Re: 12v ring main

[Nigel Orr <nigel@xxxxxxx>]

At 14:08 18/10/00 +0100, you wrote:
>Your suggestion of a 12v ring main sounds like a good idea
>and I might well drop some speaker cable down with my CAT5
>to allow for such a requirement!

It is a good idea, in some ways, but you should be aware of some
limitations.
1) Voltage drops in cables are comparatively worse at 12V than 240V, (to
transfer the same amount of power, the power lost in the cable goes up by a
factor of 400!) so it is less efficient (not a big deal at low power) and
may not work with some equipment (which needs the supply to be close to
12V)
2) Some equipment relies on having it's own isolated power supply, and will
get upset if it is sharing a supply with another piece of equipment that it
is connected to (that's a non-technical explanation, if you want more
detail on the actual problems that are likely, just ask!).  That mostly
applies to things like Ethernet hubs, but may cause problems with cameras
etc.
3) You may find that equipment suffers from more interference, and might
not work at all, due to the longer cables being more likely to pick up
noise.

On the plus side, as has been noted;
1) It's safer than mains to work with
2) You can easily put on a battery for backup if the power fails.

With all that in mind, and as you have the chance to run cables (did I
mention you can never have too much copper in the walls? :-)  ), there's
lots of reasons to run a couple of twin and earth mains-type cables on each
run too.  Preferably keep them 2ft away from other signal cables, then you
can use them for mains _or_ 12V as appropriate (or even ditch the idea and
use them for speakers!) and not have much worry about interference.

Separate 'spurs' to each point from a central supply(the mains equivalent
is a radial circuit) would be better than a 'ring' covering several points,
to minimise the current in each individual wire and so reduce
losses.  2.5mm^2 mains twin and earth is probably the best thing to use,
ideally pick a different colour from the rest of the house wiring (usually
only available in grey or white) to help work out what's what later on!

Nigel (another Nigel...)


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