Electromagnetic field (AC current) detection

Thread Starter

kadriguler

Joined May 18, 2011
3
Can we make a simple but precise circuit that locates the hidden current (cable) on the wall ?

Circuits made with BC547 (3 pcs) transistors and CD4017 integrated circuit cannot detect the current inside the wall.
 

andrewmm

Joined Feb 25, 2011
1,494
Thanks
These circuits for surface cables, with a more sensitive circuit I want to find the cables inside the wall.
can you be specific as to how far in the wall and what sort of wall are you looking at ?

Its a compromise ,
The air is full of "transmissions" that will be received,
its a question of how sensitive the detector is, and how strong the wires magnetic field is.

the hall effect one works well to a wire carrying 240 volts, at about 1 amp through about 1 cm of plaster,

Advantage of the hall one is it has low sensitivity to the E field, whilst a typical single coil system does not.
 

Irving

Joined Jan 30, 2016
1,549
The problem with hall effect is it wont find cables in conduit and its not so good with twin and earth cables as the magnetic field around live is mostly cancelled out by that around the neutral.

A commercial cable/conduit finder uses similar techniques as a metal detector - a tuned circuit used in an oscillator is compared to a reference oscillator. When the first oscillator is detuned by the presence of metal the 'beat' between the two oscillators is an audio signal.

An alternative approach is to energise a coil as part of a moderate frequency RF oscillator. Eddy currents induced in nearby metallic materials cause a power absorption and a reduced output from the oscillator.
 

andrewmm

Joined Feb 25, 2011
1,494
I'm just wondering how they distinguish between AC on and AC off ?
An interesting problem this, wonder if anyone has taken a voltage detector apart , or designs them ?
 

Hugh Riddle

Joined Jun 12, 2020
78
I'm just wondering how they distinguish between AC on and AC off ?
An interesting problem this, wonder if anyone has taken a voltage detector apart , or designs them ?
Cable detectors normally sense the electric field around a cable, which is only present if the cable is connected to the mains (i.e. 'on') and not in a metal conduit. That field is strong enough to be cheaply detectable only because operating mains cables are (electrostatically) unbalanced (i.e. neutral near ground voltage, active at mains voltage).

Magnetic field detection isn't used to detect cables because cables create little magnetic field due to their equal active and neutral currents flowing in opposite directions, and would also require the circuit to be heavily loaded.
 

Rian4all

Joined Oct 19, 2020
1
I'm interested in the answer to the question asked by the thread starter. I did not find an answer in any comment. Did I miss anything? can you detect a 240V cable that is in the wall at a distance of about 5cm?
 

Alec_t

Joined Sep 17, 2013
12,089
I have a cheap commercial cable detector (unknown brand) which can detect a 'live' mains twin-and-earth cable behind plaster-skimmed plasterboard, i.e. at a depth of about 2cm. I don't know its operating principle.
 

andrewmm

Joined Feb 25, 2011
1,494
IMHO, There was no answer as the questions was vague.

Basically there are two ways to detect a cable,
either the magnetic field generated by the current flowing through the cable
or
the classic "metal detector" that generates a magnetic field , which the wire in the wall re radiates, to be picked up.

The former system, is simple, some for of current to voltage detector , be that Hall, or a coil , and cheep,
either capacitively or inductively coupled.
BUT , does require a current to flow. If say the light is turned off, the current flowing is VERY low, and a s such is a very small field, making it hard to distinguish from the ambient field.


The latter is more complex, a transmitter / receiver and discriminator system and much more expensive.


QED.

To answer your question,
yes you can detect a cable 50mm in a wall,


BUT.

not if it has no load and with a cheep detector.
 

Janis59

Joined Aug 21, 2017
1,332
RE:"Can we make a simple but precise circuit that locates the hidden current (cable) on the wall""
Thats exactly what do the Bosch seeker. I bought it about 50 USD and it tells about 5 mm accuracy where cable is to some 20 cm afar, tells where are black metal, where are colour metal and where are a massive wood (however about wood it exactness is bit weaker). Wonder if making such thing by own is worth to spend the time.
 

Hugh Riddle

Joined Jun 12, 2020
78
RE:"Can we make a simple but precise circuit that locates the hidden current (cable) on the wall""
Thats exactly what do the Bosch seeker. I bought it about 50 USD and it tells about 5 mm accuracy where cable is to some 20 cm afar, tells where are black metal, where are colour metal and where are a massive wood (however about wood it exactness is bit weaker). Wonder if making such thing by own is worth to spend the time.
I reckon that very useful sounding Seeker product must employ pulsed metal detector technology, probably in addition to 'capacitive plate with amplifier' detection of the electric field produced by the voltage on the cable's Live wire. Developing such a tool would be a pretty demanding DIY development exercise.
 

sparky 1

Joined Nov 3, 2018
544
@Hugh Riddle

It is not an electric field alone. An alternating sine wave has a magnetic field with regard to a live AC wire having current.
It is live and It has current because the light bulb is used. The magnetic field and the hall effect are precise, capable of exact location.
The TS commented on June 21, 2020 gave a specific example then he did'nt bother any more. Could it be
The thread went in a direction of chatting about hypothetical cables, amplified capacitive plate sensors with integrated pulse metal detectors compared to the direction conversation: if it is possible to make a simple detector using a BC547
It would be courteous to start a new thread. REMEMBER be courteous to humans next time.
 
