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This is confusing me because there are two power sources in this circuit. (120 V AC and the 5V pin from the Arduino which is DC.) The output is measured at the point all the way to the right (Arduino Uno). Thanks in advance

bias circuit

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I don't see why you should be confused - plenty of circuits use multiple power sources. The two 100 ohm resistors split the arduino 5 volts to 2.5 volts. This should be hopefully simple enough. This 2.5 volts raises the secondary winding to an average level of 2.5 volts and the AC voltage from the secondary adds to that 2.5 volt bias.

Further attenuation of (only) the AC part of the signal comes from R1 and R2 and you are left with a DC voltage of 2.5 volts with an AC signal of 3.88 volts superimposed. In other words the voltage feeding the ADC of the Arduino can be as high as (2.5 + 3.88/2) volts or as low as (2.5 - 3.88/2) volts.

I calculate this as 0.56 volts to 4.44 volts and pretty normal for an ADC input of the type used in an Arduino.

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  • \$\begingroup\$ Hi, did you ignore load resistance (consider it as open circuit) for these calculation? Also, I don't understand why Vrms is 2.88V here? The ac rms voltage across the secondary winding is 6V so the rms voltage across R2 is 1.38V. So the total rms voltage at upper end of R2 is square root of (2.5^2 + 1.38^2) = 2.86V rms. So is there something wrong here? I haven't accounted for load resistance but if we include the load resistance the result should be smaller. \$\endgroup\$ – anhnha Dec 3 '16 at 4:52
  • \$\begingroup\$ Did you calculate Mrs taking into account the dc part as well. 100 Mohm in parallel with 200 ohm is still very largely 200 ohms. \$\endgroup\$ – Andy aka Dec 3 '16 at 8:08
  • \$\begingroup\$ Yes, I already considered dc part but the result is smaller than the value given by the simulator above. \$\endgroup\$ – anhnha Dec 3 '16 at 8:41
  • \$\begingroup\$ I can't cross check cause I'm babysitting all day but I don't remember any error when I double checked yesterday - did you use sqrt(aa + bb)? \$\endgroup\$ – Andy aka Dec 3 '16 at 9:51
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    \$\begingroup\$ @anhnha well, I got 2.85173 and this is certainly close enough to 2.86 when rounding errors are considered. \$\endgroup\$ – Andy aka Dec 4 '16 at 10:41

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