POWER SUPPLY HELP

dqbmethod · Jul 11, 2007

I have a question aobut powersupplys. What makes a good powersupply? and whats the differance between 3x 18 a 12v rails and 1 18a 12v rail and the other 12v rail being .5.

Can anyone claify what makes a good psu and why. Thanks

badge · Jul 11, 2007

12 volt amps are the key to having a suitable PSU. Three 12 volt rails at 18 amps each gives your system, namely your dual core processor and video card(s) 18a x 3 = 54 amps of power to work with. A single 12 volt rail at 18 amps will power a much less system (CPU and Video card). 54 amps vs. 18 amps? Easy to see. The other -12 volt rail at .5 amps is standard pretty much. The more amps you have on the 12 volt rails, the more power your CPU and video card in particular have to draw from.

Onus · Jul 11, 2007

badge :

Yes and no. You will not get 3x18A=54A on three rails. Combined, the total is likely to be considerably less, perhaps 40A-45A. Look for an indication on the PSU sticker. The 18A rating is the maximum that can be pulled through that particular rail.
These are the +12V rail(s) btw. The little one, perhaps 0.5A, is the -12V rail. I think serial ports might be about the only thing left that uses that rail.

Similarly, there's a +5V rail that may be rated at a relatively high 30A or so, and a +5vsb rail, which is the standby voltage. A few mobos reference a -5v rail (typically 0.3A-1.0A), but many PSUs don't provide it any more. I think it may have been used by floppy controllers a generation or two ago. I have read that some mobos that need it (e.g. some Abit) may actually be unstable without it, but others don't care.

dqbmethod · Jul 11, 2007

Ok so when im shopping for PSU's i should stick with ocz or thermaltake? Are there any brands to sick away form and why are they bad psu's?

badge · Jul 11, 2007

Onus :

3 x 18amps = 54 amps. 40-45 amps on a PSU rated 18 amps x 3 12 volt rails is incorrect. Are you talking about efficiency? That is important. But saying 18a x 3 = 40-45 amps is incorrect.

badge · Jul 11, 2007

dqbmethod :

What are your system specs? CPU and video card in particular?

Onus · Jul 11, 2007

badge :

No, I'm saying that most PSUs with multiple +12V rails will say that any rail can provide up to 'I' Amps, but the total of all of the +12V rails cannot exceed 'Y', where Y < I * (number of rails).

Onus · Jul 11, 2007

dqbmethod :

The PSU lists I have seen include OCZ and Thermaltake among their quality brands. FSP, and Corsair will also be there, and Seasonic, Mushkin, and PC Power and Cooling. There are others. A PSU thread here and/or other technical forums will probably have the lists. Abit's (mobo mfg) Forum has such a list as well; I'm sure there are others.

Brands to avoid include anything that comes with a [cheap] case, Rosewill, some say Aspire (mine never failed), and some say Ultra. A quick rule of thumb, if you can lay hands on it, the heavier it is the more likely quality components were used.

dqbmethod · Jul 11, 2007

Ok i have a :

amd 3500+ 939
2 gigs of ocz pc3200
7600 gt xxx
2x 160 gb sata 2
2 cold cathodes
Fan contoller and a blue led fan
480 watt thermaltake psu
All running on a k8n neo4 board

I was thinking of upgrading my 7600 to a x1900xt or a x1950xtx
but im thinknig im gonna need more power. Could anyone recommend a power supply that would fit me? Also maybe leaving room for a 4400+ 939 for future upgrades. Thanks

badge · Jul 11, 2007

Onus :

I have 25 or more psu and none of them state on what you are inferring on the unit.

badge · Jul 11, 2007

dqbmethod :

The 7600Gt uses 19/20 amps. A 1900 uses considerably more. If you can find a 500/600 watt PSU with 22 amps x 2 or 44 amps total on the 12 volt rails you should be fine. You probably can't beat the price on this one. I do not own it:

http://www.newegg.com/Product/Product.aspx?Item=N82E16817701004

mike99 · Jul 11, 2007

Have seen 420W PSU suggested for that card, so you should be OK. See if you can obtain a Power meter. In UK, Maplin sell very good model. My system only draws 275W at max. (have 420W PSU)

badge · Jul 11, 2007

here is one list from another forums.

http://www.xtremesystems.org/forums/showthread.php?t=108088

a good and adequate power suplly is worth it. a year and a half ago i bought my seasonic but recently i have been having some problems with my picture in games when in crossfire mode which i believe is my PSU.

whilst not quite the same as your situation it is a good lesson in that you need to leave yourself a bit of head room in both watts and amps. i think i have been pushing it since august last year when i bought my second gfx card.