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Hugh Riddle

Joined Jun 12, 2020
78
@Hugh Riddle

It is not an electric field alone. An alternating sine wave has a magnetic field with regard to a live AC wire having current.
It is live and It has current because the light bulb is used. The magnetic field and the hall effect are precise, capable of location.
The thread went in a direction chatting hypothetical cables and amplified capacitive plate sensors, pulse metal detectors compared to
the direction of conversation being simple and using BC547. The last directive by TS was June 21, 2020 It would be courteous to start a new thread.
REMEMBER be courteous to humans.
Can we make a simple but precise circuit that locates the hidden current (cable) on the wall ?

Circuits made with BC547 (3 pcs) transistors and CD4017 integrated circuit cannot detect the current inside the wall.
It's difficult. My ears prick up whenever sensing electric or magnetic fields is discussed. I love the area and earlier designed and developed very sensitive field detection systems, including a medical one able to determine the position of an HF magnetic field source (a tiny swallowed device that transmits temperature, pressure or ionics (pH) as it passes through a human gut) in 3 dimensions. I've also DIY used and investigated consumer mains detectors and successfully modified them to stretch their detection range to the limit.

The thread starter asked a straightforward-seeming question but, by referring to 'current', sowed and later perpetuated doubts about what they wanted discussed in my and I suspect some others' minds. IMHO, introducing Hall effect devices was a red herring.

On receiving Sparky 1's reprimand I have sought to understand what led me to state that the words 'precise' and 'exact' were out of place when discussing mains cable detection, I examined all 16 posts in the thread and saw how it had broadened into wider areas: the Bosch Seeker sounded very intriguing and kadriguler's question could probably be answered using only its simplest function: electric field (aka 'voltage') detection. A fair few wrong statements were made about V, I, and E(electric) and H(magnetic) fields (5 in Sparky 1's reprimand); nothing unusual there - many graduate electrical and electronic engineers have a poor grasp of fields and magnetics.

My answer to kadriguler's question is: a live mains cable whether or not it's carrying current, should be detectable at 5cm through intervening plaster, render or board. 10cm is a harder call (could need 10 times the sensitivity); makes all the difference if the device offers very delicate threshold adjustment. One should also ask 'Has it detected an electric field or mainly the presence of metal (e.g. conduit, pipe or rebar)?'

I have assembled below some basic facts and included one or two views, to provide useful background for thinking about and discussing these areas. May seem dry but, if carefully read, should be helpful. My nose has been squashed up against it for 3 hours, so please let me know when you spot any possible mistakes (incl grammar and spelling), so I can check, correct or edit it accordingly.

1. A conductor (wire) becomes 'Live' when a mains voltage (e.g. 240VAC) is connected to it, regardless of whether it is carrying any current.

2. A conductor connected to a voltage supply creates an 'electric field' around itself, unaffected by whether it is carrying any current, with an 'electric field strength' proportional to its voltage but which falls off steeply as the distance between the conductor and the 'electric field sensing' element (commonly a small metal plate) of the sensing device increases. Consumer AC mains 'cable sensing' devices can work in this way because a mains cable's 'Live' and 'Neutral' wires' are 'unbalanced' through being at different voltages. Electric fields are affected by building materials including wood, plaster, render and board (and their wetness) and the presence of metal (e.g pipes incl conduit and rebar).

3. A conductor carrying a current (e.g. 12A AC) creates a 'magnetic field' around itself which is unaffected by its voltage and has a 'magnetic field strength' proportional to the current it carries but which falls off steeply as the distance between it and the 'magnetic field sensing' element (often a coil) of the sensing device increases. Consumer AC mains 'cable sensing' devices can't use magnetic field sensing because the current carried by a mains cable's 'Live' and 'Neutral wires' are 'balanced' i.e. equal currents flow in opposite directions; their magnetic fields effectively 'cancel' reducing the net magnetic field strength around the cable to undetectable levels. Expensive professional cable-finding devices 'of last resort' can use magnetic field sensing to sense a cable's magnetic field but require elaborate test arrangements to drive (usually HF) current safely through the cable wire(s) only in one direction.
 

sparky 1

Joined Nov 3, 2018
544
If you bring up the net magnetic field strength (cancels) by destructive interference you must also add the constructive interference. That is why it is called net by definition. A wiring assessment should includes Phase and wire code taught is sufficient in this area.

It is a flawed hypothetical narrative. Can a simple detector for house wiring be made from a BC547 transistor capable of 10 cm depth. The narrative goes on to complicated cable measurement systems making assertions that AC house wiring is not understood by electrical engineers. That HF measurement is somehow related, that hall effect sensor do not detect because the magnetic field is too weak. That AC magnetic field is not called B field. It is not factual narrative on house wire detection regardless of how careful it is read. The theories presented are confused with other disciplines. The assumption that house wire detection and completely different study must meet a unification is beyond the scope of principals being applied. It is easy to redirect
resentful episodes look at the riots. Keeping our head straight is top priority.

The AM radio does have merit on the appropriate subject. It can also be made accurate in both distance : volume and location : position of the loop stick.
 
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