This is only a preview of the October 1999 issue of Silicon Chip. You can view 37 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Backup Battery For Cordless Phones":
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2 Silicon Chip
DC power in the home;
would it make sense?
If you refer to the battery backup project for
cordless phones in this issue, you will see that I
am plagued by blackouts in my home. That this
should be the case in 1999 in an old established
Sydney suburb is pathetic but there you are.
But it got me thinking about ways to avoid
the problem, assuming that it will continue into
the future. In my home we can at least continue
to cook during a blackout since we have natural
gas but all other power-assisted activities must
cease for the duration of the power failure. So you
start to think about providing household power
which does not involve the big power items such
as heating and cooking. Leave out refrigerators and washing machines which
use induction motors and you are not talking about huge amount of power to
be provided by batteries.
Let’s say we decided we were going to limit the number of lights to several
hundred watts and then we are left with equipment such as VCRs, TVs, computers and a host of plugpack-powered devices which run from low voltage DC.
Perhaps we are only talking about five to six hundred watts or so.
Question is: what sort of inverter would you use? Most people would think
of sinewave or modified sinewave inverters but perhaps they are not needed.
Why not just have a 12V to 250DC inverter? It would be much simpler and more
efficient than an AC inverter.
Let’s face it: most computers, PC monitors and TV sets will run from 250V DC
without any need for modifications. The DC would feed via the existing bridge
rectifier straight into the switchmode power circuitry. And incandescent lamps
and compact fluorescent lamps will happily run from 250V DC as well. That
leaves just the plugpack-powered devices (heaps of them, it seems) running at
12V or so. But why would you run them from an inverter? Why not run them
at 12V DC?
So the solution would be to run the TV, computer and lights at 250V DC and
the lower powered devices at 12V DC, straight off the same battery bank that
supplies the inverter. In effect, you would get rid of all plugpacks and run from
12V DC permanently. You would have an automatic contactor in your switchboard to switch from 240VAC mains power to the inverter’s 250V DC and you
would run a 12V DC supply around the home for all the low voltage gear. There
would be a small bonus too because you would eliminate the inefficiency of
running quite a few separate plugpacks.
Mind you, current audio equipment, VCRs and conventional fluorescent lights
will not run on DC so an AC inverter would still be required but there is no reason
why future models with switchmode supplies could not run on 250V DC as well.
What would be the cost of all this, to eliminate blackout-induced frustration?
By the time you added up the cost of deep cycle batteries, a big inverter and all
the electric wiring modifications that would be necessary, you could be up for
$4000 or more, so it is hardly practical just to stop the occasional blackout is it?
But a similar problem is faced by many people in remote locations who don’t
have the luxury of mains power. They probably think in terms of solar panels,
big batteries, DC to AC inverters and so on. Maybe the DC approach, low voltage
and high voltage, is a better way to go.
In some ways it would be a reversion to the early days of electricity before
AC power transmission became universal. It is an intriguing thought, even if it
doesn’t solve the problem of nuisance blackouts.
Leo Simpson
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