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Old March 22, 2008, 12:44 PM
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Default basic(?) psu question - dual rail combined ratings

This might be a basic question, but why is it that in some psu's with dual 12v rails you can add up the ratings to give a single 12v rating? For example, I've seen posts about a psu with 2 18a 12v rails and a person running one graphics card simply adds them up to say they have a 12v rating of 36a. Meanwhile other psu's actually state a lower combined rating on the label.

My psu is (supposedly) a re-branded 400w fsp, it has dual 12v rails each rated at 20a, but underneath the two it is rated at only 25a combined. Am I reading the chart on my psu right?

I was looking at buying a new graphics card, either 8800gt or a 9600gt, both of which some sources say a minimum of 400w psu with a 12v rating of 26a, others say 450w psu with 26a. Either way I'm guessing I'm going to need a new psu. But I was hoping that maybe since each 12v rail on what I have is 20a then maybe I could sneak by with my current psu.

Thanks for any help.
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Old March 22, 2008, 01:12 PM
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It is quite simple as long as you know the math that goes along with +12V and +12V combined ratings.

First of all you have to remember that Watts = Amps X Volts. So, that would mean 20A on the +12Vrail = 240W. See where this is going?

So, let's take a look at the PSU you are refering to. Each +12V rail has a 20A rating which would mean (in "literal" terms) that if each rail would be allowed to supply its fully alloted power (20A) to the components, that the +12V rails would draw MORE power than that particular PSU is rated for (20A on each +12V rail = 480W whereas this is a 400W PSU).

The 20A is what the OCP (Over Current Protection) circuit is set at which is a pre-determined value stating the amount your components can draw from that particular rail before the PSU will shut down. So, if your components on that rail draw MORE than 20A (480W) the PSU will shut down. This is basically a saftey feature so you do not over load your PSU. On dual rail designs (as stated by the ATX specifications) one +12V rail is dedicated to powering the CPU while the other rail is for powering all your other components that need 12V power.

What PSU manufacturers do is give a maximum value for the +12V draw (the combined value) so that components cannot draw MORE power than the PSU can provide. Yet, at the same time they provide a large enough buffer zone in the +12V rail OCP circuits so you could provide SOME power to your components.

You said your PSU has a 25A combined value but let's say for argument's sake that it was 26A. Imagine if that 26A would be divided equally into two equal 13A +12V rails. That only gives you 156W on each rail....so what happens if you are using a low-power CPU (+12Vrail #1) and a high-powered GPU along with HDs, fans and whatnot (+12V rail #2)? Basically, your CPU would have all the room in the world but the OCP on the other rail would probably shut down your PSU because of your other components drawing more power than it is set to provide.

This may or may not have made any sense but in the end it all comes down to the PSU manufacturers building in some flexibility into their +12V rail outputs.
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Old March 22, 2008, 02:40 PM
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That makes sense, thank you muchly. I've seen you posting over at a certain retailer's forum, always helpful :)

So can I assume that psu's that don't have a combined 12v rating listed have literally taken the total 12v amperage/wattage and divided it by 2? So if I'm looking for a psu with at least 26a on the 12v rail, and I find one with say 14a on each but no combined value, I can safely assume its 28a? I guess keeping in mind the total wattage of the psu. Actually most psu's do list the total wattage for each division(?) of the psu, so I guess that would be the upper limit?

For example:


So 481.5 max amps for the 12v rails (but combined with the 3.3 and the 5 - what runs off the 3.3 and the 5?)

Or maybe I can ask you this, so it regards specifically to my situation; what would you recommend for a minimum watt psu and minimum 12v amperage with an amdx2 4200, 2x1g pc5300 ram, one sata hd, one dvd (multi) burner, one card reader (sd, compact flash, etc), on board realtek 7.1 audio - considering an basic stock evga 8800gt (which recommends minimum 450watt psu with 26a on the 12v rail).

I'm on a budget, so I'm looking for a cost effective psu plus gpu combination. And I'm trying to educate myself on psu's ;)
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Old March 22, 2008, 03:38 PM
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Quote:
Originally Posted by ccm001 View Post
I was looking at buying a new graphics card, either 8800gt or a 9600gt, both of which some sources say a minimum of 400w psu with a 12v rating of 26a, others say 450w psu with 26a. Either way I'm guessing I'm going to need a new psu. But I was hoping that maybe since each 12v rail on what I have is 20a then maybe I could sneak by with my current psu.

Thanks for any help.
The most important factor here is your above statement.

If you plan on running your system with MINIMUM PSU ratings you can expect to have a very short life span of your components and run with unstable power.



I would not consider anything less than 550w or more for a single card, but thats just me. I like stable power.
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Old March 24, 2008, 05:41 AM
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Now the thread officially turns into a power supply suggestion thread

How about this psu: 535w enermax

http://www.enermaxusa.com/catalog/pr...roducts_id=107

or a CoolerMaster extremePower 650w

or an ocz stealthxstream 600w

all are available local to me for about the same price (within $10)
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Old March 24, 2008, 06:56 AM
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Quote:
Originally Posted by ccm001 View Post
So 481.5 max amps for the 12v rails (but combined with the 3.3 and the 5 - what runs off the 3.3 and the 5?)
Ah, here we come into something interesting which is the FSP-built OCZ PSUs which are based on the FSP Epsilon platform. The combined value you see there means that the combined value for the +12V rails ALSO has to take into account the amount of load on the +3.3V and +5V rails.

Quote:
Or maybe I can ask you this, so it regards specifically to my situation; what would you recommend for a minimum watt psu and minimum 12v amperage with an amdx2 4200, 2x1g pc5300 ram, one sata hd, one dvd (multi) burner, one card reader (sd, compact flash, etc), on board realtek 7.1 audio - considering an basic stock evga 8800gt (which recommends minimum 450watt psu with 26a on the 12v rail).
I see a number of options for you.

Corsair VX550
Corsair HX520
Silverstone ST50EF PLUS
Seasonic M12II 500W
Gigabyte Odin Pro 550W
Thermaltake Purepower Rx 600W
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Old March 25, 2008, 06:45 PM
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Well I found a really good deal on the Enermax I linked above (FMA II). 535watt psu with a combined 34a on the 12v rails. Its too good a deal to pass up I think.

It seems like that psu should power my system (no overclocking, stock 8800gt), I'm leaning towards it to save some money. What do you think SKYMTL? OR should I really spend an extra $40 on the ocz stealthxstream 600w?

(With all the PSU's you suggested SKYMTL, I can't get locally, and I'd have to factor shipping into the costs. Although I do like Silverstone's but no one sells them here in Halifax).
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Old March 26, 2008, 04:58 AM
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If you have EMT or Paypal I would be more than happy to sell you one of my review units. If not, I would say go with the StealthXstream but you should REALLY look at shopping online since it seems like you are overpaying for the units you are buying...
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