On Mon, 6 Jan 2014 19:39:51 +0000, Ian Jackson
Post by Ian Jackson Post by email@example.com
On Sun, 5 Jan 2014 23:15:14 +0000, Ian Jackson
Post by Ian Jackson Post by tedjrr
Assuming that the local exchange or cabinet DSLAM is still powered,
then the best approach would be to use axillary power to maintain your
router. Many domestic UPS units are low cost, and will provide a
relative period of power if the router s the only load. The
alternative is to use a car battery to power an inverter.
My router runs off a 12V, 1A wall wart PSU - and I expect many also do.
If so, then skip the inverter.
For many just running a lead in from the car parked outside plugged
into the 12v cigar lighter socket would be an option. With a simple
plug it not be a good idea to run the engine unless you know your
12volt equipment will take the higher charging voltage but stablised
power supplies to let you do this are easily available.
I doubt if the wall wart PSU for my Linksys router is stabilised. The
label simply says 12V, 1000mA. It isn't switchmode, and has a chunky
transformer. I wouldn't be surprised if the rest of the circuit is
simply a fullwave bridge and a reservoir capacitor. I expect that I
could run the router off a small floating 12V (13.4V) battery which is
being constantly trickle-charged at a rate sufficient to ensure it IS
actually slightly charging, and not being flattened by the router.
Such kit (routers, ethernet swicthes and the like) have been using
switching regulators on the DC input jacks for better than a decade
now. Long gone are the analogue regulators of yesteryear.
It's only recently that the classic mains transformer based wallwarts
used to provide unregulated DC (or even simply the low voltage AC
transformer output voltage to feed the bridge rectifier integrated
into the swicting regulator in some kit - allowing such kit to also
use unregulated DC as well as a nominal 6 to 12v AC) have started to
be usurped by the more efficient and lighter SMPSU type.
A switch or router or ADSL/Cable modem router can usually be safely
powered direct from a 12v lead acid battery because of the use of
switching regulator technology.
Even a car battery being float charged by a running engine will be
well below the limiting maximum input voltage rating of the input
capacitor (16 or 25v being typical). However, I'd be a little bit
leary of testing at voltages higher than 15 volt, even when a 25v
capacitor is used on an input labelled simply to match that of the
nominal output voltage of the supplied wallwart.
It's quite easy to verify whether a switching regulator is being used
by measuring current consumption from a DC test supply initially set
to match the labelled input voltage and observe whether the current
increases as you lower the supply voltage (and vice versa as you
If it draws more current on a lower voltage, it's the expected
switching regulator otherwise if it shows an almost constant current
draw with a very slight positive slope, it'll be an antiquated
You might be surprised that the move from a cheap 'n'cheerful
analogue regulator solution to the, at the time, more expensive
switching technology solution was driven by economical considerations.
The big problem was that analogue regulators were likely to dissipate
as much energy as they were supplying to the 'load' (in some cases,
more) which necessitated a decent sized heatsink.
The usual solution was to use the metal case itself as a heatsink.
Unfortunately, metal cases only made sense in the days before mass
market demands transformed this kit into 'cheap commodity' consumer
items where the need to eliminate the expense of a metal case became a
The manufacturers could make considerable cost savings if they could
eliminate the waste energy (heat) issue of the analogue voltage
regultor since they could then make the case out of cheap plastic
injection mouldings. Not only that, cheaper wallwarts with only 60% or
so of the VA ratings of the earlier analogue regulated kit would
provide even further savings (and had the added bonus of "being a
greener product"). It was a "Win win" situation all round for
Not every bit of kit is "Car Battery Safe" but such examples are few
and far between. A big clue to this 'danger' is if the input voltage
is marked as 6 or 7.5 volt. Most kit with a 10 to 15v input rating
should be perfectly ok with car battery power. Even the kit marked
with 6 or 7.5 volt ratings might be just as fine (just as long as the
input capacitor voltage rating isn't marked as 10v or less).
Regards, J B Good