# US5881 Hall Effect sensor not detecting AC field

Yesterday I was disappointed to learn there is no magical IC to measure current flow in a standard appliance cord.

On the advice I received, I went ahead with a US5881 Hall effect sensor and split the conductors. I then passed only one of the conductors over the over the sensor (to avoid cancellation of the magnetic field from having two conductors in close proximity).

I wired it up as shown below. The setup works fine when tested manually with a permanent magnet. But I cannot detect any pulses when attempting to measure against the AC line. I do not have an oscilloscope, so pulses are being measured using interrupts on a digital Arduino input.

(I also tried with a 100nF capacitor on the source voltage and a 4.7nF capacitor on the signal legs, as shown on the datasheet. The result was the same: no pulses when measuring the AC current.)

Is this just a matter of insufficient sensitivity on the US5881? From the other thread I linked, I kind of got the impression this chip could measure AC fields with this setup, but maybe not? Or is there something wrong with my approach?

simulate this circuit – Schematic created using CircuitLab

• Have you tried addressing the problem to the person suggesting it? – Andy aka May 20 '16 at 8:47

There are a few things wrong with your approach right now:

• The sensor is not sensitive enough. Consider the Biot-Savart law $B=\dfrac{\mu_0I}{2\pi r}$, where I is the current and r is the radius from the center of the wire. For a 120W light bulb at 120V (1 A), and the minimum separation to the center of the wire (optimistically 2 mm), you end up with 0.1mT. Compare that to the trigger threshold of your sensor (25mT nominal), and you would need 250A in order for it to trigger.
• The light is off. Without current flowing (even if voltage is present) you will not have any magnetic field to sense.
• The switch is sensitive to magnetic fields in only one dimension, and you have picked the wrong one. Again, looking at the datasheet, the field must be going through the front of the package, which would mean that a wire would need to be touching the top of the package for any current-induced magnetic field to actuate the sensor

Ideally I'd like to see a capacitor near the sensor, but that's not your problem. That sensor is really just for detecting magnets as a tachometer or an interlock/switch, not for detecting the presence of AC current.

• thanks! The sensitivity calculations are especially enlightening. I should have mentioned the light was off for the photo only (to avoid glare). And I did also try with capacitors (per datasheet, and mentioned in my post), but outcome was the same. I guess I just need to look more closely for a more sensitive chip. (I know there are off-the-shelf current transformers for this, but they seemed expensive... and wouldn't teach me as much!) – Roberto May 20 '16 at 4:37
• @Roberto You may be hard pressed to get a digital Hall effect system functional due to the signal magnitudes, especially if you have a low current device (<1A) under test. I get single-unit current transformers at less than \$3.00 at Digikey, so not that expensive. You may be able to cobble together an opto-isolator solution as well, or wire in a hall-effect sensor that is designed to be in-line with a current-carrying wire. – W5VO May 20 '16 at 4:47