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How do stud sensors actually sense current, what would cause the effect to propagate across an 8x8 foot surface area and remain strong enough to be detected?

I asked another group "what is happening", now I would like to ask your group "why". This is a question about electrical sensors included on some stud sensors used in home repair to find wiring within walls. The garage for my home was built in 2000, the garage interior is naked particle board directly nailed to wood studs, and the insulation has paper backing (not foil). When using the electrical sensor, most of an entire wall surface shows having current instead of indicating specific wire routes. Other walls in the garage do not show this effect. When I turn off the breaker for the outlets, the current disappears (so the reading is related to current flow, not a bad sensor). What is the effect the sensor is designed to detect, and why might that effect propagate across the surface of one wall and not the (similarly constructed) other walls? Thanks.

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  • \$\begingroup\$ Have you googled how do stud finders work? \$\endgroup\$ – Voltage Spike Feb 1 '16 at 22:49
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    \$\begingroup\$ Are you using a stud finder or a current sensor? \$\endgroup\$ – Samuel Feb 1 '16 at 22:49
  • \$\begingroup\$ I think the interesting thing here is that the triggering is changing based on the breaker. He is therefore doubting the basics. \$\endgroup\$ – mcmiln Feb 1 '16 at 23:22
  • \$\begingroup\$ Thank you for your comments. In this case, the capacity to find studs and the capacity to sense current are two different capabilities of the same tool. I am concerned with the capacity to sense current. How can current be sensed through another material, and how might that effect spread? \$\endgroup\$ – Franklin Hoffman Feb 1 '16 at 23:36
  • \$\begingroup\$ I was concerned with how current can be sensed through another material, and how might that effect spread. Another answer specifies the dielectric change the sensor requires perhaps being affected by a current within damp wall material. That is interesting and troubling. \$\endgroup\$ – Franklin Hoffman Feb 1 '16 at 23:50
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So there is a magnetic stud finder and an electric one. I will assume you are speaking of the electric kind. In these, there is a capacitive plate in the front end which is driven using two multivibrators. As a multivibrator is driven, the voltage on the plate drops to zero and then as it goes low, the plate charges back up. The rise time/slope determines the dielectric changes between your plate and the plate in the wall. This dielectric change is what we detect.

Since an the electromagnetic lines are disrupted by the secondary plate, this causes a change in flow of the field lines. Thinking of this in terms of your sensor, there is something blocking the field lines over the whole wall. This means that the thickness of the wall is proper in all areas. So your whole wall is a stud.

As for when the power is turned off, that's a great question. My thought would be that the wood is a bit wet. This damp wood is then carrying current. You did say it was bare, and it may be damp. Might want to look into waterproofing.

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  • \$\begingroup\$ "The rise time/slope determines the dielectric changes between your plate and the plate in the wall. This dielectric change is what we detect." So there could be a short which along with moisture in the particle board wall surface sets up what the sensor sees as a "plate in the wall", producing "dielectric change" ? (which I admit I will have to look up). \$\endgroup\$ – Franklin Hoffman Feb 1 '16 at 23:44
  • \$\begingroup\$ In this case, the capacity to find studs and the capacity to sense current are two different capabilities of the same tool. I am concerned with the capacity to sense current. \$\endgroup\$ – Franklin Hoffman Feb 1 '16 at 23:52
  • \$\begingroup\$ OK, I looked up dielectric change. It may be possible that the insulator (the wooden wall) combined with moisture has become dielectric and presents to the sensor an indication of electric current when an (unseen) shorted wire causes an electric field to form in the 8x8 foot affected area. Does that make sense? \$\endgroup\$ – Franklin Hoffman Feb 2 '16 at 0:04
  • \$\begingroup\$ It does indeed. Essentially the dielectric constant is something which changes based on the crystalline structure of an object and how it changes the band gaps. This is a property given by solid state physics. I say this to reign things back. All material has this property but with a different structure. When combining the dielectric of the water with the wall, you get an average dielectric based on the moisture content. This change in the dielectric constant allows the change in electric field. \$\endgroup\$ – mcmiln Feb 2 '16 at 4:07

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