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# PSU Guide

Discussion in 'General Hardware' started by panchoman, Sep 13, 2007.

1. ### Namslas90New Member

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Most PSU Guru's say add the rails together and subtract 30%.
For example 18 amps + 18 amps = 36 amps .
36 X.30 =10.8 amps; 36 amps - 10.8 amps leaves you with 25.2 amps.

2. ### sneekypeetUnpaid BabysitterStaff Member

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in a very short and simple anwser keakar....YES...dont know why it made sence to me ,but I always did the total math, then used the efficiency % to finish the equation. Say a PSU is 70% eff. then I figured if a PSU is rated for 50amps, when at full load it should truely provide only say 35 of those 50 amps.

3. ### keakar

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4. ### keakar

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i am guessing that the amps for the 3.3v and 5v rails are standard amounts so it is not necessary to concider them in the equation but some psu have very high amps listed on these rails so why is that?

5. ### sneekypeetUnpaid BabysitterStaff Member

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Well if you do the math that Namslas and I were just speaking of ....that 72amp rail drops down below 60 amps capability!

Hell my OCZGSX700 says 4X18amp rails...which is then 72amps on all 4 rails right? I dont think so it also is in the 60amp range as well I promise ya.

6. ### Namslas90New Member

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If you look at those PSU's and all the other PSU's over 1000 watts they have better wires, then a standard PSU. Somebody here at TPU has allready verified this I THINK is was HeavyH20, but I could be wrong.

7. ### Namslas90New Member

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Internally all PSU's are 12v so the 5v and 3.3v rails are just devided from a 12 volt rail/portion of the psu. The voltage is changed but the amps are not affected in the devision of voltage(or at least not much).

8. ### keakar

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well the 700watt ocz psu has this:

+3.3@36A,+5V@30A,+12V1@18A,+12V2@18A,+12V3@18A,+ 12V4@18A,-12V@0.8A,+5VSB@3.0A

now there must be a reason to have higher amps on the 3.3v and 5v rails than the 12v rail? why?

Last edited: Sep 17, 2007
9. ### sneekypeetUnpaid BabysitterStaff Member

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Do the math on the sticker provided by OCZ its near impossible for those numbers to be correct or I'd have a 1000W PSU...here math provided

+3.3 X 36A =118.8W

+5 X 30A = 150W

+12(1-4) X 72A(all combined) + 864

now that leave us with this 118.8 + 150 + 864 = 1132.8W ?????

now even divide with the 30% rule thats 1132.8 X 0.30=339.84W loss

1132.8 - 339.84 =792.96 W total to be delivered by this PSU.

With all that in mind then why is it when this PSU is badged by its OEM (FSP) they label it as a 600W PSU.....As stated B4 it is a ploy by manufacturers to get you on the "OMG look what this has" train of thought.

10. ### keakar

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well there is why i get so confused by it all. just because you have watts doesnt mean you have amps and the 12v rails seam to have less and less amps now because of more rails.

now if you cant go by the stated amps on the label then we are buying blind.

if we cant believe these numbers then why can we believe any ocz numbers they state?

here s the website info on it: http://www.ocztechnology.com/products/power_management/ocz_gamexstream_power_supply-sli_ready_
and the label: http://www.ocztechnology.com/drivers/GameXstream_700W_chart.jpg

Last edited: Sep 17, 2007
11. ### sneekypeetUnpaid BabysitterStaff Member

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All i can say is do alot of homework and dont just read from one place. The review database was a huge help when I purchased mine(found right here at TPU). That is where I found it was in fact a rebadged FSP. Now I purchased on old experiences (Antec NEO 550 blew up) and word of mouth branding, and lots of reading. I think after all that info processed ,I bought the best my money at the time could afford. Guess all in all it is still sort of a craps shoot, but as posted here B4 ALL COMP PARTS ARE NOT CREATED EQUAL....even same brand and model, the two of them could perform completely different!

If you want a more scientific answer , wait for Kenny as he has the fresh knowledge of electric science, hell even Namslas has a better grasp. All in all just read and take it all with a grain of salt. Do the math yourself, then buy what makes the most sence in all aspects, and hope to hell you dont get something built on a Friday @ 2:30pm.

12. ### keakar

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well to start your homework you need to know what the important facts are and where to get those facts from to make your conclusions about the subject.

everytime i get to a base guideline it is proved to be phony.

stated watts are lied about and exagerated (so you dont have acurate watts to calculate your amps)

stated amps are lied about and exagerated (so the subtracting 25 - 30% rule wont work)

wire size is not listed to even know if they are lying about the amps on them

so i ask you, how are we supposed to come to a conclusion when we dont have real facts?

Last edited: Sep 17, 2007
13. ### panchomanSold my stars!

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thanks for backing up my point

14. ### panchomanSold my stars!

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yes you do seem very argutive, if you want to argue with me, i'm sure we can start a new thread thats a battlefield for single rail vs. multi rail, but this guide is meant for contributing information to the community to help them select better psu's.

