Power Supply Voltage Railings- Explanation please?

[xerocool]

Title says it all, can anyone give me a quick rundown of the basics? Like what numbers I should look for and the difference between generic and good PS's? And what might happen with bad railings? Thanks!

 

[SocrPlyr]

well the are important b/c they are the voltages that everything runs off of, if they are wrong you can blow things up... like try hooking a flashlight that uses 2 AA batteries to a 9V battery, you'll probably blow the filament in the bulb...

Next each one should be within a few percent +-5% is resonable
The major rails are +5V +12V +3.3V and the minor ones -12V -5V
i could be forgetting one or putting them in the wrong major/minor category but this is just off the top of my head...

Josh

 

[Matthias99]

Originally posted by: xerocool
Title says it all, can anyone give me a quick rundown of the basics? Like what numbers I should look for and the difference between generic and good PS's? And what might happen with bad railings? Thanks!

The most important difference I usually see is the maximum amperage allowed on the +12V line. Really cheap power supplies will only have 10A or so, even if it's a "400W" or higher model. This is not sufficient for modern graphics cards, since other equipment in your system also uses the +12V line. You want closer to 20A (better supplies usually have 18A or so), and 25A or more if you're running a big server with a ton of drives hooked up.

The other thing they sometimes to is to limit the combined amps on the +3.3V and +5V lines too strictly. This can be a problem for newer CPUs with high-speed (and high-power) memory if you have a number of peripherals (all your assorted stuff, like PCI cards and USB peripherals, runs off these lines). However, this is rarely a problem on any "big" power supply, since cheaper ones tend to overinflate these numbers to make their overall wattage rating look bigger, and good-quality ones still have sufficient ratings.

An insufficient power supply often results in an unstable system, although the instability will often only manifest itself sporadically, while the system is doing something that requires 'extra' power, such as booting up or playing a game.

 

[xerocool]

ok, so for example, i just pulled out my PS from my comp and looked at the numbers, here they are:

Voltage 115/230V
+3.3 -> 24A
+5 -> 35A
+12 -> 12A

Current 8A/4A
-12 -> .8A
-5 -> .5A
+5SB -> 2.0A

Ok, so just from these numbers what you're saying is that my +12V rail is bad cuz it's only putting out 12A. But what do you mean limiting the +3.3V and +5V too strictly? (And why are the lower voltage rails soo much more amperage? Doesn't the 12V rail power the most important and heavy amperage stuf?)

Also, where do I find the percentages? They don't see to be on the sticker on the PS....?

 

[3chordcharlie]

The +12V rail powers your drives, videocard, etc, but it doesn't necessarily need to be HIGHER amperage - for example 12V@5A is the same amount of electrical power as 5V@12A, so comparing amperage between rails at different voltages is not very meaningful.

Common problems from too little +12V amperage would include bad drive performance (for example, when copying between drives), 'coasters' from your cd-writer (especially the older, non-burnproof ones), and I assume video card problems as well, particularly with new video cards that ask for a lot of power (I wouldn't know as I still don't own a card made this century... )

Tolerances would normally be printed somewhere on the label, but apparently aren't in your case - I would assume the supply you have is OK for an ordinary system with no overclocking, a small number of drives, and no fancy video card, though there are rumours that AMD processors hate bad power. If you want a good, reliable system though, it needs to start with a solid power supply.

 

[Matthias99]

Originally posted by: xerocool
ok, so for example, i just pulled out my PS from my comp and looked at the numbers, here they are:

Voltage 115/230V
+3.3 -> 24A
+5 -> 35A
+12 -> 12A

Current 8A/4A
-12 -> .8A
-5 -> .5A
+5SB -> 2.0A

Ok, so just from these numbers what you're saying is that my +12V rail is bad cuz it's only putting out 12A. But what do you mean limiting the +3.3V and +5V too strictly?

Well, it could be higher, but it's probably fine for something that doesn't have a super-duper fast video card and a lot of hard drives. 12V@12A = 12 * 12 = 144W; newer high-end video cards can come close to 100W by themselves, which doesn't leave much for your hard drives and the motherboard parts that use 12V power.

By "limiting", I mean that on many power supplies, you'll see something like this:

+3.3V: 30A
+5V: 40A
+3.3V and +5V combined: 50A max.

Yet they'll count this as if it is "300W" worth of power (100W on the +3.3V line and 200W on the +5V), whereas you really can't run more than ~230W total at the same time (40A@5V + 10A@3.3V)

(And why are the lower voltage rails soo much more amperage? Doesn't the 12V rail power the most important and heavy amperage stuf?)

