Mike Harrison wrote:
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.
snip

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.

'All' (the 6 I've investigated) cameras internally regulate the 12V to
5V.
(this generates heat in the camera, most are much cooler run at 8V.)
As long as the supply voltage is over 7V, then it's fine.

Ok, 200mA camera.
Assume a 2V drop is OK, and 0.5mm dia cable is used.
That's about .1 ohms per meter, or .2 for both wires.
For 200mA, and 2V, you need 10 ohms, so that's 50m.
If it was 0.5mm^2 instead, call it 100m.
I suspect it'll still work out to 2-3 times this distance.