method (without rectifying) is to add a DC bias of 2.5V so that the signal goes above and below 2.5V (but not abober 5V or below 0V) you could then read both the positive and negative signal while maintaining a safe 0-5V for the ADC. Look up adding a DC bias on Google for many references, but it's basically a voltage divider with the midpoint connected to the signal you want to bias.






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      AdvancedNewbie is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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      $endgroup$





















        0












        $begingroup$

        if u r measuring Ac Voltage corresponding to AC Main i will suggest u please used diode bridge and filter capacitor at secondary side.
        1. u will get DC voltage corresponding to AC mains
        2. In your circuit opamp o/p getting loaded because of transformer secondary winding when your AC Mains not available. that's why your reference is changing.






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        $endgroup$





















          0












          $begingroup$

          Where did you get the circuit? I'd like to see the original. It will not work at all as drawn.



          If all you want is 5v you just need your transformer connected to a bridge rectifier. The low output of the bridge rectifier will be your ground. Then you connect a 7805 regulator and a cap before and after and walla! You have a nice 5v 1A power supply.



          (https://i.stack.imgur.com/eiwzE.png)






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          TpKnet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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          $endgroup$









          • 1




            $begingroup$
            It's not a power supply. OP is presumably trying to isolate, step down, and bias the mains voltage to midscale for the analog-to-digital input.
            $endgroup$
            – Spehro Pefhany
            yesterday










          protected by Community yesterday



          Thank you for your interest in this question.
          Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).



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          6 Answers
          6






          active

          oldest

          votes








          6 Answers
          6






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          3












          $begingroup$

          Trash will couple from primary to secondary, at frequencies the opamp cannot control.



          How about this





          schematic





          simulate this circuit – Schematic created using CircuitLab






          share|improve this answer











          $endgroup$









          • 4




            $begingroup$
            Care to explain why this is better than the other one? and something about what it does?
            $endgroup$
            – laptop2d
            2 days ago








          • 1




            $begingroup$
            Its just a transformer followed by a high pass filter. Not sure why'd you use a high pass filter to try and eliminate HF noise though.
            $endgroup$
            – Stiddily
            2 days ago
















          3












          $begingroup$

          Trash will couple from primary to secondary, at frequencies the opamp cannot control.



          How about this





          schematic





          simulate this circuit – Schematic created using CircuitLab






          share|improve this answer











          $endgroup$









          • 4




            $begingroup$
            Care to explain why this is better than the other one? and something about what it does?
            $endgroup$
            – laptop2d
            2 days ago








          • 1




            $begingroup$
            Its just a transformer followed by a high pass filter. Not sure why'd you use a high pass filter to try and eliminate HF noise though.
            $endgroup$
            – Stiddily
            2 days ago














          3












          3








          3





          $begingroup$

          Trash will couple from primary to secondary, at frequencies the opamp cannot control.



          How about this





          schematic





          simulate this circuit – Schematic created using CircuitLab






          share|improve this answer











          $endgroup$



          Trash will couple from primary to secondary, at frequencies the opamp cannot control.



          How about this





          schematic





          simulate this circuit – Schematic created using CircuitLab







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 20 hours ago

























          answered 2 days ago









          analogsystemsrfanalogsystemsrf

          15.6k2822




          15.6k2822








          • 4




            $begingroup$
            Care to explain why this is better than the other one? and something about what it does?
            $endgroup$
            – laptop2d
            2 days ago








          • 1




            $begingroup$
            Its just a transformer followed by a high pass filter. Not sure why'd you use a high pass filter to try and eliminate HF noise though.
            $endgroup$
            – Stiddily
            2 days ago














          • 4




            $begingroup$
            Care to explain why this is better than the other one? and something about what it does?
            $endgroup$
            – laptop2d
            2 days ago








          • 1




            $begingroup$
            Its just a transformer followed by a high pass filter. Not sure why'd you use a high pass filter to try and eliminate HF noise though.
            $endgroup$
            – Stiddily
            2 days ago








          4




          4




          $begingroup$
          Care to explain why this is better than the other one? and something about what it does?
          $endgroup$
          – laptop2d
          2 days ago






