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I am trying to build a non-invasive current sensor using a hall effect sensor and ferrite core.

I am using a hall effect sensor sandwiched between 2 halves of a ferrite core like this: ferrite core - digi-key I grooved out the core with a dremmel so that the hall effect sensor can sit inside and allow the flux lines to pass through it.

It works well until the ferrite core magnetizes after the wire being sensed has current flowing through it long enough..

Is there a better material to use other than ferrite that will allow flux lines to pass, but not become magnetized?

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  • \$\begingroup\$ "Is there a better material to use other than ferrite that will allow flux lines to pass..." - any non-magnetic material will let magnetic flux pass through it, but you probably want something that concentrates the flux lines. What frequencies are involved? \$\endgroup\$ Jan 1, 2020 at 1:43
  • \$\begingroup\$ Yes you are probably correct, the wire continously has about 0.2amps DC flowing, then for appox 7 seconds every 5 minutes it will jump to 0.4 amps. I want to sense this change. The problem is that the sensor gets less sensitive as time goes on... I suspect this is from the core itself becoming magnetized \$\endgroup\$
    – MattG
    Jan 1, 2020 at 2:14
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    \$\begingroup\$ I'm concerned about your 'grooved out with a dremel'. Post a cross section of how the modified core is assembled with the Hall device. \$\endgroup\$
    – Neil_UK
    Jan 1, 2020 at 6:01

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The best way to use a flux concentrated DC Hall is like this

Blue is ferrite or iron, black is the two Hall devices.

enter image description here

First, note there is no milling out of pockets with a Dremel for the sensor to sit inside. This way, there is a large consistent air-gap to lower the overall permeability of the ring, to prevent it saturating with small currents. You've not provided a picture of how your Hall sits in a groove, but I suspect it allows the two halves of the core to touch. This (1) allows most of the flux to bypass the Hall and (2) with no airgap the cores saturate at low currents.

Second, there are two Hall devices, one facing up, one facing down. The electronics adds the two signals together. This tends to reject any fields in the sensed gap caused by linear external fields, while doubling the sensitivity to fields round the core caused by the current. Alternatively, you can have the two Halls facing up, and take their difference, which might be easier to do in a bridge configuration.

The gain of this configuration is linearly sensitive to the length of the airgap, for high permeability magnetic material and small airgap. Take some precautions to make the gap consistent, for instance with a non-magnetic spacer like aluminium, if you dismantle the device between calibration and measurement.

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  • \$\begingroup\$ Good answer dude \$\endgroup\$
    – Andy aka
    Jan 1, 2020 at 11:32
  • \$\begingroup\$ I second that, thank you! So to make sure I understand correctly, I would connect the outputs of the two halls together, then amplify this value? \$\endgroup\$
    – MattG
    Jan 1, 2020 at 14:56
  • \$\begingroup\$ And also, do you mind briefly explaining why the air gap prevents saturation of the core? Thanks so much! \$\endgroup\$
    – MattG
    Jan 1, 2020 at 15:10

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