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I am trying to determine if an electrical device is on in a non-invasive way using an arduino. My understanding is that devices like current transformers and Hall sensors must enclose only the hot or only the neutral wire.

Am wondering how products like the iVac work as it is clamped around both hot and neutral wires. What components could I get to achieve this?

Alex
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  • You should use a hall sensor.It is very reliable. – Miss Mulan Mar 25 '21 at 19:41
  • Do you need to measure current or just sense the presence or absence of voltage? – vir Mar 25 '21 at 19:41
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    @MissMulan, the question is how would you get a Hall (note capital as it's a bloke's name) sensor to detect current when the live and neutral currents are in opposite directions and the magnetic fields cancel out. – Transistor Mar 25 '21 at 20:12
  • @vir just the presence of voltage! – Alex Mar 25 '21 at 22:27
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    "*... just the presence of voltage!*" Alex, that's not what you asked. Presence of voltage in a cable generally doesn't tell you whether or not something is on. The switch may be downstream of where you are sensing **electric** field. You can confirm this with one of those LED screwdriver testers. You specifically mentioned current transformers and enclosure of the wires which means you are trying to monitor **current** by its **magnetic** field. These are two very different requirements with very different measurement techniques and shouldn't be confused. – Transistor Mar 25 '21 at 22:46
  • @Transistor good point - definitely want to tell if current is flowing (what you answered) – Alex Mar 26 '21 at 02:36

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From page 13 of the user manual:

  • The rear Clamp Cover has three positions for mounting, to accommodate various sizes of power cord diameter.

Clever!

  • The Mode Switch is set to OFF. The Sensitivity Control is set to mid point.
  • The power to the power tool is turned on.
  • If the ‘Tool Plus’ is detecting the current flow then the Activity LED will turn on.
  • If the Activity LED does not turn on, then the Sensitivity Control should be adjusted counter clockwise until the Activity LED turns on.
  • If the Activity LED does not come on, then the ‘Tool Plus’ should be rotated slowly around the cable or moved along the cable until the Activity LED comes on.

The last instruction above gives you the clue as to how it works.

enter image description here

Figure 1. Magnetic flux around the live and neutral conductor of a 3-wire mains lead. (Image mine.)

You are correct that a typical current transformer will see zero resultant flux around the cable. The trick in this case is to put a small sensor - probably a Hall sensor - in very close proximity to the cable and rotate it around to a point where it is closer to one of the current carrying wires. In my illustration this would be above the brown or to the right of the blue. Placing it on the top right or beside the earth wire would result in cancellation of the two fields and an inability to sense current.

At this point tighten the clamp to the cable.

You've got it working so clamp it in this position.

The Sensitivity Control should now be set to a fully clockwise position. It should then be rotated slowly clockwise.

Now set the "on" threshold.

Transistor
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  • That's pretty cool. Thanks for the reverse engineering intel. Do you happen to know if this is common practice or if there is a better way to do this (without separating the wires)? – Alex Mar 25 '21 at 22:42
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    I don't know of any other way. You've got to detect the **magnetic** field. Note my comment below your question where you seem to be confusing voltage and current sensing. – Transistor Mar 25 '21 at 22:48
  • Gotcha - thanks for the clarification! – Alex Mar 26 '21 at 02:36