          $begingroup$
          Care to explain why this is better than the other one? and something about what it does?
          $endgroup$
          – laptop2d
          2 days ago






          1




          1




          $begingroup$
          Its just a transformer followed by a high pass filter. Not sure why'd you use a high pass filter to try and eliminate HF noise though.
          $endgroup$
          – Stiddily
          2 days ago




          $begingroup$
          Its just a transformer followed by a high pass filter. Not sure why'd you use a high pass filter to try and eliminate HF noise though.
          $endgroup$
          – Stiddily
          2 days ago













          1












          $begingroup$

          That's the normal behavior for the circuit you posted. Your adc can read voltages from 0 to 5v but the ac signal (if the voltage divider is spot on) is -2.5v to 2.5v. So you mix a dc 2.5v with the ac signal and subtract the offset voltage the arduino measures to reference your measurement circuits ac 'ground' reference. So 0-127 will be negative and 128-255 will be positive. Openenergymonitor project has some great tutorials and sample code that may keep you from killing yourself literally or figuratively figuring this stuff out.



          The buffer opamp isn't needed in your circuit as you are not using much current from the reference voltage. Just Capacitor c1 would give the 2.5v divider a clean voltage. A 3pin 2.5v Ldo chip would give a more accurate reference but is overkill for a toss off arduino circuit.



          But I think the real answer to your question is that you should use ac voltage setting on your meter to test the circuit. if you use a meter in DC mode with the mains plugged the voltage will be 0 as the average of a sine wave and 2.5v with no mains signal as that is your 'ground' voltage not 0v.






          share|improve this answer








          New contributor




          Michael Valliant is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.






          $endgroup$


















            1












            $begingroup$

            That's the normal behavior for the circuit you posted. Your adc can read voltages from 0 to 5v but the ac signal (if the voltage divider is spot on) is -2.5v to 2.5v. So you mix a dc 2.5v with the ac signal and subtract the offset voltage the arduino measures to reference your measurement circuits ac 'ground' reference. So 0-127 will be negative and 128-255 will be positive. Openenergymonitor project has some great tutorials and sample code that may keep you from killing yourself literally or figuratively figuring this stuff out.



            The buffer opamp isn't needed in your circuit as you are not using much current from the reference voltage. Just Capacitor c1 would give the 2.5v divider a clean voltage. A 3pin 2.5v Ldo chip would give a more accurate reference but is overkill for a toss off arduino circuit.



            But I think the real answer to your question is that you should use ac voltage setting on your meter to test the circuit. if you use a meter in DC mode with the mains plugged the voltage will be 0 as the average of a sine wave and 2.5v with no mains signal as that is your 'ground' voltage not 0v.






            share|improve this answer








            New contributor




            Michael Valliant is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.






            $endgroup$
















              1












              1








              1





              $begingroup$

              That's the normal behavior for the circuit you posted. Your adc can read voltages from 0 to 5v but the ac signal (if the voltage divider is spot on) is -2.5v to 2.5v. So you mix a dc 2.5v with the ac signal and subtract the offset voltage the arduino measures to reference your measurement circuits ac 'ground' reference. So 0-127 will be negative and 128-255 will be positive. Openenergymonitor project has some great tutorials and sample code that may keep you from killing yourself literally or figuratively figuring this stuff out.



              The buffer opamp isn't needed in your circuit as you are not using much current from the reference voltage. Just Capacitor c1 would give the 2.5v divider a clean voltage. A 3pin 2.5v Ldo chip would give a more accurate reference but is overkill for a toss off arduino circuit.



              But I think the real answer to your question is that you should use ac voltage setting on your meter to test the circuit. if you use a meter in DC mode with the mains plugged the voltage will be 0 as the average of a sine wave and 2.5v with no mains signal as that is your 'ground' voltage not 0v.






              share|improve this answer








              New contributor




              Michael Valliant is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.






              $endgroup$



              That's the normal behavior for the circuit you posted. Your adc can read voltages from 0 to 5v but the ac signal (if the voltage divider is spot on) is -2.5v to 2.5v. So you mix a dc 2.5v with the ac signal and subtract the offset voltage the arduino measures to reference your measurement circuits ac 'ground' reference. So 0-127 will be negative and 128-255 will be positive. Openenergymonitor project has some great tutorials and sample code that may keep you from killing yourself literally or figuratively figuring this stuff out.



