Hard to say what she meant, English wasn't her first and might not
have been her second language.
I took it meant that some power supplies had an input of 60 volts
which I found confusing but after the long time I had been online, I
really didn't want to pursue it.
It could be that whomever she went to for an answer just made it up.
Wouldn't be a first time for that.
The units would be watts, a power rating. A 125W brick would
presumably be physically bigger than a 65W brick. State of the
art power conversion equipment has a "watts per cubic inch limit",
as to what is physically possible. The watts is what makes it warm.
As Gene says, the DC voltage flowing from the adapter, could be
the same as the highest regulated voltage inside the computer.
Some mini-ITX motherboards work that way. But on a laptop,
they use a voltage which is higher than the highest regulated
voltage needed, which means the laptop takes in 18-20V, but
uses it to charge say, a 14.4V battery pack or provide 12V to a VCore
circuit or whatever. There's a whole chain of "voltage-handling"
in the box.
The advantage of using a higher DC voltage, is that reduces the
current flow. Say you want a 65W adapter. You arbitrarily set
the intermediate (adapter cable) voltage to 18V. 65W/18V = 3.6 amps.
The wire of the adapter cable, has to have a wire gauge selected,
to carry that level of current flow. The barrel connector that
plugs into the laptop, needs that 3.6 amp or more rating too.
The trade-offs are, a higher voltage means fewer amps, but a higher
voltage eventually becomes a safety concern. There are standards
that define what voltages are "safe" for consumer goods.
http://en.wikipedia.org/wiki/SELV#Separated_or_safety_extra-low_voltage_.28SELV.29
Bumping up the watts, bumps up the amperes, makes the barrel
connector potentially larger. That is, if you wanted a
common voltage for both adapters, so they were interchangeable.
A 65W adapter could be 18V at 3.6A, while a 125W compatible
adapter could be 18V at 6.9A. Being the same voltage, you
could interchange the two adapters (connector willing).
The amps rating, is a maximum amps, and at any given time,
the laptop could draw 0..3.6A, any value between those limits.
I get the impression, some battery charging circuits use
linear regulation techniques. And something in there can
overheat, if you use a slightly different adapter voltage.
It doesn't have to work that way. But if the laptop designer
knows the provided wall adapter outputs between 17.5V and 18.5V,
the designer could look at that range of numbers, and decide
whether a linear or an SMPS technique was needed for
conversion, to charge the battery. And in the interest of
shaving a few dollars off product cost, could in the scheme
of things, limit the adapter to a small range of voltages.
With SMPS, you could arrange the computer to run off a wide range,
like 12V to 30V, without smoking anything. If, on the other hand,
they take advantage of the well-regulated adapter, they can give
the laptop a strict 17.5 to 18.5V input voltage range.
Then, it's on the owners head, if they buy a 20V Ebay adapter,
and the motherboard fries a month later.
So when you buy an adapter, it may be difficult to get a
precise spec for the laptop, as to what it was designed
to tolerate. For the engineer who designed the laptop,
they had a 17.5V and 18.5V voltage range to work with
(the provided adapter), and that's all they really cared
about. They didn't worry about "can I plug that into
the marine battery system on my cabin cruiser". The laptop
just has to work with the provided adapter.
Paul
