Seperate PSU's?

[Logistics]

I've often wanted to run a power supply only for hard drives and CD-ROM's as well as fans and then have a power supply dedicated soley to powering the motherboard and it's cards. Is this possible? I keep hearing stuff about a said rail dropping too low because an opposing rail has no load. Is this what will happen if I do this?

[starfury1]

Re: Seperate PSU's?

it has been done and there are discussions around on it in both these forums and others
Ideally you should only power MB from 1 and peripherals from the other.

make sure both are grounded together, usally bolting them in the same case takes care of that
but beware you never mix any + or - rails of them together.

personally I think, if you think you need to use 2 psu your better of getting one big more powerful psu

You need a way to switch them both on which is another consideration
(easy enough done though)

on the rail dropping its the way the supplies regulate the outputs by monitoring
the output of 1 rail only (think, not sure its the 5 Volt)
what you need to do then and is put some constant resistive load on that rail.

there is a post on these forums that did discuss this

would have to find it, when I do I'll post the link

HTH

Cheers

[starfury1]

Re: Seperate PSU's?

here's one thread on it

there is info on the net...but apply a little bit of grey matter to what you read too...some ideas might be just plain dangerous

there is info out there on tiering the supply rails of each supply together too...
be careful here cause it needs to be done correctly.

Generally speaking

if you are thinking of doing that..just buy a bigger PSU is my advice

[Logistics]

Re: Seperate PSU's?

I remember long ago on a different board I was talking about this with other users, but the purpose is to remove things like the HDD, CD-ROM's and fans because their mechanical features add garbage to the power lines of the PSU. I believe it was a thread about improving a PC as an audio transport.

I've been through these threads and I still don't see what the problem is. The only oddity I can think of is that the PSU's might still see a piece of eachother due to the ground leads of the drives data cables, but they could be lifted.

[999999999]

Re: Seperate PSU's?

Do not try to isloate the grounds you WANT them to have a common ground.

The problem with audio is typically when it's analog output and the noise contaminates that, as it is a high (enough) frequency that it is not rejected so well by active DAC or opamps powered by it.

You are not going to have the problem from the hard drives and fans as much as the larger current subcircuits like video card or motherboard, CPU power subcircuit. If you really want an audio solution just use an external DAC, like a USB type, with it's own (preferably linear) supply.

Generally people use two supplies to split up current some, when one supply is underpowered for the whole system. In that case you would look at what division of hardware allows each PSU to run within it's capability sharing the loads as much as reasonable.

Whether one rail drops low because it has a higher percentage load than the PSU's feedback network was designed to use for regulation, or not, depends on the PSU and exactly what you're powering. Most often that means the PSU regulates mostly from the 5V rail and will need a certain % load on that rail, a % of the load you are powering from the other rail(s), so that the rails you needed stay within tolerance.

Generally speaking, unless you were going to power something with an unusually high 12V current, like peltier(s) or only certain video cards, you should be able to connect the PSU and if you find it is shutting off or the rails aren't balanced well, then you would add some load to the underloaded rail (typically 5V rail). Power resistors are a common load to use, but they do create a bit of heat and need isolated, ideally you would just shift the loads around so these resistors weren't needed.

In other cases the PSU might detect an unused rail (like 3.3V), having no load, was floating too high and the PSU would shut off. In that case a far smaller load (higher ohm) resistor would be needed, which wastes less power & creates less heat so it seems a more reasonable thing to do.