[PLUG-TALK] Wall warts, Laptop AC adapters, and efficiency

[Keith Lofstrom]

Don't throw away working laptop AC adapters!  

If you have an old laptop to be recycled, chances are it has an
AC adapter with a smart, efficient switching regulator in it.  These
are a safe and efficient way to convert line voltage to well regulated
DC power.

The wall warts AKA power bricks that come with most consumer
electronics are dumb and inefficient (but cheap).  They typically
contain a highly resistive transformer and some diodes.  The
resistance is on purpose - if the output of the wall wart is shorted,
the resistance limits the current and keeps the wall wart from catching
fire.  However, the resistance also makes the device inefficient.  The
overall combination of wall wart and device is quite intolerant of 
power line upsets, brownouts, etc.  They also draw around 3 watts with
no load, and run at less than 50% efficiency.  But, they are cheap.

The switching regulator in a laptop adapter contains sophisticated
electronics that converts the power from high voltage AC to low 
voltage DC much more efficiently.  It deals with shorts by sensing
and limiting current, so it actually runs cooler when shorted.  With
no load, it draws a fraction of a watt.  It runs at 80% efficiency
or better.  It can tolerate wide line voltage variations (often
100 to 240 volts), and can ride out power glitches that will cause a
wall-wart-powered device to reset or hang.  A switching adapter can
save a few dollars a year in electricity costs compared to a wall wart.

It is possible in many cases to find a used laptop AC adapter that
can provide the same voltage as the wall wart currently powering a
device (sometimes the power plug needs to be re-wired).  Since wall
warts produce widely-varying unregulated voltage, it is often possible
to replace a "12 volt" wall wart with AC adapters between 8V and 16V.
For example, my Linksys WRT54G access point uses a wall wart rated
for 12V at 1 amp, but can tolerate voltage between 7V and 22V.  I
can power it from an IBM Thinkpad AC adapter (with matching barrel
plug) that produces a regulated 16 volts.

So, don't throw away those efficient AC adapters!  It may be possible
to set up a repository of them at a place like Free Geek, and develop
a chart showing what consumer devices they might be compatable with,
and send them back out into the world to be reused, where they can
save a little power and a little hassle for others.

Keith

-- 
Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs