[Ewrt-devel] [Fwd: [ptp-general] Powering Linksys WAPs]

Irving Popovetsky
Fri May 7 13:13:21 PDT 2004


For the hardware geeks here.  Keith did some cool hardware hacking at
PLUG last night.  Here are his results:

-----Forwarded Message-----
From: Keith Lofstrom <keithl at kl-ic.com>
To: general at lists.personaltelco.net
Subject: [ptp-general] Powering Linksys WAPs
Date: Fri, 07 May 2004 10:19:57 -0700


Technical stuff:  skip down to "What does this all mean" if you
are an "eat dessert first" kinda person.  Exciting possibilities!


The Linksys BEFW11S4 V4 802.11B WAP/Router, and the WRT54G V2 802.11G
WAP/Router, are designed to be powered off a 12 Volt,  1 Amp, 
center-conductor-positive 5mm diameter power plug.

The power tolerance for these units is quite wide, though.  I did some
work with a regulated bench power supply, and looked at the circuit
inside, and learned that you can power these units off a *regulated*
supply anywhere between 4V and 27V, with a safe area probably between
7V and 22V.  Among other things, this means I can power one of these
WAPs off the AC adapter for my Thinkpad laptop (16V nominal).  It also
means I can power one off my 130Watt-hour NCharge portable battery for
a LONG time ( 20 hours for the WRT54G, 50 hours for the BEFW11S4 ).

Inside the WAPs is a circuit called a "bucking" or step-down regulator,
based on the Anachip AC1501-33, capable of 40V operation.  The input
capacitor is a 25V device (if this was replaced with a 50V device, and
there is plenty of room for it, the WAP could take 40V).  The regulator
is designed to provide a constant 3.3V for internal circuitry, using
power from any external voltage in the above-mentioned range.  This allows
the unit to work with the cheezy 12V/1A wall warts provided by Linksys
(which can ripple between 9V and 16V under load).   In general, wall
warts are pretty sloppy, and most of the numbers below assume something
better controlled.

The BEFW11S4 draws about 2.4 watts idling (with ESSID broadcast) and
2.6 watts transmitting (an FTP download), while the WRT54G draws 5.4
watts idling and 5.6 watts transmitting.  The current drain appears
"bursty", matching the increased power into the transmitter during
packet transmission, as you would expect.  The buck regulator inside
draws enough current at any given voltage level to supply that power,
so an idling BEFW11S4 draws around 400mA at 6V and 100mA at 24V.  This
is called a "negative differential resistance" in electronics-speak,
and can cause instabilities with the wrong sort of power source. 

One typical "wrong power source" that people here might see would be a
Power-Over-Ethernet kludge with too much resistance in the wire.  For
example, if you power the WRT54G unit from a 24V source with 25 ohms
of series resistance, you can theoretically provide the needed 5.6W
as 12V.  However, during startup the input voltage has to move from
0V up to that 12V, and while passing through 3.3V, for example, the
unit might be attempting to draw 1.7A, which is more than the 25 ohm
resistance can provide.  This is probably why some folks have had
better luck with a regulated power source - those have lower internal
resistance added onto the resistance of the interconnect wire.   

My guess is that the power source will need to temporarily provide
2 amps at 3.3V to the WRT54G, and 1 amp at 3.3V to the BEFW11S4,
during startup. So with a 24V source, that is a maximum resistance
(including connectors and eventual corrosion) of 10 ohms for the
WRT54G, and 20 ohms for the BEFW11S4 .  With a 12V source, the
maximum resistances are 4 ohms and 8 ohms.  An RJ45 with "green scum"
on it, copper corrosion pretty common in Oregon, will have higher
resistance.  The Linksys 12V/1A wall warts have an internal resistance
of 2 to 5 ohms that adds to that external resistance.  Yikes!

Well, enough calculation nonsense;  
----------------------------------
What does this all mean?
----------------------------------

First, it means that for POE situations you may be able to use a >15V
laptop AC adapter as a power source.  Even the older ones have
significantly lower internal resistance than Linksys wall warts.  All
the IBM Thinkpads since the 560 have used the same power plug,
compatable with the Linksys units, and all their adapters provide
more than enough power to drive a Linksys.  Prepackaged and ready
to go, and there are plenty on Ebay.

Second, it means that you are compatable with the supplemental
external notebook batteries.  My 130 Watt-hour, 3 pound NCharge
battery from Valence Technologies (www.valence.com, $300 from Fry's
with one adapter) can be connected to different adapters for different
laptop plug/voltage combinations.  So you could get one $25 adapter
that works with your laptop, and if that is not suitable for the
Linksys, you could get a second adapter that is Thinkpad and
Linksys compatable, and interchange adapters as needed.

Third, this makes projects like Sam Churchill's Wifi bike project
pretty darned easy.  Sam could use 2 WRT54G's running the custom
EWRT code, one device configured in client mode and connected to
a high-gain antenna for the uplink, the other configured as a
Personal Telco portal.  A Y splitter could power both units off
the same NCharge battery, and multiple batteries can be connected
in tandem if Sam needed more than 10 hours. 

Sam, if you want to follow this up, you might try contacting Valence
for a donation of a battery or two;  they will be pleased to know
about this new application of their product.  They might appreciate
it even more if you include some JPEGs of photogenic 20-something
females posing with your wifi bike.  :-)

Enough for now.  Forgive the long-winded writeup, but the possibilities
that all you imaginative folks may come up with based on this info
has me really jazzed ...

Keith

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



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