using PC psu as power source

Lots of Pc power supplies need to have some load on the 5V line too and the oututs are quite noisy

A PC's power supply is designed to provide much of its power on the 5V supply. If it's totally unconnected, there may be operational problems. Also, such power supplies aren't always happy with such a light loading. You'd need to experiment.

Graham

power

Thats easy to fix, put a resistor across the output and some caps to get rid of noise.

Circa Fri, 18 May 2007 19:47:10 GMT recorded as looks like "Andy C" sounds like:

I get the impression from your question that you intend to use the 12V from a functioning computer. If so, forget it. The power supplies are designed to run a motherboard and a few peripherals, and won't have enough reserve power to be used for this kind of external load. Others have touched on the load requirements if the PS is to be run independently.

Stand-alone 12V 5A supplies are plentiful and inexpensive, and far less hassle.

A 50mA load ?

Graham

yes, that is exactly what i had planned - we have a dedicated PC to run the software for the CCTV cameras and i was hoping to run the cameras from one of the 12v supplies from this functioning pc...

how so ? the computer PSU boasts 500watts - wouldn't 4amps @ 12volts be a puny external load ? cheers, andy

Most of these ratings are fairly ficticious.

Graham

Hi Andy I'm a totaly newbie here, but I would find out what uses 12V on this computer, and if it is things like floppy and CD drives (I'm too lazy to look it up :) and you are not using these and there is 50W of 12V power to spare, it might work. But your computer might run a little hotter (well PS, anyway) so I would be looking for devolution in this system, ie, having a separate PS for the cameras. Just my idle thoughts.... jack

How big, how many watts, how hot? How's he going to run his computer with this setup?

jack

The 12 volt supply itself must be rated at 3 amps (or whatever) at least.

might work better.

If the cameras need a negative supply that could be a problem.

Check that with the camera running there's no voltage between the battery negative and the ground of the video output.

Bye. Jasen

Recently there's more power provided on the 12V. Much of it is downcoverted to below 2V on the motherboard to run the CPU.

there's not much in a modern PC that runs at 5v

power

this one is going to be running a PC at the same time as the cameras so that shouldn't be a problem.

Bye. Jasen

Circa Sat, 19 May 2007 00:57:01 GMT recorded as looks like "Andy C" sounds like:

That rating is for the entire supply. Most of that rating will be for the

Furthermore, you will have no isolation on the 12V rail between the computer peripherals and the cameras you want to use. Noise and instability from any one device will affect all other devices on the rail. So you are faced with doing a complete load analysis on the computer and the cameras in order to find out whether or not this idea is feasible.

All that instead of spending $25 on a 12V 5A dedicated switching supply. It just doesn't make sense, unless you are more interested in experimental engineering than creating a workable, stable system. What if something unpredictable happens with the bus you have routed external to the computer, and you wipe out your hard drive? To me, the cost-risk-benefit analysis is weighted very much in opposition to the shared bus idea.

Circa Sat, 19 May 2007 00:59:50 GMT recorded as looks like "Andy C" sounds like:

I'm not sure what the point of posting those links is, but what strikes me from reading the "Discussion:" is that there are a hell of a lot of people out there playing with electricity who don't know jack s*it about it. You did well to post your question here, and I hope you take the advice you've been given. I see from other posts that my estimate of $25 for a separate supply was very much on the high side. That makes the decision even more of a slam-dunk, IMO.

yes, that makes good sense, thanks everyone - i will buy an independent supply for the cameras - i was just thinking it would be convenient if it was possible as they would power up and down with the PC, the cables are already routed to the PC and at face value the '500watt' PC PSU seemed ample- thanks again, andy

Circa Sat, 19 May 2007 21:50:44 GMT recorded as looks like "Andy C" sounds like:

Consider getting a switched power strip, and plug the computer, monitor and camera power supply into it. Make turning the strip on and off part of the routine of starting and stopping the computer. Good luck! :-)

works for me for a 200 watt supply for no load testing. i power my ex modom, a hub and some fans, has run for years now.