Dodgy voltage detector

[tabbypurr]

, well known, and I described it upthread. Nothing you've done has even beg un to contradict what I said.

Capacitive 'non contact' volt detectors measure the voltage between their d etecting tip and the field around them. When a human holds the thing, the h uman's stray capacitance keeps them not far from 0v, usually. In which case the detector works fine.

But if the human is off the floor and say leaning on a mains wire, or a wal l with a wire under the surface, human can be at higher voltage. If human v oltage goes high enough, they are near live voltage, albeit at minimal curr ent via stray capacitance. In this situation the capacitive meter will give inverted readings, live gets reported as ground and ground as live.

Wrapping yourself with a live wire raises your voltage, but to what extent we don't know, as we don't know human to wire capacitance or human to surro undings capacitance.

NT

[Andy Burns]

Quite likely to be so called "100 volt line" speaker wiring ...

each speaker has a transformer to tap-off its signal.

[Max Demian]

Sometimes "extension loudspeaker" sockets in valve radios had a high impedance output - i.e. connected to the primary of the output transformer, so one end would be at HT potential to earth - 250V typically. Moving coil extension speakers would have a transformer, perhaps offering high or low impedance with different sockets. I assume this was for the benefit of horn speakers, which may have been high impedance.

[tabbypurr]

le, well known, and I described it upthread. Nothing you've done has even b egun to contradict what I said.

detecting tip and the field around them. When a human holds the thing, the human's stray capacitance keeps them not far from 0v, usually. In which ca se the detector works fine.

all with a wire under the surface, human can be at higher voltage. If human voltage goes high enough, they are near live voltage, albeit at minimal cu rrent via stray capacitance. In this situation the capacitive meter will gi ve inverted readings, live gets reported as ground and ground as live.

t we don't know, as we don't know human to wire capacitance or human to sur roundings capacitance.

cates live all the time)

Do that then unwrap it bit by bit until it just stops indicating live. Then when you approach it with a neutral wire it should indicate live. Approach it with a live wire and see what it thinks too.

NT

[Bill Wright]

Typos

"resister"

'operators' should be 'operator'.

'matt' should be 'mat'

"Now one suggestion was that it might be possible to invert the test result, so its possible..." should be 'it's'.

Interesting stuff!

Bill

[Terry Casey]

It wouldn't matter as, in this case, the speaker wiring was disconnected from the amplifier so completely floating.

We were probably disconnecting the amplifier for repair or possiblyy reconnecting it after repair but it is now a very, very long time ago ...

[John Rumm]

Yup sorry, was lashed together in a hurry!

ok fixed em...

[John Rumm]

ok just tried that...

If I lay a live wire on the bench, then place the handle end on top of it (half a turn if you like), then it indicates live when in free space.

Now if I place a hand close enough to the handle end, it will then stop indicating live. You can adjust the hand spacing so that you are right on the threshold of detection. If at that point I then offer it a different live wire to its probe end, then it still indicates live when in proximity. It does seem remarkably hard to fool.

[Robin]

Your experiments remind me of - and replicate - the behaviour of a gold leaf electroscope[1] and the way a pre-charged leaf would fall if an opposite charged rod approached but then rise again as it moved even closer.

But I'd caution the OP not to expect such sensitivity with a cheap LAP. Mine performs best when held. And when held gives a pretty good indication of the presence or absence of the potential (sic) touch voltage. And that's what I want in most circs. Eg if I am about to open metal conduit I don't actually care if it is live relative to earth; I want to know if it is live relative to _me_.

[1] probably too old-fashioned a device for youngsters like you and Tim with your "modern maths" :)

[John Rumm]

ISTR our physics lab at school was the proud owner of one... Did some experiments with it including demonstration of the photo electric effect ;-)

[Rob Morley]

...

We did "new maths" and old science. Lots of mahogany and brass in the physics labs. :-)

[Max Demian]

I assume there were Leclanché and Daniel cells on a high shelf.

[tabbypurr]

You're not going to get the thing at live potential by sitting one end on a wire. It always has capacitances to anything & everything around it, the live wire is closer but also far less area than everything else.

I don't know where mine is so can't experiment, but I know exactly how it works.

NT

[John Rumm]

It gets it "live enough" to cause it to indicate live.

The upshot with mine[1] is that if you get the handle end into a field strength near enough to that of a live wire, then it indicates live. If the handle field strength is less than "live", then when the tip encounters live it will also indicate live.

I think that represents the most favourable behaviour since if used correctly[2] it won't tell you something is safe when its not, and will always tell you something is dangerous when it is. The only area for error seem to be the possibility of either not being able to get a reading at all (because it permanently indicates live), or getting false positive from a neutral / earth (i.e. telling you something is dangerous when its not)

(One thing I have not tested, is what happens when you want to detect a live at the tip that is at a lower potential than that of the handle (mine covers 90V - 1000V, so you could for example want to test 110V site wiring while coupled to a 240V field). My guess would be based on the artefact observed from the experiments of it seeing N & E as live when its in an elevated field, is it would probably indicate live in this case as well)

[1] Keep in mind these observations are all based on one particular model of one makers range - others may be different. [2] i.e. Test on known live before and after your actual test - preferably in the same location.