              The buffer opamp isn't needed in your circuit as you are not using much current from the reference voltage. Just Capacitor c1 would give the 2.5v divider a clean voltage. A 3pin 2.5v Ldo chip would give a more accurate reference but is overkill for a toss off arduino circuit.



              But I think the real answer to your question is that you should use ac voltage setting on your meter to test the circuit. if you use a meter in DC mode with the mains plugged the voltage will be 0 as the average of a sine wave and 2.5v with no mains signal as that is your 'ground' voltage not 0v.







              share|improve this answer








              New contributor




              Michael Valliant is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
              Check out our Code of Conduct.









              share|improve this answer



              share|improve this answer






              New contributor




              Michael Valliant is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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              answered yesterday









              Michael ValliantMichael Valliant

              111




              111




              New contributor




              Michael Valliant is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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              New contributor





              Michael Valliant is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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              Michael Valliant is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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                  0












                  $begingroup$

                  When you disconnect the adapter there is no current flowing in secondary coil, as there are two 12k resistors which make voltage divider by ratio of 1:2 so in the mid point of it there will be 2.5 v which is applied to the non inverting terminal of op amp which is set to unity gain so the output of op-amp will b 2.5v and as the is no current flowing in the resistor R1 the voltage drop will be 0v so the 2.5v is seen on the Arduino Input pin.






                  share|improve this answer








                  New contributor




                  Yash Jasnani is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                  Check out our Code of Conduct.






                  $endgroup$


















                    0












                    $begingroup$

                    When you disconnect the adapter there is no current flowing in secondary coil, as there are two 12k resistors which make voltage divider by ratio of 1:2 so in the mid point of it there will be 2.5 v which is applied to the non inverting terminal of op amp which is set to unity gain so the output of op-amp will b 2.5v and as the is no current flowing in the resistor R1 the voltage drop will be 0v so the 2.5v is seen on the Arduino Input pin.






                    share|improve this answer








                    New contributor




                    Yash Jasnani is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                    Check out our Code of Conduct.






                    $endgroup$
















                      0












                      0








                      0





                      $begingroup$

                      When you disconnect the adapter there is no current flowing in secondary coil, as there are two 12k resistors which make voltage divider by ratio of 1:2 so in the mid point of it there will be 2.5 v which is applied to the non inverting terminal of op amp which is set to unity gain so the output of op-amp will b 2.5v and as the is no current flowing in the resistor R1 the voltage drop will be 0v so the 2.5v is seen on the Arduino Input pin.






                      share|improve this answer








                      New contributor




                      Yash Jasnani is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                      Check out our Code of Conduct.






                      $endgroup$



                      When you disconnect the adapter there is no current flowing in secondary coil, as there are two 12k resistors which make voltage divider by ratio of 1:2 so in the mid point of it there will be 2.5 v which is applied to the non inverting terminal of op amp which is set to unity gain so the output of op-amp will b 2.5v and as the is no current flowing in the resistor R1 the voltage drop will be 0v so the 2.5v is seen on the Arduino Input pin.







                      share|improve this answer








                      New contributor




                      Yash Jasnani is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                      Check out our Code of Conduct.









                      share|improve this answer



                      share|improve this answer






                      New contributor




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                      answered 2 days ago









                      Yash JasnaniYash Jasnani

                      1




                      1




                      New contributor




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                      New contributor





                      Yash Jasnani is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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                      Check out our Code of Conduct.























                          0












                          $begingroup$

                          The 'non-midpoint' ground of the secondary transformer should have the same potential as the ground of the Arduino just like analogsystemsrf has done. Otherwise, who knows what kind of voltages you could see at the ADC of the Arduino?



                          Also, since this is AC, and the 'positive/top' side of the transformer would go negative with respect to the secondary ground, so you need a rectifier so the ADC pin does not go below ground. A series cap like analogsystemsrf would eliminate DC bias.



