On 02/07/2012 12:39, Jim K wrote:
Mate's Dell Laptop (oldish) has flat batt and won;t boot.

Plug in to mains charger - won;t charge - batt light on lappy "winks"
once per insertion of the jack, then nought.

Lappy won't boot either when plugged in to the charger...?

Had batt out & makes no difference except the batt light "wink"
disappears when you shove the jack in.

Got charger here now, plugged in, green LED on tranny box is lit,
multimeter on the DC jack end - measuring 0.01V - it's rated on the
tranny label at 19.5v 3.3A....

These days they are almost always switched mode PSUs.

Seems something not right - but :-

do laptop chargers "sense" and adjust their output somehow?? meaning I
can't actually say 100% it's the charger that's shagged & just acquire
a new one...?

All help appreciated

They can and do vary. Some will always provide nominal voltage at the
plug even under no load and others require a certain minimum load and no
short circuits before they will drive the output high.

A sort of self protection against being left unplugged on an office desk
full of loose paperclips and staples.

A 22R 10W resistor (80p from Maplins seriously overrun so getting hot
quickly) or sacrificial automotive 12v lamp in series with multimeter on
the 10A range should give you a quick go/no-go indication.

--
Regards,
Martin Brown