On Fri, 08 Oct 2004 13:17:43 +0100, "Dave Plowman (News)" wrote:

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

Note the word 'supply'. But you've missed out DC.

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?

No, you'll need a power supply. The transformer is only part of this.

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.

Volts x Amps gives you Watts. And VA can be treated as the same as watts
for this sort of use.

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?

No. These put out AC. Your cameras want DC

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.

They're for low voltage lighting. Which is AC. Pretty well all electronic
devices require DC.

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?

Maplin etc do them in all flavours. However, 8 cameras at 200 mA each will
take a total of 1.6 amps, which is rather too much for a wall wart - due
to size restrictions.

They do a free standing 12 volt 2 amp SMPS type for 20 quid - N95AT

On a bad day, a switchmode PSU and a sensitive camera could have interference issues - SMPSs are
inherently more noisy than linear (e.g. wall-wart type) PSUs. Whether this is enough to cause
visible noise in the image will only be apparrent once you have set everything up...
Having a central PSU and long DC cable runs may also introduce significant ground loop issues -
again using a SMPS may make this worse.
Bottom line, it will work, but it _might_ cause noise in the images.