When is cross load an issue?

[you2]

Vaguely a long time ago I remember reading that certain power supplies would not function correctly if you did not have load on both rails (something like driving a cpu but no gpu). Unfortunately I don't remember the details or cause the psu to shutdown...

However, since I am setting up a machine without a gpu (2500K) I thought I would ask the qeustion in case it is an issue.

 

[theAnimal]

Cross load means high 5V and 3.3V with low 12V or vice versa. If you buy a good quality PSU it will not be an issue, nor would having an unloaded 12V rail.

 

[Zap]

It usually wouldn't be a problem as long as the PSU isn't a crappy one to begin with and is of a "modern" design with most of the power on the +12v (if you are using it on a "modern" computer).

The motherboard draws power from all the rails so as long as it is hooked up, you are good to go. The "problem" comes when you are doing something strange, such as using two PSUs in one system, and are using group regulated PSUs (all cheap ones, even lots of the better ones that aren't super high wattage). For instance if the PSU is only powering the graphics cards, then it is only asked to deliver +12v. This is a cross-load situation that would make crappier PSUs fall over and die (and provide lousy voltage regulation in the meantime). Better PSUs can handle it just fine, and the best ones (usually higher wattage) have zero issues due to being independently regulated. This means everything in the PSU runs off +12v, and the minor rails (5v, 3.3v, etc.) are all running off a little PSU within the big PSU.

 

[you2]

Ok thanks. I think I'm going with the seasonic x 400 or FSP gold 400; kind of hate spending $120 on something like this and there are no reviews on FSP gold 400; but i take it from your description neither of these are going to have an issue.
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Bit surpise there aren't a few cheap 400 gold but I guess it is a niche market.

 

[yottabit]

I have an FSP Group 460w 1u power supply and it is a very good unit, noticed a big increase in efficiency going from my non 80plus 600w ocz (dropped 20 w at the wall)

the fact that it's 80plus certified means it was tested and will hold up at ful load. You can look up the test on the 80plus website to see the results

the only thing I'd be concerned about is noise levels, but that's only if it's a concern to you. And make sure it has the appropriate connectors and length you need

also with no gpu 400w will be way overkill... If you never want a gpu you'd probably want a 250w psu

 

[Meghan54]

the fact that it's 80plus certified means it was tested and will hold up at ful load. You can look up the test on the 80plus website to see the results

Actually, that's not what that testing means. 80plus testing only shows what the efficiency at various outputs from a specific power supply is, not can it sustain a certain output level nor the electrical quality (voltage regulation, noise and ripple generation, etc.) of the output.

 

[yottabit]

Well, they test the efficiency at near 100% load, so you at least know that it doesn't explode at its rated power so in my eyes that weeds out a lot of the bottom-of-the-crap-barrel psu's

Also if you check the actual testing document, they will show you the voltages and current drawn at each load:
http://www.plugloadsolutions.com/psu_reports/FSP TECHNOLOGY_FSP400-60EGN_ECOS 2223_400W_Report.pdf

There's the report for FSP Groups 400 W Gold PSU. The voltages seem fine to me at 398W draw

Noise and ripple aren't shown but at least it gives you a fairly good rough idea of the quality of the PSU

Also, if you read through their testing protocol (http://efficientpowersupplies.epri....er_Supply_Efficiency_Test_Protocol_R6.5.pdf):
"Prior to power measurements, the UUT shall be allowed to operate at each load point for at least
15 minutes in order to allow the power supply to reach a steady state of operation. A steady state
of operation has been reached if the total input power reading over two consecutive five-minute
intervals does not change by more than ± 1%"

The PSU is subjected to the full load for at least 15 minutes

Lastly, FSP Group publishes the Ripple & Noise thresholds (along with detailed drawings, and other information) for their PSUs on their website:
http://www.fspgroupusa.com/fsp40060egn/p/848.html

I don't know how to interpret those values as to whether they are acceptable but if anyone more knowledgeable than me would like to chime in I'm sure that could help the OP

IMO an 80plus rating of any sort is a good indicator of the PSUs quality. The PSUs I think everyone is afraid of are the ones that can't handle sustained full load (at least without a large voltage drop).

 

[theAnimal]

The 80Plus tests are done at room temperature which is not what you would find in a normal PC. From what I understand some manufacturers cheat the test by cherry picking and/or modifying the test sample they send in.

Those ripple & noise thresholds are simply the ATX standard, and I would not want a PSU that approached those levels.

 

[you2]

Well part of the problem is the series is new:
FSP AURUM GOLD 400
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I thought about getting a smaller psu but I shoudl be drawing at least 20% and I didn't find anything with good efficiency lower than 400W.
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However I am open to suggestions.

 

[yottabit]

I dug around a bit but couldn't find any either :/ Anand did a review of a Huntkey 300W Gold supply but nowhere to find it.

If you want to save money I'd say go with a smaller non-Gold rated PSU, perhaps a 300W Bronze PSU. However given how low your power consumption is in the first place, worrying about the efficiency isn't going to make a world of difference (unless you're living off the grid or something)

It's when you get into the 800, 1000 W PSUs that efficiency is of the greatest concern

 

[Zap]

From what I understand some manufacturers cheat the test by cherry picking and/or modifying the test sample they send in.

This is true, unfortunately. At least this practice is better than the other one... of using 80Plus badging on a PSU that has never been tested. Yup, there have been "fake" 80Plus PSUs on the market.

 

[you2]

Ok thanks. i guess I'll go with the hcg-400 or fsp 400; the hcg-400 is $40 if I wanna mess with rebate and buy it this weekend but it isn't really just about the money; I like the idea of the system running cooler and using less current.
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Just that the seasonic price of $130 seems unreasonable. Guess there is not much demand for these psu; nor a whole lot of competition.