Legit Reviews

Sorry if this is a bit longwinded. Fuel pump woes notwithstanding (http://forums.legitreviews.com/about14943.html), since I work with computers daily I need to know more about PSUs and the ever-increasing demands made by PCIe and SLI/crossfire variations thereof. Up until all these high-powered video cards, a power supply was a power supply was a power supply for the most part anyway. Obviously some manufacturers were better than others but generally speaking they pretty much did the job. But in reading specs on graphics cards and doing more builds with HD 3850s, etc. I'm worried more and more about the PSUs we use on a daily basis here at the shop.

For example I'm looking, eventually anyway, to buy an EVGA 8800GT which requires a minimum of 20A 12V according to the manufacturer. My current PSU is a two year old Antec 500W which puts out a maximum of 18A on one rail and 15-17 (can't remember exactly) on the other. Thus far I haven't put any extreme demands on it but it has wokred flawlessly 24/7 for the last two years.

Another example, about a month ago I did a build for a customer with a C2D E6750, 2 GB G.Skill PC6400 RAM, 150 GB Raptor HDD, and HD3850 grpahics. We used a stock PSU (APower 400 W) on this and so far not a hitch even though the card specified a 450W or higher PSU. Customer ordered this strictly for gaming: CoD4, WoW, Halo, Crysis and miscellaneous online games that his wife plays.

In looking at PSUs for myself, to find one with specs of 20A or higher per rail, I have to get into 800w or better units that, frankly are more cash than I'm prepared to spend for my needs since the card alone demands more power than the rest of my system. Single rail PSUs, the very few I've found on Newegg, have more amperage on the single rail of course but the reviews on them are horrible for reliability.

So my question, (finally! you say), is do "modern" PSUs apply more amperage to the rail that requires it or is each rail limited to its max? And, in audiophile parlance, how can you tell which PSU's specs are "peak" vs "rms". Are higher-end manuf. such as Antec more towards continuous output?

Thanks for reading all this blather and thanks in advance for your thoughts!
Jack

most high end one are more rms as you say. as for the amps question and why you cant find a psu thats below 800w with a 20 amp rail, you have to rember something:
watts=volts X amps, so if 12x20=240, 240x4 (cause most highend supplies have 4 12 volt rails nowadays)=960, plus the 5v rails and the -12v rails, it all adds up. (not that just about any psu is gonna have 20amps on all 4 rails, i just did that for simplicity)

Most PSUs are strictly limited to the max amperage of what is listed on them (and i think its generally a peak rating, as the max wattage drawn from rails is usually less than the available amps x rail). If power is not used on one of the other rails, and another rail needs more power, it can't have it, and as such it has trapped power.

There are some PSUs however that can dynamically shift the unused power from one rail to the other (only on the same voltage rail though) when it is needed. This will be advertised as a feature as it is so uncommon with regular PSUs. There are yet further PSUs that just have one huge 12v rail, like on some of PC Power & Coolings PSUs (and those are good PSUs).


The PSU recommended specs by ATI/Nvidia are for the whole system (not just the card), and as such don't have to be all on one rail. So your 500w Antec, with its 18a & 17a, should be treated as having 35a of 12v power, which is plenty for the card and system.

The downside to the split rails (even counting them as one for the purpose above) is that you have to make sure that the load is spread across the different (12v) rails, so plug the PCIe (graphics card) on one rail (i.e. 12v1), and the main board/cpu power on another rail (i.e. 12v2). If you don't know what wire connects to which rail, this becomes tricky, and it is better to get a higher rated PSU.

Hope that helps,

Dan

Thanks to both of you! It's amazing the things you forget when you don't use them on a daily basis (like the fundamental laws of electricity. d'oh!! )! Years ago I used to design and install high-end car audio and competition systems. I can't believe I didn't apply those same principles to PSUs. But as the saying goes "if you don't use it you lose it..." You both answered my question and a few more, to boot. I really. really appreciate it! Thanks guys!
Jack

No worries ^_^

Dan