In article ,
"listerofsmeg" says...
Hi all,

I am setting up a CCTV system, which will have about 8 cameras.
Each of these cameras requires a 12V 200mA supply.

After a bit of research, it appears I can have one transformer (say
12V 2A), and take LV power lines off for the cameras in parrallel.
Does this seem right?

As long as the transformer output is rectified, and preferably regulated
too.

Ok, my problem appears to be finding a transformer!
Firstly, they seem to have different power ratings. Some are in amps,
some are in Watts, and some are in "VA"s whatever they are. Anyone
know how to work out one from the other.

Watts = Volts x Amps
VA is like Watts but for an inductive rather than resistive load.

Secondly, everywhere I look seems to have these strange light, white
thin transformers that are primarily used to low voltage lighting. Can
I use one of these to power my cameras? After googling a bit there are
reports that these throw off a lot of RF interference which is no good
to me. Also I will have some pretty long cable runs (20 meters), and I
have read that some have restrictions on cable length.

Pass.

What I was really after was a good old "black lump" transformer that
plugs into a mains socket, but I can't find any of these online at
all! Are they called something different?

Power supply or DC mains adapter rather than transformer, as it includes
a rectifier and possibly a regulator. You'll not easily find a wall wart
that supplies 12V at 2A. An alternative cheap source might be an old PC
- the switch mode PSU in all but the oldest PCs will easily provide a
couple of amps at 12V, so if you can hide it away somewhere that might
be an option to consider. Bear in mind that an SMPS requires a
_minimum_ load in order for it to work properly, so don't just hook up a
couple of cameras, connect them all before powering up. And obviously
get the PSU checked by a competent technician if you're in any doubt
that it's working properly, before you blow all your cameras.
If the cameras are any distance from the PSU you should also probably
use a fairly substantial cable to connect them to reduce any voltage
drop.