# How does this current sensor work (with only half magnetic core)?

I have been working for sometime with current sensors (current sensor goes all around the conductor to get a voltage reference corresponding to the amperage), however I just found this little device that claims to be patented (I couldn't find the patent).

It has come to my attention that the system measures current just as a current sensor (sensor around the conductor), and it measures voltage with the "little plated pins", in order to provide relevant information, apparent power, power factor, current, etc...

The datasheet claims that it has been patented (it does not provide patent number) according to what I know, the current sensor shouldn't work as intended since it does not close the loop so the readings should not be accurate, and yet the device claims a 2% accuracy.

I have also been looking for any articles about what kind of device or technology is behind this device to verify its accuracy however I haven't found anything about it nor about current sensing techniques involving incomplete magnetic cores.

I'm clueless about what's going on here so I have come here to ask (hopefully I can get a lead); what am I missing here?

• My guess is that it measures the current by sampling the magnetic field strength with a hall effect sensor. They don't necessarily need a core around the conductor and they can measure DC in addition to AC, but they need to be pretty close to the wire in order to be accurate. An example chip: melexis.com/en/product/mlx91206/… – jms Jul 8 '16 at 4:48
• Also, take a look at Fluke T5-600. Same principle. – winny Jul 8 '16 at 5:46
• Hall cells are the most likely technology. (I long long ago saw a meter with a U section trough along the back of a moving iron movement - you laid an insulated car battery cable in it and it measured cranking amps (ie 100's of amps). It used the magnetic field of the straight wire to deflect the moving iron meter movement :-) – Russell McMahon Jul 8 '16 at 11:56

"Closing the magnetic loop" improves the sensitivity of the meter, but not necessarily its accuracy.

The magnetic field strength around a wire is proportional to the current flowing in it. However in open air this field is too weak to measure with a conventional meter. A loop of high permeability magnetic material such as silicon steel attracts the field into itself and makes the signal as much as 10,000 times stronger. This can then be transformed to higher voltage for measurement with a meter. But the magnetic core also suffers from hysteresis, eddy currents and saturation, all of which can reduce measurement accuracy.

Alternatively the field could be measured directly using a Hall effect sensor. With good amplifiers and temperature compensation a Hall effect sensor can accurately measure far weaker magnetic fields than a moving coil meter. Without using a core the magnetic field will vary more depending on orientation and distance from the wire. But if the sensor can be accurately positioned then there is no reason why it can't be just as accurate as a clamp style meter.

Devices such as the MLX91208 have a small amount of magnetic material embedded in them to concentrate the field, both to increase sensitivity and reduce the need for accurate positioning. Since they don't completely 'close the magnetic loop' the gain is less than a clamp-on core, but the large air gap may actually improve accuracy.

The 'little plated pins' in the Wi-Beee may have embedded Hall sensors with magnetic material around them them that performs a similar function to the concentrators in the MLX91208. The Wi-Beee is precisely positioned over the terminal block in close proximity to the wires, so it should be able to get good sensitivity and accuracy.

It doesn't matter if the core is closed or not, theoretically, you don't even need an iron core.

But a core bundles and contains almost the entire magnetic field of the conductor. This not only shields the field from outer influences to some extent, it also leads the entire field through the secondary winding.

Now have a look at the field of a horseshoe magnet. It contains (nearly) the entire flux in its turn. So if a conductor is placed deep inside the slit of a horseshoe-shaped core, the magnetic field would look like the same.

Compare this to your device. It seems it consists of four horse-shoe shaped cored stacked in two rows, the lower row for the first and third, the upper row for the second and fourth conductor.

Another benefit of ferromagnetic materials is the high amplification of the magnetic field by that factor of $\mu_r$. Usually in the order of 1, the value is in the order of 10'000-50'000 (and beyond) for those materials. By using an open core, this effect is reduced a little. Ampere's law says:

$I=\oint H\,ds=\oint \mu_0\mu_r B\,ds$

Or in words: The auxiliary field summed up along a closed path gives the current through the surface enclosed by this path. If the path goes only 75% through iron and the rest through air, the measured field inside the iron is also just 75% of that in a closed core (minus some losses). That's not that dramatic.

Of course, the device needs to be calibrated to give correct readings.

It is of course possible to fool the device. I guess placing an iron bar onto the fingers from behind would increase the readings a lot, since this reduces the air gap and increases the integrated field. But since the extent of the field of a horseshoe magnet isn't that large, ferromagnetic material placed just nearby, not directly at the fingers shouldn't influence the readings a lot.

(And of course, it doesn't matter if the current is measured by a secondary winding or by a hall effect sensor, which also would be placed in a small gap in the iron core.)

• Good user name for a magnetic field answer :-) – Russell McMahon Jul 8 '16 at 11:53
• @RussellMcMahon: Yes, I'm a Tesla per squaremeter ;-) But that name is quite common here. – sweber Jul 8 '16 at 12:38
• And I'm a red headed son of Mahon (but only in name) - but nobody knows. – Russell McMahon Jul 8 '16 at 14:29