So I am trying to use this circuit to read in voltages from the wall and input it into the Arduino Uno. Would an F-1X transformer work for this application?

my circuit

  • \$\begingroup\$ Depends what part of line voltage is important to you. ie. you need to specify. To some it's; Vavg Vmin, Vmax, Vrms, Or some other parameters like to be able to detect 1 cycle Mean to compare with a threshold for a dropout of 1 cycle , the typical holdup time for a good ATX supply. Or for others, anything more than x Vp with an RC delay of 3T. or others detection of a 3kV to 6kV transient and detect the number per day..like Florida..so many specs are possible,... which YOU must do first... then design \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 4 '16 at 23:08
  • \$\begingroup\$ Well the goal is to examine the frequency content. I want to be able to record voltages from the wall, input it into the Arduino (hence the rectifier), and feed it into my machine learning algorithm that detects the power grid the signal originated in. \$\endgroup\$ – user6132277 Nov 4 '16 at 23:55
  • \$\begingroup\$ What is the frequency content information needed by your machine learning algorithm that identifies power grids? What voltage accuracy and precision and sampling rates will you require? Do you need voltage vs current phasing information? What surveys of the existing literature have you already done? (If I were doing this, I'd already have dozens of datasets collected with COTS data recorders or oscilloscopes and I'd have already thoroughly tested my learning algorithm on them, offline. I would have a very clear idea what I wanted regarding a circuit by this time.) \$\endgroup\$ – jonk Nov 5 '16 at 0:09
  • \$\begingroup\$ A 1 or 2 PPM oscillator chip is only a few bucks so you can measure frequency error and voltage error compared to a reference sine of 5% peak then locked in a PLL and thus have greater resolution on Power quality both in amplitude and frequency use the phase error compared to reference 50 or 60 Hz with a time interval counter on beat frequency. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 5 '16 at 1:01
  • \$\begingroup\$ If you are talking about the utility mains power distribution in a developed country, you should know that the entire power grid in your country (or region) is precisely phase-locked together or it would be unable to function. The frequency of the mains power is the most stable parameter and is probably MORE stable than what you are trying to measure it with. If you want to measure other parameters, check out: atmel.com/Images/… \$\endgroup\$ – Richard Crowley Dec 6 '16 at 20:44

A transformer is required for safety (to isolate from the high voltage input). A common doorbell transformer can be directly connected to AC; if yours is NOT a doorbell transformer, it may lack internal protection (fuse link, resistance) against short circuits, and an external fuse and maybe a limit resistor are recommended.


simulate this circuit – Schematic created using CircuitLab

The Arduino analog inputs have a permissible range of 0 to +3.3V when using the builtin 3.3V regulator as analog reference, so R2 and R3 sets DC level, and R1 and the S.A.T. (select according to test) resistor will attenuate the input voltage. I'd aim for 2V peak-to-peak, indicated by AC voltage on the S.A.T. resistor of 0.7 VAC. Don't trust the transformer ratio (because doorbell transformers, while functional, are not always accurate).

There's no need for diodes, the AC is easier to read without adding complications.


It highly depends on the accuracy of your reading and as you are using a 36:1 transformer means that you will have a greater uncertainty compared to lets say a 20:1 transformer.you are also using a 100 mega ohm resistor which in best case scenario has 5% uncertainty which means it can be anywhere from 120 to 80 mega ohms which is a huge difference.I suggest you measure the resistance of your resistor accurately rather than focusing on the transformer as the transformer has a much smaller impact on the accuracy of your reading.Based on the manufacturing website "Input Voltage (recommended) 7-12V" so if you work only with a 120V AC input then the best transformer would be (120/7 to 120/12) which I recommend the first one as it is on the lower boundary of your input tolerance and connecting the output voltage of the rectifier directly to your input,but if you really want to use a resistor to be extra sure about your board remaining safe then you can use a 1:1 potential divider with as low as a value as would feel safe with (using two 1K ohm resistors should be more than enough) and then just multiplying the obtained value by 2.I can help you more if you provide more detail on what is the range of the voltage that you want to measure and how accurate you want it to be.

  • \$\begingroup\$ I don't know what you mean by "120/7" \$\endgroup\$ – user6132277 Nov 5 '16 at 14:07
  • \$\begingroup\$ Also the 100 MOhm resistor represents an analog pin in the Arduino \$\endgroup\$ – user6132277 Nov 5 '16 at 14:08
  • \$\begingroup\$ 120/7 is 120 divided by 7 which (when rounded up) is 18 and shows the reduction ratio,which means you can use a 18:1 transformer and still be fine.It helps you to reduce your errors by a factor of 2 if you use 18:1 instead of 36:1.You can also use a 10:1 to be on maximum accuracy but it presents the danger of damaging your board which is not recommended.Long story short, you can use any transformer which has an output voltage which is accepted by your board and the lower the reduction ratio,the more accurate your reading.Any more questions? \$\endgroup\$ – OM222O Nov 6 '16 at 10:07
  • \$\begingroup\$ Also read the answer from @Whit3rd as he seem to have more experience with Arduino than me,I just gave advise on the basics of what happens when you change the values of different things. \$\endgroup\$ – OM222O Nov 6 '16 at 10:12

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