15. ### panchomanSold my stars!

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guys, i have one question to ask you, lets say i have a cpu using 7 amps of the 18 amps that are being supplied to it on its own rail, and lets say i have a compressor or someother power hungry thing on another rail demanding 22A on the 18A rail, what happens? if i'm correct, the amperage cannot be rerouted from the cpu rail to the rail with the compressor because the max the rail can handle is 18A, and the compressor is demanding 22A. what happens when you want to use that compressor? if i'm right, then the the psu wont be able to power that compressor, because it cannot supply 22A on a 18A rail, let along that the 10A that isn't being used on the cpu rail cannot be rerouted to another rail. however if i used the compressor on a single 60A rail, it would work, correct? also because i'm just a kid in high school and i cant understand half the stuff, you guys can rewrite your own rails paragraph and pm it to me with links to back up what you're stating and i'll put the best one up, otherwise i will not lean either way on the rails idea, and leave it to the reader to decide.

16. ### keakar

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think of it like this, all the power is in one bucket where you have lets say 50 amps

your rails are limited to lets call it 18 amps, now you say you used 8 amps from one rail

now all the rails can each give you up to 18 amps but no more then that because they have a regulator that wont allow more then that.

any device can use power from the 12v rail up to that 18 amp limit.

things like video cards drawing lots of power are spread out over the rails so the pci bus is giving it amps and the 12v rail is giving it amps so it doesnt draw all the amps from the plug but also from the pci socket so no one leg needs to supply more than 18 amps.

that is the general explanation but im not totally clear on specifics as to exactly how the amps are divided up.

17. ### panchomanSold my stars!

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what about running a compressor or a tec unit? i dont think the pci bus can supply a compressor extra amps which the psu cant supply to it due to rail limitations. and i wonder why we need to buy seperate single rail psu's for a tec system? why cant we just have a dedicated rail on the psu for a tec?

18. ### keakar

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because a single rail psu isnt limited to 18 amps, it can run higher amps if it has heavier wires and this is why the extra psu is needed so it can supply the higher amps needed.

also im just guessing here but i believe seperating the power supplies is a protection thing so if there is a failure it wont take down the whole system.

19. ### panchomanSold my stars!

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i had a dual rail psu before, and the only thing that died was the hdd, and dont try to tell me that only the hdd was on one rail...

tecs require a lot of power, and a capped rail cant supply it, but a single rail psu can supply the tec with enough.

20. ### keakar

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there are no certaintees but to limit what is connected will limit what can be damaged. it can take out nothing as easy as it takes out everything. i seen it take out just the case fan already so there is pure luck as to what if anything can be taken out.

as for single rail, that is exactly what i just said. you asked why they use a seperate psu and that is why, to get enough juice because a single rail puts out more amps because it is not limited to 18 amps output.

21. ### panchomanSold my stars!

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there we go, so if a single rail psu can work safely and not have any overloads, idk whats so good about a multi psu.

22. ### keakar

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well were it has trouble is in more total amps so it adds limited amperage rails so you get higher total amps but not more amps for use on any one rail

so the extra amperage from the power supply is passed through the board

assume you have a psu with total of 54 amps on 4 rails limited to 18 amps each,

lets say the cpu uses 8 amps so you have 10 amps left that can be used from that leg

so the board sends it to the pci bus to use

now you have a video card needing 20 amps, but no one rail can supply it because of the 18 amp limits on each rail. what happens is the video card takes the 10 amps available from the pci bus and uses 6 of those amps and then the second rail gives it the remaining 14 amps it needs to get its 20 amp total. this has used up two of the four 18 amp rails and used 28 amps.

this now leaves you with only two rails remaining to power the rest of the computer but you only have a total of 26 amps left to use from the original 54 amps. (54-28 = 26 amps left)

now the rest of the computer is powered on these two remaining rails but again neither one of which can give more than 18 amps so it is divided up as balanced as you can.

most people wont need more than two rails and unless you have a real high powered video card (8800 ultra) you wont need a 4 rail psu.

if your system isnt very power hungry a single rail would do for powering your sytem but i think at least 2 rails is the way to go because later if you make future video upgrades you dont want to have to replace the psu. really power hungry high end video cards or SLI or crossfire setups is the only reason to use 4 rails IMHO.

23. ### panchomanSold my stars!

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okay so i'm just going to do it neutral aight, it doesn't really matter so, both types of psu's are very good so. and the topic is now closed, along with modular, if you want to correct something else or contribute to the brand/oem list by looking up the oem of the psu you have, that'd be great. i'll be finishing up the guide sometime this week when i find time(probably the weekends)

24. ### keakar

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yes keeping it neutral would be a good idea because each type of psu has its good and bad points so weather you use single, dual, or quad rail psu, each can be a good choice depending on the application your using it on.

25. ### panchomanSold my stars!

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