Thanks Stranger. Nice.

Onus · Jul 11, 2007

badge :

The very PSU you link below has that on it!
http://www.newegg.com/Product/ShowImage.aspx?Image=17-701-004-11.jpg%2c17-701-004-02.jpg%2c17-701-004-04.jpg%2c17-701-004-03.jpg%2c17-701-004-06.jpg%2c17-701-004-05.jpg%2c17-701-004-07.jpg%2c17-701-004-08.jpg%2c17-701-004-09.jpg%2c17-701-004-10.jpg&CurImage=17-701-004-04.jpg&Depa=1&Description=MSI+TurboStream+600W+ATX+12V+V2.2%2f+EPS+12V%2f+BTX+600W+Power+Supply+90+-+264+V+Safety+%2f+EMI+Approvals%3a+FCC+certification+-+Retail

It lists each +12V rail as 22A, but then has a max of 480W. 12V * 22A = 264W; 264W * 2 = 528W, but the max is only 480W.
Furthermore, as is often the case, when the loads on the +3.3V and +5V are considered, there are additional maximums across all these rails.

badge · Jul 11, 2007

Onus :

Each 12v rail has 22amps max. That's right. It doesn't mean, as you are trying to insinuate, that 44amps means anything other than 44 amps. Why do now quote watts (12v x 22a = 264 watts, but the max is only 480 watts. 480 watts on the 12 volt rails? Sure, the psu is rated at 600 watts. There are other voltages than just 12v on the psu. Obviously. I think you have your figures somewhat mixed up. You can refer to efficiency and make some sense, but to say a PSU with 18amps x 3 12 volt rails + 40 to 45 amps is incorrect. Where do you come up with that figure? Obviously you do not know more than the manufacturers building psu. So what exactly is your point? You think the mfg.'s are misrepresenting their products? I don't think you know as much as you proclaim to know. Giving information like you are giving is misleading in the least.

RetiredChief · Jul 11, 2007

Jtt283 vs Badge - Jtt283 Winner. Just Kidding,
Just looked up specs for a Thermaltake 600 W. (a Good PS) They show 4 12 Volt rails @ 18A (18A x 4 x 12V = 864 Watts - Hey the PS is only 600 W) The limit for all 4 +12 rails is only 48 A. This is further componded; This would only leave 24 W for the +3.3V and +5V rails. So the real life max +12V Current is less than the 48 A.

Very few PS labels show relationship between +3.3, +5 to the +12 V.
Did run across one that, inaddition to MAX +12V Power, also reflexted Which +12 V rail, and Max pwr for the combo.

Criminal89 · Jul 11, 2007

Onus :

Ah yeah, jtt283 is right on this one badge. You need to go ahead and realize that he knows more about what he is talking about then you do. Anytime you are dealing with a multiple rail power supply, you are going to have voltage and amperage drops across the rails.

badge · Jul 11, 2007

RetiredChief :

Jtt283 said: quote: 18 amps x 3 12v rails = 40 to 45 amps. I say 54 amps available is telling the OP the correct information.

badge · Jul 11, 2007

Criminal89 :

So could tell jtt283 how to explain what you claim? Can you explain how 18amps x 3 12volt rails = 40/45 amps?

Criminal89 · Jul 11, 2007

badge :

The following is quoted from an article I came across a few months ago:

So what's all this rubbish about multiple 12 volt rails?
If you've paid much attention to current (2006) power supplies then you've probably noticed that most of them have more than one 12 volt rail. A conventional dual rail ATX12V power supply has two 12 volt rails: 12V1 and 12V2. According to the ATX standard, 12V2 is the 12 volt rail which powers the CPU and is provided by the 4 pin 12 volt cable. 12V1 is the 12 volt rail used in all other power supply cables and powers everything but the CPU. A few motherboards don't follow the ATX standard on what is powered by 12V1 and 12V2. EPS power supplies can have as many as four 12 volt rails and have many combinations of which rails powers which devices.

If a PSU needs more 5 volt power then they just build a higher-capacity rail which can supply more current. So why do you see power supplies with two, three, or even four 12 volt rails? Why not just have one bigger 12 volt rail which can supply more power? Well, that's going to take some explanation.

