[PLUG] I need a router...

[Keith Lofstrom]

Keith Lofstrom wrote:
> WRT's - good units.  Shit power supplies. 

On Sat, Feb 18, 2006 at 12:51:12AM -0800, AthlonRob wrote:
> 1)  The cause of the instability in these things is power related - are
> the wall warts only giving crappy power when the line voltage drops or
> other such issues with the line?

I'm not sure whether they would always work with "good clean power",
but there is no such thing coming from real power wires.  The worst
things that sag power lines are induction motors, such as are found
in refrigerators and furnace blowers.  Move to a neighborhood without
motors and winds and you are fine ...

(we just had a 0.2 second or so power glitch that shut down some
equipment while writing this).

Beyond that, if the connections get resistive due to corrosion, the
edgy nature of the devices means that what is adequate voltage during
install may be inadequate during use.  In "mechanical engineer" terms,
it is like designing in a material with wide variance and a safety
factor of 1.5 .  When the same 10 cents can buy either more reliability,
or a million more transistors worth of function, what would you expect
a manufacturer to chose?

> 2)  Will providing more conditioned power to the wall warts cause them
> to be reliable enough for stable operation?

There are items called "constant voltage transformers" that will take
care of brownouts (voltage sags) but not short blackouts.  It is all
a matter of energy storage, and that is why the AC adapters for laptops
are superior.  These typically store about half a second of power for
a 50 watt laptop, which means about 4 seconds of power for a 6 watt WRT.

Measuring these glitches is difficult, so I am only doing some
educated speculation.  However, things like a WRT will typically
have some circuit that does "power on reset" and puts the unit into
a known state if the power to the unit ramps from zero to usable in
a fraction of a second - the sort of thing you get when you plug it
in or close a switch to it.  If the power dribbles up slowly, or if
if goes down to "not enough to function right" but not "low enough
to trip the power on reset", then you can end up with a circuit in
an indeterminate (and nonfunctional) state.

> 3)  Will my el-cheapo UPS provide conditioned-enough power to keep a
> wall wart happy for longer?

No.  Cheapo UPSes do something called "failover";  if the voltage
drops below a certain level they throw a big relay and turn on the
inverter from the batteries.  A short glitch or voltage sag in the
line passes right through.  Again, there is not enough "energy bucket"
in the WRT to ride through even a short glitch, and the power provided
by the wallwarts is only marginally adequate under good conditions.

> 4)  If not, is there anything I can purchase that *would* be enough to
> keep Wally the Wart happy?

You can purchase used AC adapters for Thinkpads off Ebay for like
$10.  If you scour the surplus stores (I saw something that would
work at Surplus Gizmos on Wednesday) you can probably get something
for $5.  I imagine Free Geek has a wad of unmated laptop AC adapters
that produce >10V (adequate for a WRT).

A non-cheapo continuous-regulation UPS would keep the P.O.S. wallwart
happy.  That would cost hundreds of dollars.  These always run the
input power through a battery charger, and back out an inverter, 
running continuously.  They are energy-inefficent, but very reliable.
You can tell you have one of these because it is always humming and
warm, and you shelled out a lot of money for it.

You need an energy bucket somewhere.  A laptop AC adapter is the
easiest form.  A 12V battery and a charger is another form (not a
car-connected battery - huge voltage spikes!).    Wiring a big
25V capacitor onto the wire from the transformer to the WRT would
be another form.  Or buying a more expensive router with a very
well designed power-on-reset circuit (that always engages when the
voltage is inadequate for function) would be yet another way to
accomplish the task.

As we move into winter, and the furnace blowers engage more often,
sagging the power in the building, we can expect more system
failures.  I like laptop AC adapters to deal with this, because
they are plentiful and cheap on the used market, and store enough
energy to survive a short sag.  I like laptops as routers because
I want my routes to survive 5 minute power outages.  I like UPSes
not so much, because you don't find out the batteries are dead
until the power goes out (unlike Martha Stewart, I don't test my
smoke alarms and UPSes monthly).

Powerwise, it is a jungle out there.  Looking at wall voltage with
an oscilloscope dispels any notion that you are buying a sine wave
from the power company - it looks more like a dog that lost a fight.
It costs money to design a power supply that can always work with
such crap coming in, and you will not find such power supplies in
inexpensive consumer-grade gear;  lamentably, you will not always
find good power supplies in very expensive stuff.  The worst power
supplies *catch fire* and *emit flames* when they are mistreated.

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