Amps on dual 12v

S

Silent Assasin

[H]ard|Gawd
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Aug 1, 2004
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Ok, so with my new case build I've been starting to look into even buing new parts. But anway, If I do happen to build a new computer, I want to get into SLi. Previously my general consensious was to go with no less then 30amps on the 12v rail. Now almost all of the sli certified with native 24pin connections have dual 12v rails with no more then 20 amps per rail. So what should should I be looking at as far as amperage. I'd prefer not to spend any more than $150 on the psu if I happen to build the new rig. I'm mainly looking at the OCZ line at the moment.
 
In your case, then look for the maximum combined +12V wattage on the particular PSU that you're buying (if such a figure is given), and then divide that by 12. That will give you the maximum combined +12V amperage.

That's the reason why 20A plus 20A almost never equals 40A.
 
so if you have a 500 W power supply and the +3v and +5v rails add up to 150 watts, you can safely assume that the remainder is divided between the two 12v rails? That would leave 350 watts for 12v = 29.2A.

More than enough power right?
 
vidor said:
so if you have a 500 W power supply and the +3v and +5v rails add up to 150 watts, you can safely assume that the remainder is divided between the two 12v rails? That would leave 350 watts for 12v = 29.2A.

More than enough power right?
Click to expand...

NO!!!!

Look on the label. There's going to be a 12V1 rating in amperage, a 12V2 rating in amperage and a total combined wattage.

You can't just subtract the 3.3V+5V combined from the total wattage because you can't fully load the 3.3V+5V and the 12V at the same time.

In reality, you'd probably get a number a lot lower than what the PSU can actually do. On the other hand, you're not taking into consideration the -12V and +5VSB (which likely make up for about 20W.)
 
The labels I see all list a max amperage output on each rail but I dont see a combined max wattage for the 12v rails.

For a single card setup, a 500w amp with 16/18 A on the 12v 1/2 rails is plenty regardless, right?
 
Probably... but what power supplies are you looking at? Every (decent) power supply I know of shows combined wattage for the 12V rails, except for Antec. But then Antec just puts it at the bottom of the graph below the 3.3V and 5V combined rating.
 
sorry, I dont mean to hijack the OP's thread. I've read most of the FAQ's but I'm new to this so bear with the stupid questions please :rolleyes:

i'm looking at this this aspire 500W thats on sale. I budgeted for ~750 dollars and forgot about the PSU. I'd like to get one under 50. Going into

Asus SLI-premium (or BFG's NF4U mobo, havn't decided yet)
AMD 3500+ Venice
BFG 6800 GT OC
1 IDE HD
DVD player
CD-RW
1 sata HD

no real plans to overclock (never done it before) and no plans to use SLI (I want the premium for the heat pipe ;) ).

Seems to me that system should come in around 380 watts. the aspire has 16/18 A on the 12v rails. Enough or not?

Thanks for the quick responses
 
vidor said:
sorry, I dont mean to hijack the OP's thread. I've read most of the FAQ's but I'm new to this so bear with the stupid questions please :rolleyes:

i'm looking at this this aspire 500W thats on sale. I budgeted for ~750 dollars and forgot about the PSU. I'd like to get one under 50. Going into

Asus SLI-premium (or BFG's NF4U mobo, havn't decided yet)
AMD 3500+ Venice
BFG 6800 GT OC
1 IDE HD
DVD player
CD-RW
1 sata HD

no real plans to overclock (never done it before) and no plans to use SLI (I want the premium for the heat pipe ;) ).

Seems to me that system should come in around 380 watts. the aspire has 16/18 A on the 12v rails. Enough or not?

Thanks for the quick responses
Click to expand...

Actually, the Aspire is one of the "cheaters" in the PSU industry. Its voltage regulation is +/-10% at that wattage and those current ratings, which is clearly well outside the ATX12V 2.0x spec. That PSU would have been a decent 350W unit - because the ATX12V 2.0x standard requires a voltage regulation tolerance of no greater than +/-5%, which that Aspire PSU just meets at 350W.
 
would my build need more than that PSU, if you label it as a decent 350W?
 
vidor said:
would my build need more than that PSU, if you label it as a decent 350W?
Click to expand...

