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emonPi with SCT-013-005 CT

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Hi all!

I need to measure small amounts of power consumption coming from a device like a television or a PC monitors. I think that the SCT-013-000 sensor sold with emonPi is too big for this purpose, so I bought an SCT-013-005 sensor which is suitable for small amounts of power consumption.

I read some other threads in this forum to understand how to connect alternative CTs to the emonPi. Since the SCT-013-005 CT already contains a burden resistor, I simply removed the 22Ω burden resistor from the emonPi and connected the sensor to the emonPi.

According to this guide (https://learn.openenergymonitor.org/electricity-monitoring/ctac/ct-and-ac-power-adaptor-installation-and-calibration-theory) I’ve also calibrated the sensor with a value of 5.

However, when trying to measure the power consumption of a pc monitor, I’m getting 0 W, -1 W and 15 W respectively in off, standby, and on states. Except from the on and off states, I can’t understand why I’m reading a negative value while the monitor is in standby.

Am i doing something wrong while calibrating the sensor?

No, I think you have the calibration correct, but 15 W is still a very low power even for a 5 A c.t. With that, the maximum power is more than 1 kW, so you are trying to read 1.5% of full scale. I think you were lucky reading 0 W when the monitor was switched off, and the -1 W is noise that is being picked up and measured.

You could try to increase the sensitivity still more by taking the wire through the c.t. several times. If you have 5 turns through the c.t., you will appear to multiply the current by 5 times, and to correct that you must change the calibration constant to 1.0 That is because 1 A flowing in the wire goes through the c.t 5 times, so it looks like 5 A.

I suggest you try as many turns as you can - maybe 10? I use 20 turns and 5 A when testing the 100 A c.t.

Thanks for your reply.
I will try your suggestion and see if i can get better readings.