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help in understanding PSU

post #1 of 5
Thread Starter 
hi guy's

iam just about going to buy Seasonic M12II 620W

i didnt know about PSU's , that much ..just did a little research read some

articals but there somethings which i couldnt understand i the diagram of the psu and the rails

i got diagram pic's from Seasonic for exaple , the S12d series 750w+850W



look on the the maximum load of the +3.3v &+5v it's valued 150 why?

isnt 3.3v*24a=79.2, &5v*30a=150 >150+79.2=229.2?


*notice how in the +12v ,-12v,+5vsb rails are "correct".....


now look at the pic of the seasonic m12d series 750+850



look, you can see its similar to the s12d series

except its two +12v rails , each rail 38A ......so> 38a*2=76a >>+12v*76=912

*notcice in this series unlike the previous it has diffrent ampers in the +12v rails.. for the 750w & 850w

*its the same issue with the 850W

*notice the combined load this series and previous its the same, although the
s12d series got other ampers in the +12v

*both series, if you look on the total power it says 750w , quick math says its 918.6.. (same problem with the 850w)...who right?


look at this picture of seasonic s12IIBronze



again the issue with the +3.3v & +5v combined load problem


guys hope didnt dig to you much but i would really like to understand it
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post #2 of 5
It's not a problem, but it can be confusing. All it means is that no matter what, you cannot draw more than 150W from the +3.3V and +5V rails. The rating is almost always combined for those two rails, especially today since they provide little power to the system compared to the 12V rail.
The amperage rating for a rail does not correspond to its actual capacity. Rather, the sticker tells you how the OCP circuit on the rail is calibrated, i.e. when the PSU will shut down. For example, if you try to draw more than 24A from the +3.3V rail, the PSU will (or should) shut down. If you tried drawing, say, 20A from the +3.3V rail and 20A from the 5V rail, the PSU should also shut down because you're trying to pull more than 150W from the two rails combined. That's basically just the way it works.
For the same reason, if you see two ampere ratings for the same rail, e.g. 12V1 38A and 12V2 38A, you should never add them together. 12V1 will shut down if you try to pull more than 38A from it, and 12V2 will also shut down if you try to pull more than 38A from it, but either way you cut it, you can't draw more than 744W or 62A from both rails combined.

You should read the articles there if you want more complete explanations: http://www.overclock.net/power-suppl...ml#post9110838
post #3 of 5
Thread Starter 
Quote:
Originally Posted by Shub View Post
It's not a problem, but it can be confusing. All it means is that no matter what, you cannot draw more than 150W from the +3.3V and +5V rails. The rating is almost always combined for those two rails, especially today since they provide little power to the system compared to the 12V rail.
The amperage rating for a rail does not correspond to its actual capacity. Rather, the sticker tells you how the OCP circuit on the rail is calibrated, i.e. when the PSU will shut down. For example, if you try to draw more than 24A from the +3.3V rail, the PSU will (or should) shut down. If you tried drawing, say, 20A from the +3.3V rail and 20A from the 5V rail, the PSU should also shut down because you're trying to pull more than 150W from the two rails combined. That's basically just the way it works.
For the same reason, if you see two ampere ratings for the same rail, e.g. 12V1 38A and 12V2 38A, you should never add them together. 12V1 will shut down if you try to pull more than 38A from it, and 12V2 will also shut down if you try to pull more than 38A from it, but either way you cut it, you can't draw more than 744W or 62A from both rails combined.

You should read the articles there if you want more complete explanations: http://www.overclock.net/power-suppl...ml#post9110838

hi shub


you said "The amperage rating for a rail does not correspond to its actual capacity' but it does, when its single rail doesnt it?

and about the total power what does it mean? that all the power i can use at

once from all the rails?
Edited by OverKilla - 1/19/11 at 12:35pm
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post #4 of 5
Right, if it's a single rail then the amperage rating should stand, sorry about the confusion. And yes, max power means all rails combined.
post #5 of 5
Thread Starter 
Quote:
Originally Posted by Shub View Post
Right, if it's a single rail then the amperage rating should stand, sorry about the confusion. And yes, max power means all rails combined.
about the multiple 12v rails


how the power shared between the CPU, GPU, etc..

does each of them use one rail or mixed usage?

another one is that you said about if i use one rail or combined rails like +12v ,+3.3v &+5v much above the indicated "combined load" , the psu will likely would shut down .

what if i use all the rails on the PSU until the limit (without going over the OCP) , which will be much than the "total power"

theoritcly it shouldnt shut down isnt ? cause i wont pass the limit per rail
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HITACHI 82.3GB IDE + Maxtors 160GB SATAII 7200RPM Windows XP PRO SP3 32BIT Acer V193W b "19' Seasonic M12II 620W 
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My System
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CPUMotherboardGraphicsRAM
AMD Phenom II X4 955 GIGABYTE GA-MA785GT-UD3H (rev. 1.0 Intergrated ATI Radeon™ HD 4200 Kingston DDR3 2GB 1333Mhz CL 9 
Hard DriveOSMonitorPower
HITACHI 82.3GB IDE + Maxtors 160GB SATAII 7200RPM Windows XP PRO SP3 32BIT Acer V193W b "19' Seasonic M12II 620W 
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