SCT-013-000 very confused!

Makes sense, Thank you so much! I’ll sample the analog input on the final circuit to make sure it stays in the A/D converter range!

Good day ladies and gentlemen,

I am a newbie in this group and also new with Arduino. Since 2 weeks ago, I have spent my time to build an electricity data logger for my study in college.

At this moment, I have no 240V AC to 9V AC adapter, so I decided only to use the SCT-013-000 sensor to get the current value based on the diagram in the link below:
CT sensors - Interfacing with an Arduino
While reading the content in this link, I noticed the following phrase:

If the CT sensor is a “current output” type such as the YHDC SCT-013-000, the current signal needs to be converted to a voltage signal with a burden resistor. If it is a voltage output CT you can skip this step and leave out the burden resistor, as the burden resistor is built into the CT.

The SCT-013-000 that I bought is printed (on the sensor itself) with information saying that the sensor produces output = 1V when the input is 100A. Based on my understanding, this means that the SCT-013-000 that I have is voltage output CT, NOT current output CT. Therefore, I conclude that I don’t have to use a burden resistor for my voltage output CT, as mentioned in the previous link.

I tried to double-check about this on the manufacturer’s website on the following link:
100A Split core current transformer SCT-013
There is the data sheet for SCT-013-000 in this link. Inside the datasheet, I found the schematic diagram for both current output type and voltage output type. Based on my understanding of the schematic diagram, the current output type is equipped with TVS diodes while the voltage type is equipped with TVS resistor, am I right? Therefore, I can omit the burden resistor for my situation because TVS resistor in my SCT-013-000 acts as burden resistor, right?

If my assumption about I don’t have to use burden resistor is correct, then the estimated value for the TVS resistor is calculated below:

There are 2000 output coils in SCT-013-000. If the input current, Ii is 100A, therefore Ii x Ni = Io x No, hence output current, Io = Ii x Ni/No = 100A x 1/2000 = 0.05A
If the input current 100A produces produces 0.05A and 1V at the output, that means the TVS resistor, Rtvs = Vo/Io = 1V/0.05A = 20Ω

I tried to use multimeter to read the value of resistance between the output terminals of my SCT-013-000 and obtained the value 16.6Ω. However, I think this measuring method is not accurate because the TVS resistor is connected in parallel with the 2000 output coils of my SCT-013-000. If the multimeter supply DC current/voltage to measure the value of resistor, the 2000 output coils will behave like an inductor supplied with DC current/voltage, which is a short circuit. Dismantling the SCT-013-000 just to disconnect the TVS resistor from the 2000 coils will damage the sensor. Therefore, at this moment I decided to stick with the TVS resistor = 20Ω, which is estimated and explained in the previous paragraph.

I use the code below for my Arduino UNO kit, which is downloaded from Open Energy Monitor website:

// EmonLibrary examples openenergymonitor.org, Licence GNU GPL V3

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

void setup()
{
Serial.begin(9600);

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
}

In one of the replies above, I noticed that Mr. Robert.Wall said that the value of the calibration constant should be near to the burden resistor value. Does this means that I should also revise my calibration value in the script above from 111.1 to 20 for my case?

Long story short, I would like to double check the following 2 things:

  1. I don’t have to use burden resistor because the SCT-013-000 that I have is voltage output type that comes together with TVS resistor.
  2. I have to revise my calibration constant in the code above from 111.1 to 20 if it is estimated that the value of TVS resistor for my SCT-013-000 is 20Ω
    Thank you very much and have a nice day.

Below is the photo of my SCT-013-000. The information saying that the sensor produces output = 1V when the input is 100A is printed on the sensor.

I just read one of the posting by Mr Robert.Wall about reading the Resources > Building Blocks article that explains the calibration theory. Below is the link:
Calibration Procedure

However, I found another additional information on the following link:
Installation and Calibration
This link teaches us to calculate current constant for both current output CT and voltage output CT. The formula is shown below:

current constant (for current output CT) = (maximum input current ÷ maximum output current) ÷ burden resistor value
i.e for SCT-013-000 current output type with maximum input current = 100A, maximum output current = 0.05A, and 18Ω:
current constant = (100A ÷ 0.05A) ÷ 18Ω = 111.11

current constant (for voltage output CT) = (maximum input current ÷ maximum output voltage)
i.e for SCT-013-000 voltage output type with maximum input current = 100A and maximum output voltage = 1V
current constant = (100A ÷ 1V) = 100

Based on the formula above, I don’t have to worry about the value of TVS resistor and don’t have to use burden resistor if I use the voltage output SCT type. All my questions in the previous 2 postings are answered. Thank you very much and sorry for disturbing. Have a nice day.

See About split core CTs SCT-013 family - #5 by foxfaisal

Throughout the above, you write “TVS resistor”. The component is not a resistor, its full name is Transient Voltage Suppression Diode. It is a 2-terminal semiconductor device with a highly non-linear V-I characteristic. Because of that, you will not read a sensible value of resistance. I suggest you look up the data sheet for one to see exactly what it does. It is connected across the secondary winding of a current transformer to clamp the voltage to a safe level should the burden become disconnected. It is most important that it does not shunt current away from the burden, and therefore it must conduct negligible current at the normal operating voltage across the burden resistor.

