A weird thing with an electrical screwdriver

[adder1969]

Did you read the instructions? The light will only light when you touch the end as you're completing the circuit. Why it lights on all three wires is somehting else though!

[Drake]

I was mystified by something I experienced today. I'm sure any electrician will know the answer. Sorry if its a naive question.

I was using one of those electrical screwdrivers with abuilt-in diode for identifying live wires. I wanted to disconnect a security light on my house but I wasn't sure which circuit it was connected to. I concluded the light was wired into the lighting circuit, because it failed to come on after I pulled the main fuse from that circuit. But just to play safe, I touched the screwdriver onto the terminals in the junction box which the light was connected to. The diode didn't light up. However, when I touched the brass end of the screwdriver's handle, it lit up, quite brightly. So I found my volt meter and tested the wires to see if there was any voltage there. No voltage registered. Can anyone explain why the diode lit up when I touched the brass end? And it lit up the same, whether I touched the screwdriver onto the live, the neutral or the earth terminal.

Regards,

Drake

[Lurch]

On Thu, 10 May 2007 20:30:52 +0100, Drake mused:

Basically, those neon screwdrivers are absolutely crap. Don't ever use it as a means of testing for power, ever. It's as safe as the wet finger test.

[Bob Eager]

Several naive bits actually. Did you read the instructions?

Not a diode - a neon. A fairly massive difference. If it were a diode you'd get a shock!

The NEON wouldn't light without sometjing else to complete the circuit. You're only giving it one contact - the blade.

That's the other contact. It needs only a tiny current - which is just as well. That's what you are meant to do.

The neon is very high resistance and so nothing will show.

As explained above.

These things can be lethal:

1) They fail without warning. Don't ever rely on them if they say there ISN'T a voltage. 2) If they get damaged and short (unlikely, but...) you can get a serious shock.

Use a voltmeter. You had one, so use it!

[Ron Lowe]

Whist I agree with everything you say, modern voltmeters can be misleading too.

Modern DVMs have such a high input impedance that they can find stray voltage on longish runs of cable which is actually of such high source impedance that it will drop to zero volts as soon as any ( even megohm ) load is put on it.

So they can indicate a voltage as present when it's not really. ( Safer than the reverse, of course! )

I trust my old 20Kohm/v AVO-8 for this kind of thing more than my fancy all-singing Fluke DVM.

[mike]

"Ron Lowe" wrote in news:46438294$0$646$ [e-mail]:

AVO-8 even....

Respect ;)

mike

[Bob Eager]

I agree...good point. I was rather assuming a cheap low impedance one...!

Indeed. I use a cheap one I got from Maplin many years ago. Friend of mine has an 8, but then he had a summer job at Avo for years!

[Drake]

No, I've never read the instructions. I don't think it came with any.

Not actually true with this particular screwdriver. If you touch it onto a genuinely live contact, the red light lights up brightly whether you have your finger on the end or not.

Yes, I hope someone here can offer a possible explanation.

Drake

[Bob Eager]

It doesn't require much leakage for the neon to light.

Leakage again...voltmeter is more reliable (but note that a cheap one is probably better here).

[Drake]

Funnily enough, as I mentioned to adder, it does light up if you touch it on a genuinely live conductor - even if you don't have a finger on the end!

Anyway, thanks to all for the input. I'd still like to know why the thing lit up when touched on junction box terminals after the fuse for that circuit was removed, and on all three too (including earth) - when a volt meter registered 0 volts.

Drake

[The Medway Handyman]

What do you think of these?

[Dave]

I had this problem when measuring the flap/slat asymmetry detection voltage on an aircraft I used to work on (it was an AC voltage wave form. 400 Hertz) The modern volt meters used to measure the residual voltages in the wiring. Reverting back to the milliamp hungry meters cured this and I could get a proper reading

Never believe a reading from a modern meter, unless you know what you are doing.

Dave

[Bob Eager]

[John Rumm]

sure I would trust them as a sole means of identifying energised circuits.

[John Rumm]

Its called inductive coupling. The wires probably run some distance in proximity to other wires which are live. Although there is no physical connection between them the alternating magnetic field produced by the current flowing in the live circuit will induce a voltage in the disconnected one. (same principle by which a transformer works)

[Newshound]

Sense at last!

Yes, you need to be careful with neon screwdrivers. A false negative may mean a broken screwdriver but if you check that it is working every time you use it the chance of getting zapped is very low (and if you short every potential live to earth before touching it it is even lower). You can often recognise a false positive caused by induction because they don't light quite so brightly. I prefer my cheapo "electronic" screwdrive which buzzes on a live because it will do this through insulation, but I still use neons. And I always check that it's working before trusting it.

[John Rumm]

I gave up using my one after confirming that I had pulled the correct fuse with the thing, and then did the live to earth check "just to be sure". The big blue flash and the vaporising of the end of the screwdriver was enough to convince me never to trust one again! (the lack of a blade also rendered that one less useful as well)

[Dave Plowman (News)]

It's a bit big for simple volt measurements like this, though. Pretty well any analogue meter will do - or simply fit a shunt to the Fluke. My Model

8 stays on the shelf these days - too expensive for general use outside the workshop.

For general 'are there volts present and how many' type uses, this is my device of choice:-

avoids having a high input impedance where it's not needed.

[Dave Plowman (News)]

If it's a red light it's likely one of those volt stick devices which is active in operation. It has a small amp built in. With these you really do need to read the instructions.

[Drake]

Ah, yes. I know for a fact that the cable leading to that circuit does run close to other cables for some distance. Thanks for the education!

Drake