Metalclad Laptop Tingle

[polygonum]

Partner has a MacBook Air. And the other evening she realised that if yo= u =

ever-so-gently brush your hand across the case, there is a sort of very = =

mild electric tingle. Although it varied in strength a bit, it did seem = to =

be present wherever you touch the case, with it folded up or open, on th= e =

inside around the screen or keyboard, anywhere.

A bit of surfing later found that this is a widespread phenomenon across= =

many metal-clad lappies of various makes. Eventually even found a soluti= on =

posted =E2=80=93 use the long mains lead. (For those who, like me, don=E2= =80=99t have a =

Mac, their power supplies have the choice of a slide in duckhead which =

allows the lump to plug straight into a 13A socket. Or a longer cable =E2= =80=93 =

more like a kettle lead =E2=80=93 which allows the lump to be a metre or= two away =

from the 13A socket.)

Sceptical, I could not think why that would work and assumed it was =

mis-guided advice. Until tried. It worked. The tingle dropped away to th= e =

level where you are not sure if it has totally disappeared or not.

The longer mains lead is also made of a peculiar material =E2=80=93 it l= ooks like =

normal flex but is strangely less flexible and doesn=E2=80=99t even feel= like =

normal PVC outer insulation. In some ways it is a bit like some car HT =

leads! So I start thinking that this special lead has some magic =

properties that somehow dissipate whatever is causing the tingle.

Partner then tried a much older lead =E2=80=93 and this was most definit= ely like =

any other mains flex. So I was surprised when that too reduced the tingl= e =

to nothing.

Trying the same on a MacBook Pro had exactly the same effect =E2=80=93 e= xcept that =

the tingle was much less noticeable in the first place.

So, to the questions. What causes the tingle and why does using the long= =

mains lead stop it happening? And why is the new mains lead so peculiar?=

[John Rumm]

Yup, common problem...

Was the mains lead three core?

[polygonum]

the lump to be a

I got the impression it was pretty common. And it is not even that big a= =

deal.

The plugs have metal earth prongs - but where the lead/duckhead connects= =

to the lump there are only two connections - looks like a fairly standar= d =

figure-of-8 plug/socket. I doubt the mains lead has three cores but cann= ot =

tell for sure as both ends are moulded on.

[The Natural Philosopher]

Its probably the other way about. The mains lead DOES have an earth, but the plugtop does not.

Now although te mains is isolated, its not QUITE isolated - typically there will be RFI capacitors from live and neutral to the case - which is not earthed except when using the extender. So there is a high impedance 115v on the case. Not enough to be dangerous, unless the live cap goes short circuit. But enough to blow a serial port in at least one case to my certain knowledge.

Frankly I think its bloody dangerous and would disallow all metal cased equipment from running unearthed, but still connected to the mains by at least one component capable of going short circuit.

Macs are all s**te anyway.

[Nick Odell]

The Wikipedia describes the MagSafe connector pinout as

Ground > V+ > Control > V+ > Ground

...which surprised me. Have Wikipedia have got it wrong (not unusual) and have written Ground when they mean 0v or somesuch? Or is the Mac grounded rather than isolated?

Out of interest I've never noticed anything anyway but I've just gone over my metal-cased not-a-Mac and satisfied myself that the PSU is completely isolated and that there are no voltages on the case trying to seek an earth. (Digital and Analogue meters both used to account for impedances)

Out of further interest, I'll take my meters round to my son's place this weekend when I'm dog-sitting there: he's got a MacBook Pro which, again, I've never noticed anything unusual about, but it woudl be interesting to check it out.

Nick

[polygonum]

ows the lump to be =

Well there are definitely only two metallic connections on the lead end = or =

the duckhead.

[polygonum]

Sorry but I don't get how there is 115v on the case! The wire to the magsafe adaptor is most definitely low voltage. And the entire lump-to-thin-low-voltage-wire-to-magsafe is idneitcal whether connected to duckhead or longer "extension" lead.

(Or should that be extra low voltage?)

[The Natural Philosopher]

OK its this away. Maybe.

I am saying that one of the 'low voltage' wires connects to the case. possibly by a capacitor, possibly directly.

Now if that wire and the whole LV power supply is ALSO connected at one side to mains earth by a proper cable then thats fine and dandy. BUT there is no reason why if its NOT connected to earth the whole computer and its low voltage supply cant be, like a bird sitting on a 33KV cable, at any damned voltage with respect to earth that whatever cockeyd design apple have done, puts it. Do you see?

And, furthermore, IF they have some mains filtering either in the duck-head or wherever the mains stops that connects the standard twin RFI capacitors between live and LV 'earth', and neutral and LV 'earth' so as to involve the case as a screen, that puts LV earth plumb halfway at 115V unless LV earth is *actually connected to mains earth*.

My suggestion is that this is exactly what they HAvE done, and the duckhead doesn't actually earth a damned thing - either because its designed that way or because its faulty.

