Gaming 12V to 5V (PC psu)

[Scorpei]

As the topic sais I want to change a 12V rail to a 5V one on a PC PSU I have.

The reason for this is that the PSU I have only has 12V lines (it's a PSU specifically for SLI setups), but I don't need as much 12V. I currently have a 300W passivly cooled PSU installed which barely pulls my system (server). As such I want to add a thermaltake Power Express 250W to take some of the strain off of it. However it only supplies 12V lines so I can't hook it up to for example the hard drive or DVD drive (or currently even the GPU for that matter as it wants molex).

Now I'm not sure how to go about this however or if it is even a wise idea

. Any resistor would be a bad thing not in the least because it would probably get hot as hell

. So I guess I want to use a DC/DC converter but I'm not sure which one would be appropriate (and which wouldn't get too hot). Anyone have any ideas

?

 

[FAST6191]

Very odd, lack of 3.3V is nothing new but lacking 5V even..... What motherboard form factor is it if I may ask; most form factors call for a 5v somewhere at least so it might be worth looking especially if you are already poking around inside it*, you said server though so I would not be surprised if something fun existed

*it may exist but it does not mean it is a good idea to try drawing massive current.

A sandbar resistor might be better if you want to go down that road but personally I would go for a Zener diode, circuit of choice:
http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm
Do it right and you win a very clean voltage as well which is always a plus in computing.
My only problem here is how much you want to load it (hundreds of milliamps is typical rating and most 5V rails I see get into that world, up into the amps world and I would suggest looking at the automotive grade stuff or running parallel circuits)

Failing that you can buy a straight up voltage converter in IC form (current may still be a problem although they are more easily in the amps range) and I am also seeing the return of micro (well large capacitor size) transformers for just this sort of thing.

If I remember I will speak to some friends (12V is all the rage in boats these days as are low voltage/low power things like LEDs).

 

[Athlon-pv]

http://www.overclockers.com.au/wiki/Power_Supply_Unit

Buy a new one there "good" PSU 's out there that are silent and cheap. Have this one as a backup

 

[Scorpei]

Thanks for the tips so far

. I think I was slighly unclear though, I have a standard ATX2.0 PSU of 300W which can't take the load of the system (I had to downclock the CPU in order to get it stable, which I was planning on doing anyway, and currently have no optical drive attached as that was too much). In order to compensate I want to add the SLI specific power supply but that only supplies 12V lines (2 rails, 21A total with 25A peek).

So to offload the normal ATX PSU I'd like to connect things like the hard drive, optical drive and video card to the 250W SLI PSU but that also requires a 5V line.

Naturally I could also replace the ATX PSU all together but seeing as it is a passivly cooled one (and higher W passive ones are quite expensive, a lot more then the extra 250W PSU) I'd like to keep it. The system is on 24/7 and I sleep next to it

.

 

[FAST6191]

Dual PSU then.

Assuming both have the same reference voltages (they should but stick a meter across the positive terminals anyway).

Is it not worth hybridising the PSUs and using the SLI to to drive the (or some) 12V wires for things that need it and pulling 5 for the molex from the existing one; 12V rails are usually power and 5V is for the logic and is back in the 10s of mA (even at 5V).

Also externally powered caddy?