I would like to make a device to see how much power some AC motors eat up. My guess is to go with the current transformer as I already have one SCT013. But, as I am not quite sure which one I have, I came here.

On the CT there is SCT013 and under it says 100A: 50mA. I searched it a little and get that this one is SCT013-000. Correct me if I am wrong. If so, then this one as output has current.

Let's start with questions. :slight_smile:

  • Do I need a burden resistor across L and K, and if I need it, which one?
  • Does it matter the direction of AC lines?
  • Does it matter L or N line?
  • Can it measure small currents?

What I tried?
According to

I made the same setup. I use two 10K resistors as a voltage divider, 10uF cap, and burden resistor. On the schematic, it says for 5V (Uno) I need a 33 ohms resistor. As I didn't have this one, I used 22R and 10R in series. Everything soldered, no breadboard. With a 5V LED bulb (AC current 220V) and curent_only sketch from the EmonLib library, I get 0 (nothing, zero) on analog input A1. I made the exact setup as on the page I provide. Something I did wrong, but not sure what.


  1. Yes, choose the value so the max current (50mA) produces the voltage you want to see. What voltage is up to you.
  2. No.
  3. No, if the wiring is correct. Safer to use L.
  4. Yes, but the device that you use to measure voltage must then be sensitive enough, and the current must be above the noise level of your circuitry.

Please post clear, detailed images of your actual hardware wiring, as it is likely at fault since the project you linked to is unlikely to be that badly broken.

My bad. The day was… long.

5W (not 5V) bulb on 220V gives some 20 amps. Uno has a 10-bit ADC. So maybe it is so small to even measure it. I gotta try with something more power-hungry. Or use some ADS1115 or similar.

Must have been a really really long day. 20 amps at 220 volts is 4,400 watts.

It was. :slight_smile:

There was a 5V/5W typo on the first post. I know, that's a huge difference. Sorry.

What I did, is I made two turns of a wire through the CT. There are some readings. Not sure are what they should be.

So, I've got 5W LED bulb on 220V. Live wire has two turns inside CT. Sketch curent_only gives me this:

91.63 0.40
93.42 0.41
104.00 0.45
85.15 0.37
85.36 0.37
85.61 0.37
87.44 0.38
101.31 0.44


The first column is Apparent power and the second is IRMS. Can this be what is expected?

I'm not going to go hunting for your sketch. You should post it here.

Ok, there is some progress.

First of all, sorry. I thought it is ok just to point to a library. The only one as far as I know related to CT. This is the sketch current_only.ino I am talking about.

/ EmonLibrary examples, Licence GNU GPL V3

#include "EmonLib.h"                   // Include Emon Library
EnergyMonitor emon1;                   // Create an instance

void setup()
  emon1.current(1, 111.1);             // Current: input pin, calibration.

void loop()
  double Irms = emon1.calcIrms(1480);  // Calculate Irms only
  Serial.print(Irms*230.0);	       // Apparent power
  Serial.print(" ");
  Serial.println(Irms);		       // Irms

First things first.
When I switched to 1400W/220V iron, I get the results. They are not quite precise, but I think calibration will do the job. I will leave it for now.
5W/220V LED bulb was not the one that I will be checking with CT in the first place.

Right now, I wonder if there a way to take, say, 3 of these CTs with ESP8266. I am familiar with PCF8591, but for a regular job. Here things are not that simple. For me at least.
Let's say the above sketch is ok. The analog pin is declared inside the setup as you can see. If I connect the output pin of my CT circuit to an A1 of the PCF8591, how to assign that pin to the function in the setup emon1.current(1,111.1).
The functions of the PCF8591 library don't give that option.

While taking a look at other PCF8591 libraries, I saw that for what I need, they act the same.

Or I am wrong...

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