As I said 115v was exactly what I measured on metal surround of an RS232 port that blew up another port when connected to it, precisely because the 'two wire' power supply had a mains filter going to the computers metal frame which wasn't earthed, there only being two wires.

[Brian Gaff]

The effect is electrostatic and is there because of the alternating current in the first place. You get it on hi fi as well, if no earth is present. In them olden days theere used to be an earthed screen to stop it, but now... as for the leads affecting it, no idea, but presumably the further away the less likely, as its a high impedence effect. Brian

[andrew]

No - there are 3. The metal tab that the lead locates on is the third. On t= he long lead, this is connected to the earth pin in the plug, but the duckh= ead doesn't have the connection. If you look in the groove that goes over t= he tab, the long lead is metal lined, the duckhead isn't. There have been v= ersions of the duckhead that had the earth connection, but they are not ver= y common. A

[John Rumm]

Apparently, another spectacular "form over function" gottcha is that the stud connection on the PSU often comes with a thin plastic cover stuck on it, to keep it looking nice and new and shiny when out of the box. If you don't peel this off, it can end up insulating the earth connection!

[whisky-dave]

I get a very similar thing on my iMac, usually when dragging my feet across= the carpet on touching the metal keyboard I get a small 'shock' soemtimes = enough to make me retract my hand quickily.

[John Rumm]

Indeed it will be a low voltage in the sense that there will only be a

0V and a ~16V connection. However all that ensures is that the 16V connection is 16V DC higher than the 0V line. Those could be floating anywhere with respect to earth.

What often happens is that three wire mains comes in, and two wide DC goes out. The -ve or ground of the DC is often tied to the mains earth in the PSU. That is fine until you take away the earth connection from the PSU. At that point the DC output is at best left floating. At worst, you have something like this on the mains input:

(have a careful look at the circuit diagram on the side of the tin)

The filter ensures that noise generated in the PSU (switching designs can generate lots) gets quenched before it gets out of the PSU and back up the mains. However if that earth connection is left unconnected, you can see it is connected between the incoming line voltage between a pair of caps. That will cause it to rise to half mains potential. If the 0V line out of the PSU is in some way connected to that earth, then that is the source of the tingle.

The output caps in that one linked to above appear to be 2200pF, so the reactance at mains frequency will be substantial, at 1 / 2 x pi x f x c = 1.45 Mohms. So the RMS current is limited less than 160 uA

Its identical, but I would expect in one configuration it has an earth connection on the input, and the other its does not.

its under 120V DC, so yes.

[whisky-dave]

They could be but rarely are, don;t forget that actually Earth and Neutral are connected together back at the power station.

A bit like in a boat then or on a plane they don;t have eqarth connections as such. Are you saying Macs or any laptop with a metal case is dangerous when not connected to earth ?

No that's not really what it for it's to stop interenece to things like radios TVs the same things are put on drills and motors.

It could be but the tingle could be static getting back to earth.

Not really life threatening then.

[The Natural Philosopher]

until and unless one of the caps goes 'short', no.

[mathewjamesnewton]

Not just metal cased laptops - I've had exactly the same phenomenon on at least two plastic machines too.

Mathew

[John Rumm]

I think we may be talking at crossed purposes...

I am referring to the 0 and 16V ELV supply that comes out of the power brick on the laptop side of the supply. Its a 16V potential difference between the pins, but that is not a guarantee that there is no potential difference between say that 0V line and mains earth (or local earth for that matter)

Yes. Or the output of a transformer or generator for example.

No its not generally dangerous, because although the voltage present can be relatively high, the maximum current that can be drawn in down in the tens of micro amps (see the example calculation in the previous post). However it can give the slightly "odd" sensation when you touch the metalwork.

[John Rumm]

That is rather different, in both sensation and cause.

What you are experiencing there is an electrostatic discharge. You would get this with any equipment that is connected to earth (or a water pipe or even a largish lump of free standing metal).

Basically you have used friction to accumulate a significant charge on your body (usually at many tens of thousands of volts!) As you approach something with the ability to conduct that charge away, the electric field strength caused by the huge potential difference becomes so strong as to cause the gap between you and it to break down and become conductive. After the "zap" you have eliminated the accumulated static charge and would feel no further sensation.

The mains coupling type of effect the OP described, feels more like a vibration as you stroke your fingers over the metal surface of the device, and it will continue all the time it is powered.

[The Natural Philosopher]

As in :

'I cant get a shock from this 1.5V battery! Its only 1.5V!' 'Oh yes you can!' 'Prove it!' 'Hold one end and touch the other end onto that 11KV cable crossing the field there then'

FTTSSTT!!

[polygonum]

Aha! Could have sworn that inspection of both had revealed no metallic part that could make contact. Now think I must have looked at duckhead, put it down, looked at duckhead again, when that should have been lead end.

Yes - absolutely, there is a hidden connection.

Any idea where to get an earthed duckhead?

And that would make the Wiki description of magsafe almost right - "grounded - if connected via long lead".

Now need an explanation of the odd material the newest lead is made from.

Thank you.