Re: Any power supply experts?

Duncan Kennedy <nospam@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

I'm very please with my very new 13" MBP (the all aluminium one) but I
found a strange phenomenon. When using the power supply with the little
3-pin plug part slotted directly in, I get a slight "burrr" (OK I'm a
Scot so that illustrates the effect best) when passing the back of my
hand over the case, just touching. If I swap to the supplied extension
cable that burr has gone.

It sounds as though there is some capacitance between one wire of the
mains and the aluminium of the casing. This could be stray capacitance
or it could be a capacitor deliberately placed there to earth the
aluminium at radio frequencies, so that it acts as a screen to prevent
any radio interference inside the power supply from getting out.

Inevitably a deliberate capacitor passes a small amount of current from
the mains, but the capacity is very tightly specified so that this
current is harmless. The capacitor is often made up of two units in
series, so that a single failure cannot create a dangerous situation.

With a three-core mains cable, I would expect the aluminium to be
connected to earth, with no extra capacitor, so the current is taken
directy to earth. It is possible that equipment which could finish up
being used on non-earthed circuits might not have this connection (even
though it has a three-pin connector) and would be fitted with a
deliberate capacitor.

Most likely the two effects you describe are caused by a difference
between the two leads:

a) The 'tingly' one has a missing, broken or corroded earth
connection.

b) There is no earth connection inside the power supply and the
polarity between the two leads is different, in which case the
capacitor(s) may go to the live wire when using the 'tingly' cable and
the neutral with the other one.


Years ago, some of the 'pizza-box' Macs had mains connectors with short
pins which would only just touch if they were rammed fully home. I
haven't seen that mistake on any recent models.



--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
.

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