Power draw is measured in Watts (Watts = Volts * Amps). So your *wattages* are:

3.3V: 80W (3.3 * 24)
5V: 175W (5 * 35)
12V: 144W (12 * 12)

This is a "400W" or "430W" power supply, most likely, but you'll have a hard time finding a mix of parts that can actually draw 400W from it. Here's what an Antec TruePower 430 has:

Output: +5V@36A, +3.3V@28A, +12V@20A, -5V@0.5A, -12V@1A, +5VSB@2A

It has a 240W output on the 12V line, which is much better for a high-end system these days, as much of your power draw *is* on the 12V line (hard drives, optical drives, video card, motherboard power regulation). The 5V and 3.3V drive your CPU, memory, and USB/PCI/serial peripherals, which are unlikely to total to more than 200W even in a fully-loaded system.

Edit:

For illustrative purposes, I found a too-cheap 500W PSU, the MGE 500W SuperCharger

Output: +3.3V@28A,+5V@38A,-5V@0.5A,+12V@17A,-12V@0.8A,+5VSB@2.0A

+3.3V: 28A (93W)
+5V: 38A (190W)
+12V: 17A (204W)
-5V: .5A (3W)
-12V: .8A (10W)
+5VSB: 2A (3W)

Total: 503W. But there's no way you're ever going to need 300W on your 3.3V and 5V lines (excess capacity is not a bad thing, but here it's basically just marketing), whereas you can *easily* pull more than 200W on your 12V line if you have enough drives and/or a fast video card.

Here's the much nicer Antec TruePower550:

Output:+3.3V@32A;+5V@40A;-5V@0.5A;+12V@36A;-12V@1A;+5VSB@2A

+3.3V: 32A (106W)
+5V: 40A (200W)
+12V: 36A (432W)

Total: 738W. First off, you'll note that this is more than 550W -- there are probably some restrictions on how much you can draw from different rails simultaneously that Newegg just isn't listing. But this is a serious, heavy-duty server power supply.

 

[xerocool]

yeah, i'm asking all this cuz my comp freezes when i play video games like unreal 2k4 and warcraft3 and lineage2. it'll happen at random intervals and the whole entire computer locks up and i can't do anything but a manual reboot.

and i bought an MGE Vigor 400W ps yesterday at Fry's cuz it was $20 - $15 MIR. i'm gonna go check the rails on it, but if the MGE Supercharger 500W isn't good enough, I doubt this one will be.

btw, i'm running:

P4 3.06b
2x Optical (CDRW, DVD but CDRW is disconnected currently)
4x 7.2k RPM HDD (20, 160, 160, 200 but one of the 160's is disconnected currently)
Radeon 9700 Pro
2GB Dual Mushkin PC2700

and i emailed Ati, cuz i thought it was a driver issue, but if i'm my hardware is sapping too much power, i might have to go buy yet another PS.


edit:

so i just checked my rails on the vigor 400W, they are:

+3.3V -> 25A
+5V -> 32A
+12V -> 18A

-5V -> .3A
-12V -> .8A
5V SB -> 2A

and the +3.3V/5V limit is 160W.

so i have 12*18 = 216 W of power on my 12V rail? do you guys think that's good enough for my current setup (also, i want to reactivate the 160GB HDD and the CDRW too).?

also, what do the -5V, -12V, and 5V SB numbers mean, what are they used for?

thanks guys!

 

[Matthias99]

Originally posted by: xerocool
so i just checked my rails on the vigor 400W, they are:

+3.3V -> 25A
+5V -> 32A
+12V -> 18A

-5V -> .3A
-12V -> .8A
5V SB -> 2A

and the +3.3V/5V limit is 160W.

so i have 12*18 = 216 W of power on my 12V rail? do you guys think that's good enough for my current setup (also, i want to reactivate the 160GB HDD and the CDRW too).?

18A is probably OK for you, although you might want to check the power draw on those hard drives. The manufacturer should have the info on their website somewhere.

also, what do the -5V, -12V, and 5V SB numbers mean, what are they used for?

thanks guys!

-5V and -12V are used as part of the signalling for old-school serial ports (thus why they have <1A of current available). I don't believe they are used for anything else anymore, and I think the new BTX power spec gets rid of them entirely.

+5VSB is StandBy power -- often used for things like Wake-On-Lan functionality. It's a small amount of power that is provided even when the system is nominally "off".

Basically, none of them are terribly important nowadays.