# Non-invasive current sensor noise problem

I'm working on a project where I'm measuring current with a split core CT and feed the signal to an Arduino analog input. The problem I'm facing is that I get noise from nearby systems. Example is when I wire up a small electrical pump (about 250mA), and hold my split core current transformer next to it (without clipping it onto the wire!) I still get readings on the analog input.

This will be a big problem as I'm planning on sensing current at locations where there will be other wires conducting current around. The split core CT I'm using is the SCT-013-005.

Does anyone have any solution for this problem? Is it simply a poor quality current sensor that is not isolated from outside electrical noise? I have not yet tried to measure current with the CT, and have another component nearby drawing current.

OK so you've got this pump that presumably has got hundreds of turns to create enough magnetism to rotate the armature. The pump takes 250mA and the magneto motive force (MMF) is amps multiplied by turns. The magnetic field emanating from this pump has a driving force of 25 ampere turns (that's 0.25 amps x 100 turns) and maybe there are more like a thousand turns. It could be as high as 250 ampere turns.

On the other hand, when you feed a single wire (that's one turn) through the CT to measure maybe 40 amps peak$^1$ of household consumption, the MMF is 40.

You hold the CT close to the pump and you get a reading - are you still surprised? It's called leakage flux and the CT's core is not a perfect rejector of leakage flux that hits it.

$^1$ this will be different depending what your AC voltage is.

• I'm not surprised that you get readings from the electrical magnetism induced from nearby components. But i've seen other projects where they use CTs to measure power consumptions like this, so how do they avoid all that noise from giving you faulty readings? – user43753 May 30 '14 at 21:59