                          Edit: Another method (without rectifying) is to add a DC bias of 2.5V so that the signal goes above and below 2.5V (but not abober 5V or below 0V) you could then read both the positive and negative signal while maintaining a safe 0-5V for the ADC. Look up adding a DC bias on Google for many references, but it's basically a voltage divider with the midpoint connected to the signal you want to bias.






                          share|improve this answer










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                            0












                            $begingroup$

                            The 'non-midpoint' ground of the secondary transformer should have the same potential as the ground of the Arduino just like analogsystemsrf has done. Otherwise, who knows what kind of voltages you could see at the ADC of the Arduino?



                            Also, since this is AC, and the 'positive/top' side of the transformer would go negative with respect to the secondary ground, so you need a rectifier so the ADC pin does not go below ground. A series cap like analogsystemsrf would eliminate DC bias.



                            Edit: Another method (without rectifying) is to add a DC bias of 2.5V so that the signal goes above and below 2.5V (but not abober 5V or below 0V) you could then read both the positive and negative signal while maintaining a safe 0-5V for the ADC. Look up adding a DC bias on Google for many references, but it's basically a voltage divider with the midpoint connected to the signal you want to bias.






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                            $endgroup$
















                              0












                              0








                              0





                              $begingroup$

                              The 'non-midpoint' ground of the secondary transformer should have the same potential as the ground of the Arduino just like analogsystemsrf has done. Otherwise, who knows what kind of voltages you could see at the ADC of the Arduino?



                              Also, since this is AC, and the 'positive/top' side of the transformer would go negative with respect to the secondary ground, so you need a rectifier so the ADC pin does not go below ground. A series cap like analogsystemsrf would eliminate DC bias.



                              Edit: Another method (without rectifying) is to add a DC bias of 2.5V so that the signal goes above and below 2.5V (but not abober 5V or below 0V) you could then read both the positive and negative signal while maintaining a safe 0-5V for the ADC. Look up adding a DC bias on Google for many references, but it's basically a voltage divider with the midpoint connected to the signal you want to bias.






                              share|improve this answer










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                              $endgroup$



                              The 'non-midpoint' ground of the secondary transformer should have the same potential as the ground of the Arduino just like analogsystemsrf has done. Otherwise, who knows what kind of voltages you could see at the ADC of the Arduino?



                              Also, since this is AC, and the 'positive/top' side of the transformer would go negative with respect to the secondary ground, so you need a rectifier so the ADC pin does not go below ground. A series cap like analogsystemsrf would eliminate DC bias.



                              Edit: Another method (without rectifying) is to add a DC bias of 2.5V so that the signal goes above and below 2.5V (but not abober 5V or below 0V) you could then read both the positive and negative signal while maintaining a safe 0-5V for the ADC. Look up adding a DC bias on Google for many references, but it's basically a voltage divider with the midpoint connected to the signal you want to bias.







                              share|improve this answer










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                              edited 2 days ago





















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                              answered 2 days ago









                              AdvancedNewbieAdvancedNewbie

                              33




                              33




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                                  0












                                  $begingroup$

                                  if u r measuring Ac Voltage corresponding to AC Main i will suggest u please used diode bridge and filter capacitor at secondary side.
                                  1. u will get DC voltage corresponding to AC mains
                                  2. In your circuit opamp o/p getting loaded because of transformer secondary winding when your AC Mains not available. that's why your reference is changing.






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                                    0












                                    $begingroup$

                                    if u r measuring Ac Voltage corresponding to AC Main i will suggest u please used diode bridge and filter capacitor at secondary side.
                                    1. u will get DC voltage corresponding to AC mains
                                    2. In your circuit opamp o/p getting loaded because of transformer secondary winding when your AC Mains not available. that's why your reference is changing.






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                                    $endgroup$
















                                      0












                                      0








                                      0





                                      $begingroup$

                                      if u r measuring Ac Voltage corresponding to AC Main i will suggest u please used diode bridge and filter capacitor at secondary side.
                                      1. u will get DC voltage corresponding to AC mains
                                      2. In your circuit opamp o/p getting loaded because of transformer secondary winding when your AC Mains not available. that's why your reference is changing.






                                      share|improve this answer








                                      New contributor




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                                      Check out our Code of Conduct.






