power supply

[Jim March]

On Mon, Oct 19, 2009 at 8:03 AM, mike havens <bmike1 at gmail.com> wrote:
> so the prick is the power supply? Thanks for the help.... I am so thankful I
> found you guys. You all are so helpful and friendly.

Well mostly :).  OK, here's how power management really works in a laptop.

The "brick" turns AC wall power into DC.  I just picked up a Dell
brick off my floor, it's putting out 19.5v DC.  That's the main power
supply, what most people just call "the power supply".  Like most of
these bricks, my Dell brick can take in foreign-spec AC (220v at 50
cycles) in addition to US-spec.

Once DC gets into the laptop, a much much smaller DC-to-DC power
supply turns it into 12v, 5v and usually 3.3v or so.  Sometimes 2.8v.
It's very, VERY uncommon for that part to break, because it's not
under as much stress.  The reason the main power supply (AC-to-DC
"brick") is external at all is because it heats up from the amount of
work it does...and because they blow up a lot, the makers want to be
able to quickly swap them with no screwdriver involved.  By the time
power gets all the way into the laptop past the brick, a lot of the
"heavy lifting" power conversion is already done by the brick.  The
laptop is being spoon-fed something very easy to digest.

If the DC-to-DC internal power supply blows, you're screwed - it's
part of the motherboard most of the time.  But you're also having an
astonishingly unlucky day if that happens, I've never seen it myself.

There's one more circuit involved: the "battery charge controller".
This takes DC in and spoon-feeds it in and out of the batteries,
detecting when the batteries are full and chopping power when they
are.  Good ones slowly back off the power as it gets close to full.
That circuit is built into the battery pack itself, so swap batteries
and you swap that.  The laptop can run without it, and if that charge
controller goes bonkers it can cause problems such as you're having.
Hence as a test run it without the battery, eliminate that as an
issue.

The reason you have the charge controller in the battery is to allow
different capacity battery packs.  You can order three different
grades of battery off of Dell for example for my laptop, and each will
have different charge controller settings for their respective
internal battery arrays.

That's how laptop power works.

Desktop power supplies are simpler: they take AC in, put out the
various DC types the system needs.  Even then most motherboards will
have a small DC-to-DC power supply on board to feed very clean and
precise stuff to the CPU and in many cases vary the CPU voltage under
software control.

Jim