Go with at least a 400W PSU, in your situation. I've heard of systems whose PSUs burned out and sometimes take a couple of expensive components out with the PSU when a given rig tried to run with the total wattage and/or amperage that's barely enough for that config.

Also, good 400W PSUs don't exactly come cheap. In fact, they don't cost anywhere near as low as $40 (unless it's a Fortron model); more likely, a good 400W unit costs about $60 to $75. And more typically, in the $30 to $40 range you'll more likely find "400W" PSUs which perform like other companies' 250W to 300W PSUs.
 
vidor said:
sorry, I dont mean to hijack the OP's thread. I've read most of the FAQ's but I'm new to this so bear with the stupid questions please :rolleyes:

i'm looking at this this aspire 500W thats on sale. I budgeted for ~750 dollars and forgot about the PSU. I'd like to get one under 50. Going into

Asus SLI-premium (or BFG's NF4U mobo, havn't decided yet)
AMD 3500+ Venice
BFG 6800 GT OC
1 IDE HD
DVD player
CD-RW
1 sata HD

no real plans to overclock (never done it before) and no plans to use SLI (I want the premium for the heat pipe ;) ).

Seems to me that system should come in around 380 watts. the aspire has 16/18 A on the 12v rails. Enough or not?

Thanks for the quick responses
Click to expand...

That is a BAD example. Remember I said "decent" power supplies will tell you max combined wattages (and if they are in fact decent, it's a continuous wattage rating and not peak.)

It may or may not power your rig, but there's no piece of mind because you have NO IDEA what that power supply is capable of doing.

Try another one. ;)
 
The FSP.. No comparison.

You didn't notice that the Thermaltake only has ONE 12V rail?
 
So, my question was kind of evaded.... How is it that you should determine minimm amps for dual rails? I will most likely end up buying the OCZ powerstream but I'd like to know for future refference if I find a descent PSU that is cheaper.... Like my current minum requirement is 30A on the single 12v rail, but when I started looking at dual 12v, I have no idea what amperage to look for.
 
Silent Assasin said:
So, my question was kind of evaded.... How is it that you should determine minimm amps for dual rails? I will most likely end up buying the OCZ powerstream but I'd like to know for future refference if I find a descent PSU that is cheaper.... Like my current minum requirement is 30A on the single 12v rail, but when I started looking at dual 12v, I have no idea what amperage to look for.
Click to expand...

Your question wasn't evaded. E4g1e answered it in the first response.

For dual 12V rails, you need to look at the "combined 12V rails" rating. That will be in watts, so you'll want to divide by 12V to get an Amperage rating you can use for comparison purposes.
 
Dave,

Is the letters "C" and "V" worn off your keyboard yet?

;)
 
E4g1e said:
Definitely the FSP, which is on the list of Cheap, but Good Power Supplies. That particular Thermaltake isn't.
Click to expand...
Only 1 TT PSU is on any recommend list!
$119 & Shipping > MODULAR <
Thermaltake W0057 PurePower 500W Power Supply - Retail
Output +3.3V@30A, +5V@30A, +12V1@18A, +12V2@18A, -12V@0.5A, +5VSB@2A
Approvals CB, CE, FCC, UL, CUL, TUV
Features Dual PCI Express connectors, 4 SATA connectors, and nine 4-pin power connectors, 5.25" Drive Bay embedded a Total Watts Meter Module to read power consumption
http://www.newegg.com/Product/Produ...53024&ATT=Power+Supplies&CMP=OTC-pr1c3grabb3r
Click to expand...
And this is overkill for this system.
It is also the same price as the OCZ PowerStream 520W & the OCZ is MUCH BETTER! ;)
 
davidhammock200 said:
I have a collection of old "industrial" IBM & DEC keyboards, they last forever! :p
Click to expand...

I can hear you all of the way over here in Tampa. CLICK CLICK CLICK CLICK..
 
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