                                      $endgroup$



                                      if u r measuring Ac Voltage corresponding to AC Main i will suggest u please used diode bridge and filter capacitor at secondary side.
                                      1. u will get DC voltage corresponding to AC mains
                                      2. In your circuit opamp o/p getting loaded because of transformer secondary winding when your AC Mains not available. that's why your reference is changing.







                                      share|improve this answer








                                      New contributor




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                                      share|improve this answer



                                      share|improve this answer






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                                      answered yesterday









                                      Aditya Suresh RajpurkarAditya Suresh Rajpurkar

                                      1




                                      1




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                                          0












                                          $begingroup$

                                          Where did you get the circuit? I'd like to see the original. It will not work at all as drawn.



                                          If all you want is 5v you just need your transformer connected to a bridge rectifier. The low output of the bridge rectifier will be your ground. Then you connect a 7805 regulator and a cap before and after and walla! You have a nice 5v 1A power supply.



                                          (https://i.stack.imgur.com/eiwzE.png)






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                                          • 1




                                            $begingroup$
                                            It's not a power supply. OP is presumably trying to isolate, step down, and bias the mains voltage to midscale for the analog-to-digital input.
                                            $endgroup$
                                            – Spehro Pefhany
                                            yesterday
















                                          0












                                          $begingroup$

                                          Where did you get the circuit? I'd like to see the original. It will not work at all as drawn.



                                          If all you want is 5v you just need your transformer connected to a bridge rectifier. The low output of the bridge rectifier will be your ground. Then you connect a 7805 regulator and a cap before and after and walla! You have a nice 5v 1A power supply.



                                          (https://i.stack.imgur.com/eiwzE.png)






                                          share|improve this answer








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                                          $endgroup$









                                          • 1




                                            $begingroup$
                                            It's not a power supply. OP is presumably trying to isolate, step down, and bias the mains voltage to midscale for the analog-to-digital input.
                                            $endgroup$
                                            – Spehro Pefhany
                                            yesterday














                                          0












                                          0








                                          0





                                          $begingroup$

                                          Where did you get the circuit? I'd like to see the original. It will not work at all as drawn.



                                          If all you want is 5v you just need your transformer connected to a bridge rectifier. The low output of the bridge rectifier will be your ground. Then you connect a 7805 regulator and a cap before and after and walla! You have a nice 5v 1A power supply.



                                          (https://i.stack.imgur.com/eiwzE.png)






                                          share|improve this answer








                                          New contributor




                                          TpKnet is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                                          Check out our Code of Conduct.






                                          $endgroup$



                                          Where did you get the circuit? I'd like to see the original. It will not work at all as drawn.



                                          If all you want is 5v you just need your transformer connected to a bridge rectifier. The low output of the bridge rectifier will be your ground. Then you connect a 7805 regulator and a cap before and after and walla! You have a nice 5v 1A power supply.



                                          (https://i.stack.imgur.com/eiwzE.png)







                                          share|improve this answer








                                          New contributor




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                                          share|improve this answer



                                          share|improve this answer






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                                          answered yesterday









                                          TpKnetTpKnet

                                          913




                                          913




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                                          • 1




                                            $begingroup$
                                            It's not a power supply. OP is presumably trying to isolate, step down, and bias the mains voltage to midscale for the analog-to-digital input.
                                            $endgroup$
                                            – Spehro Pefhany
                                            yesterday














                                          • 1




                                            $begingroup$
                                            It's not a power supply. OP is presumably trying to isolate, step down, and bias the mains voltage to midscale for the analog-to-digital input.
                                            $endgroup$
                                            – Spehro Pefhany
                                            yesterday








                                          1




                                          1




                                          $begingroup$
                                          It's not a power supply. OP is presumably trying to isolate, step down, and bias the mains voltage to midscale for the analog-to-digital input.
                                          $endgroup$
                                          – Spehro Pefhany
                                          yesterday




                                          $begingroup$
                                          It's not a power supply. OP is presumably trying to isolate, step down, and bias the mains voltage to midscale for the analog-to-digital input.
                                          $endgroup$
                                          – Spehro Pefhany
                                          yesterday





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