# Can I use this CT for power metering

I have one of these

https://images.app.goo.gl/5M6VCmHZCvNACKP49

It connects to a transmitter powered by 3x AA batteries and sends the signal to a display that shows current, daily, weekly and monthly power consumption

I’d like to connect the CT to an Arduino

I’ve checked the current delivered by the CT using a multimeter and a load of 9W (led bulb); when the bulb is on I get 0.6 uA (micro A) 0.000006 A
Considering 9W/220V = 0.04A
0.04A/0.000006A = 6800

Can the CT really have 6800 windings?

Here is a general guide to connecting a CT to an Arduino.

Yes, I followed exacly that guide, but I’ve tried different burden resistors with no luck

I’m not sure if mine already has one, or if it’s even a real current transformer

I’ve attached some links and some pictures (ignore the extra pair of blue and red wires that I’ve soldered in parallel for ease of connection)

https://www.geotogether.com/consumer/product/ct-clip-sensors-pack-2/

Ok, I was using a 100k and a 10k by mistate for the voltage divider; I'm now using two 10k resistors for the voltage divider and a 150 Ohm burden resistor

I've assumed 6500 turns; supply voltage 5V; current range 75A (written on the sensor), which is 106A peak

What I'm getting using a calibration factor = 1000

emon1.current(3, 1000); // Current: input pin, calibration.
}

void loop()
{
double Irms = emon1.calcIrms(1480); // Calculate Irms only

is an RSM of about 3 when there is no load and about 4.5 with a 46 W incandescent bulb

Shouldn't the RMS be 0 with zero load no matter what calibration factor I'm using?

Quote: "Shouldn't the RMS be 0 with zero load no matter what calibration factor I'm using?".

Yes, it should. That tells me your CT is not shielded and is acting as a capacitor, picking up the AC voltage still on your wire. Remove the wire you are measuring and see if the RMS goes to zero.

Paul

about 4.5 with a 46 W incandescent bulb

Calibration against such low load currents will be very unreliable. You should do a proper calibration over a range of line currents. For example, use a 1500W room heater as a significant load.

Excellent multipoint calibration tutorial.

Paul_KD7HB:
Quote: "Shouldn't the RMS be 0 with zero load no matter what calibration factor I'm using?".

Yes, it should. That tells me your CT is not shielded and is acting as a capacitor, picking up the AC voltage still on your wire. Remove the wire you are measuring and see if the RMS goes to zero.

Paul

it still does not go to zero

jacoscar:
it still does not go to zero

ONe last chance, wrap the CT with aluminum foil, not shorting anything. IF still not zero, the CT is not your problem.

Paul

Wrapping the CT with aluminium foil didn't help; disconneting it didn't help

I've tried with a TTGO Display and I get the same problems; furthermore, if I remove everything from the breadboard and only leave the two 10k resistors as a voltage divider, the rms I get when I connect the input pin to 5V is 0, when I connect it to ground is 0 as well, but when I connect it at the mid point between the 2 resistor it's not 0!

Also, printing the voltage value at the mid point gives me something around 2600, instead of the expected 2048 (4096/2)

Try reversing the two resistors in the voltage divider. You can also measure the actual resistance of the two. They should be identical, but seem to not be.

Paul

The multimeter shows 10.00k and 9.99k
Even by swapping them I get the same reading: 2600-2700

I also measured between 5V and ground and the multimeter shows something like 4.87V
I think this shouldn’t matter, right?

This is what happens when I switch the load on and off