Dear Mr Robert.Wall,

Thank you for your feedback. I have explained how did I came up with the term ‘TVS Resistor’, which is available in the following links:

  1. The first explanation
  2. The second explanation

Thank you very much.

I’m implementing a second CT on the same Arduino device. What I did is just create another circuit, on the same 5V out, It seems my readings on the two CTs are different, is what I’m trying to do valid or should I make adjustments?

Basically, I have two dividers, two capacitors and two burden resistance and I split +5V to go on both circuits, one CT going to A0, the other on A1

What you’ve done sounds correct - have you compared it with the emonTx circuit diagram?

You should expect a small difference between the two. The tolerance on the CT is ±3% over the range 10% - 120% of rated current, to that add the tolerance of the burden resistor, and then it’s possible that the two sets are at opposite ends of the tolerance band. So you could have 8% difference between the two with 1% resistors. If your current is below 10 A, then the accuracy of the CT is not specified, and although my measurements indicate that generally it is well-behaved, there’s no guarantee what it will read.

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So, with a meter I get these values :

C1 : 1.9 x 124 = 235W
C2 : 0.8 x 124 = 99W

The Uno actually gives me :

C1 337.49
C2 181.21

Quite a stretch isn’t it?

My Burden calculation : (5 / 2) / 0.05 = 50 Ω, I used 47Ω

The calibration I used is (100 / 0.05) / 47 = 42.55

On the second topic, here is the resultst I get when swapping the two CTs, does that seem to be in the tolerance range? It would seem so :

CT1 156.36
CT2 338.75

<<swap>>
CT1 347.45
CT2 149.77

<<swap>>
CT1 154.67
CT2 338.59

Are the numbers 1.9 and 0.8 the rms currents that you read with your meter? Why are they different? If you are comparing two inputs, you should put both CTs on the same cable so that you can compare like with like.

What load are you using? You should be using an electric heater that does not have a fan inside, or an electric kettle.

Change those things and try your test again. Until I know exactly what you are doing, and it is the same for both inputs, it is very hard to identify your problem or problems.

Yes, these are the number for each phase, sorry I should have said. This is a US installation with 2x110 phases.

When I swapped the two CT it was the idea actually, instead of moving both on the same cable I’m swapping them to see if they approximately read each other’s value.

On the load that’s a good point, I don’t know actually, It’s a weird electrical installation, I’m actually measuring power for the people living in the basement so I can’t really ask. I’m not aware of anything fancy they are doing but I think they do use space heaters.

What makes me think I’m probably just fine is this :

  • The Orange line is CT1+CT2
  • Tthe Green line is the reading counting the revolutions of the meter (or “truth”) minus (CT1+CT2)

Look at how the orange line jumps up and down (assumed space heater), and how the green line doesn’t seem to be impacted by this. To me it proves that I have the right balance, if it wasn’t the case, I would observe some of the orange line “spilling” on the green and have some kind of impact. Not sure I’m making any sense here…

To give you a little bit of background around that project, it all started with a $5000 coned bill in December, following a reading after more than a year of estimate. There was no way our home would use what was the equivalent of $400 every month for the last 16 month, so I started investigating.

The landlord “shares” the meter in the house for our family and people living downstairs (this is quite common in 3 stories homes here) and we established that I pay coned and he reimburses use 15% of the bill. We were ok with that, but didn’t think they would use space heaters as the heating is gas!! Apparantly not enough…

So I will gather some more info and have all the materials to go to my landlord with substantial information for a reimbursement.

Many thanks again!

I don’t understand “coned” - presumably American and not real English?

The second but major point is, if you are going to argue over payments, you better be certain that what you are saying is totally accurate. Hence, you need to carry out the full calibration as accurately as possible given the test gear that you have available to you - or that you can hire. If you are talking about a substantial sum, then it may be worth your while to do so.

Sounds like he’s referring to ConEd, i.e.Cosolidated Edison, his energy supplier.

Lol, tanks Bill. I live in NYC, I assume everyone knows everything about NYC :slight_smile:

Let’s say the project is 75% fun and 35% arguing with the landlord. The point is not to have a discussion on the penny, but to show that the 85% of the bill I’m asked to pay is actually more like 50%.

I’m scratching my head on the Wemos D1 mini now, trying to find the best arrangement of the components on the board, will update the thread when I’m done with pictures of the mess :slight_smile:

Robert Wall said:
you better be certain that what you are saying is totally accurate.

Not to mention that any system he uses to determine a monetary amount may need to be revenue grade.
Depends on state law, local regs, AHJ rules, etc.
AHJ = Authority Having Jurisdiction, usually the energy supplier.

That amount displayed is the overall consumption, that part I know is very precise as it’s based on the revolutions of the meter, the rest has much more margin indeed!

I wanted to thank everyone helping me in this forum. I have the system running for a while and I’m very happy with the result. Here is an example of the dashboard I was able to setup with the data coming from :

  • Light sensor counting the revolution of the meter disc
  • 2xCT, one on each phase going to the basement appartement that is not mine

These two devices feed Watt values to a Graphite Database, the dashboard are made with Grafana.

I will probably be able to get my money back, and it was a lot of fun to put that together! So thank you especially @Robert.Wall !

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So what is the resolution? Did you get a reimbursement? Enquiring minds want to know!

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