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I'm trying to validate my wiring for a variation of this circuit:

SCT 013-015 based Energy Monitoring Circuit

which is taken from https://www.poweruc.pl/blogs/news/non-invasive-sensor-yhdc-sct013-000-ct-used-with-arduino-sct-013

Before I solder everything up on a more permanent form on protoboard, I'm trying to validate that I have my variation of the design correct.

I'm using a 10μF capacitor and 10kΩ resistors instead of the 470kΩ ones in the diagram (which based on my understanding of the circuit, should not matter).

I'm trying to test that the voltage shifts up and down by the correct amount when load is placed on the split core using a simple 9V battery so I should see it move up and down from 4.5V instead of the 2.5V in the circuit as diagrammed, which corresponds to this:

Circuit as wired

All of this shouldn't affect the basic operation of the circuit from what I can tell, however I can't get it to register anything on the multimeter.

My breadboard looks like this:

Breadboard Circuit View One

Breadboard Circuit View Two

And I can't get it to work right using the multimeter at all. I get a sub-1V reading from the white wire corresponding to the analog output that would go to the Arduino.

This is well below the expected >2V lowest reading I should be getting with the 9V battery I'm using to test, and it doesn't shift at all when I put the circuit under load. I've also checked that the 3.5mm socket wiring is ok and the quick and dirty soldering I've done for it is 1Ω and 0.7Ω on each of the relevant output wires.

Is it my circuit that is broken? Do I need to measure this using an Arduino and some quick programming because this sort of circuit just can't be properly tested with a multimeter? Or do I need to use the multimeter a certain way to test this circuit?

Edit:

Here is the data sheet for the SCT 013-015 split core transformer. It does not quote any value for the internal burden resistor so I can't include that in any diagraming, I can only refer to the operating behaviour.

Here is the "wiring" of the split core transformer to the AC circuit:

split-core tranformer clamped onto AC live wire

And here is a re-done photo of the circuit with less stray wires:

breadboard-v2

  • Red is +9V
  • Green is the fluctuating voltage that is supposed to make it possible to measure the power usage
  • Black wire from the breadboard to the ground pin is obviously ground.
  • The other black wire is the connecting wire between the SCT and the capacitor and the centre of the voltage divider which should be at 4.5V.
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  • \$\begingroup\$ It looks like this circuit is designed to level-shift a current transformer to be centered around the power supply midpoint, right? Do you have anything connected to the primary of the CT? \$\endgroup\$
    – Hearth
    Commented Nov 23, 2021 at 15:46
  • \$\begingroup\$ Thats it in a nutshell, and yes I did try and make it clear that I'm putting load on the primary. The split core transformer (SCT-013-015) is clipped around the live wire of a regular AC power cable which is connected to a 90W load of regular 240V AC \$\endgroup\$
    – Techdragon
    Commented Nov 23, 2021 at 16:02
  • \$\begingroup\$ Usually the debugging would involve testing the transformer separately as a first step. Apply a waveform and check the output of the windings. You don't need any prototyping setup for this. Can you confirm that the transformer is working as it should? \$\endgroup\$
    – Syed
    Commented Nov 23, 2021 at 16:41
  • \$\begingroup\$ Testing the split core transformer with the multimeter connected to the two relevant pins on the SCT-013-015's 3.5mm jack, registers between 100𝜇A and 200𝜇A of of fluctuating AC current using my 90W test load. \$\endgroup\$
    – Techdragon
    Commented Nov 23, 2021 at 17:02
  • \$\begingroup\$ There are bunch of wires which are not described and no CT seen. Edit the schematics and write the wire colors. Where is the burden resistor and its value? Datasheet of the CT? \$\endgroup\$ Commented Nov 23, 2021 at 18:38

1 Answer 1

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The reading on your DMM mostly depends on the input impedance of your DMM. You have depicted two circuits, one with 2 x 480 kOhm divider on the other with 2 x 10 kOhm. Now, the 2 x 480k is pretty big source impedance that will be affected by the measuring circuit impedance - ADC or DMM.

You can improve the circuit by driving the reference voltage by low impedance source. This can be done with the use of an opamp buffer.

schematic

simulate this circuit – Schematic created using CircuitLab

I’m rapidly getting the impression this kind of circuit is impossible/impractical to “test” with basic equipment like a multimeter.

Not only for DMM but for ADC, too. However it depends on the input impedance f measuring circuit. Higher the input impedance, lower is the influence on the circuit VS. the impedance of the source (R1||R2) = 470k/2 or 10k/2 from your schematics. In case of using opamp, the source impedance is very low in the range of few tens of Ohms.

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