I used to design embedded electronics which are small computers which control various kinds of machines. I still build hobby projects from time to time so I have loads of power supplies laying around. Of course, most of those are "real" power supplies - not PC power supplies. Okay, technically PC power supplies are actually real but since they come with such incomplete specifications it's hard to know what they can really do. Real power supplies tell you exactly what the PSU can do: input voltage range, minimum and maximum current, load regulation, output ripple, temperature derating curves, over voltage and current limits. You name it, they spec it. And when they say 12 volts at 40 amps at 50C, they're not kidding. At least as long as you avoid crappy ones. If the PSU has multiple outputs then they explain all the dependencies between them. So if you need to increase one rail to 10 amps to get 20 amps out of another rail, they always tell you in the specs. They tell you if there's a total wattage limit among combinations of rails. If it's a really nice PSU, there are no dependencies. They just operate as independent rails. The specs are very thorough because you need to know these things to select the right PSU.

And then, there are PC power supplies. Most PC power supplies, even many good ones, would be more truthful if they stopped refering to "specifications" and used the term "marketing hokum". I'm not going to delve into that subject here because it would involves pages and pages of cussing. And if you're looking for a PC PSU which doesn't have dependencies between it's rails, keep dreaming. They've got dependencies. They just rarely tell you what they are. If you get a good PSU then it may actually meet the vague and incomplete specifications on the label. If you get a bad PSU then the wattage ratings on the label can best be described as a work of fiction. PC power supplies actually do have real specifications. They just don't publish them. So when you buy a PC PSU, it's hard to know what you've really got.

Multiple current limited 12 volt rails derived from single rail PSU
This kind of PSU only has one set of circuitry inside the PSU which generates 12 volts. But it is split into separate 12 volt outputs each of which have their own current limit circuitry. If any one of the 12 volt outputs exceeds its current limit then the PSU shuts down. For example you could have a dual rail supply which has a single internal 12 volt rail which can deliver 30 amps. Then inside the PSU it's split into two separate rails each of which has a 20 amp limit. If you try to draw more than 20 amps from either of the 12 volt rails then the PSU with shut down. If you try to draw more than 30 amps of total current from both of the rails then it will also shut down (assuming that the internal 12 volt rail also has a current limiter).

badge · Jul 11, 2007

dqbmethod :

OP as I stated from the beginning, the amount of AMPS on the 12 volt rail(s) are the key to having a power supply good enough to run a dual core CPU and a high powered video card(s). Look to the brand name units as opposed to a cheaper no name unit. Personally, I wouldn't risk my expensive system components with a cheaply constructed/manufactured PSU. As far as your TT 480 watt psu, it's the amps that come into question, not the watts regarding you upgrade plans. I will add, I think the major manufacturer listed product specs regarding the specifics of their PSU's capability are generally more accurate than not. HTH.

Onus · Jul 11, 2007

On the 2-rail PSU you listed 2 * 12 * 22 = 528, but the max was 480.
480W / 12V = 40A, not the 44A you'd get from 528W / 12V.

If it seems I'm jumping around between Amps and Watts a lot, that comes from the second equation in Ohm's Law: Power in Watts = Voltage in Volts * Current in Amps.
(The first one is Voltage = Current * Resistance)
PSU labels can be confusing because they often list outputs in Amps but maximum limits in Watts (especially as multiple rails may be involved).
An analogy could be your house wiring. You can plug a 1500W space heater in to either of two outlets (1500W/120V = 12.5A), but if you plug one into each outlet, the 25A combined load pops your 20A circuit breaker.

badge · Jul 11, 2007

I might go along with the idea that multiple railed psu comprise total amperage available in some further calculated manner. I always look to the 12v rail amperage when buying a PSU. Efficiency, active pfc and price certainly make a difference in my consumer habits. As Mike9 stated, many of my systems use only 225-280 or watts, so 12v amps availabe are the key for me.

Onus · Jul 11, 2007

Example: A video card may suggest that it requires a 400W PSU. This is based on the assumption that on a typical 400W PSU, sufficient amperage is available on the +12V rail(s). It would be much nicer if video card manufacturers stated instead that they needed a certain number of amps. The problem is, while we can figure all that out, the yard-ape playing WoW might know he has a 350W PSU, but nothing about rails and amps.

RetiredChief · Jul 11, 2007

Very good Qoute Criminal89. I think that what I satated in my short post Is explaned by your post.
In case you want to question my Knowlege - Fair to good on computers - But on Electronics, Very well qualified!

My credentials are as follows:
Have taught circuit analysis and troubleshooting ofseries regulated, and parallel (Shunt) regulated low and high voltage P/S.
Was Head of the electronics dept. of a community college.
In 86 I went to work a NASA as an electron tech to work with the design engineer for Space Flight qualified Power supplies.

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