Finally, A complete PSU Guide!

Question

Welcome Boys and girls. Throughout all my pleasureable experiance on this wonderful forum I have come accross quite a few knowledgable members who have gotten the wrong conclusion on how Power Supplys work. I have taken it apon myself to figure these things out and have found a very non speculative article that basically explains, thoroughly, how PSU's work. I have also got this information validated by an Electrical Engineer.
I will explain it to you, the way he explained it to me.
To start out, forget all the termonology or idea's you once thought about a powersupply. From now on you will never look for amount of wattage when determining your PSU. Wattage means jack.
Lets play pretend.
Lets imagine we have a large complex, in this complex is a suffistocated installment of pipes and water ways.
Amps is how much water pressure is in the pipes.
Volts is how big the pipe is.
Generally you will see less water pressure (amps) in a larger pipe (Volts)
(Defination of PSI) PSI = 1 pound per square inch = 6 894.75729 Pascals
Lets say this building has 100 psi in water pressure.
This building also has 50 water faucets.
For the time being we turn on 1 of those faucets. Does our water pressure drop?
Yes it does, but not by much.
Lets say we turn on 25 of those faucets. Our water pressure is diminished extensively and causes weird fluctuation in our pipes and water travels much slower.
Example, if one of those faucets are open water gets pushed out at a strong steady rate right?
However, if all of the faucets are on then water moves out much slower like a smooth stream or a small drizzle, as apposed to a strong spout.
Now, our building is only allowed 100psi of water pressure. It does not matter whether or not we have 25 faucets open, 10 faucets open or all 50. We will only ever get 100psi and no more.
Do we all understand?
The same thing can be applied to a rail on a powersupply. It works EXACTLY the same way.
Now I am going to explain to you why Wattage is a waste of time in measuring.
Instead of making a decision based on our Wattage we should be making decisions based on the amount of Water pressure in our pipelines! What was that? Did I loose you?
Let me re-illistrate. Again pretend a pipleine is a Railing, like a 12v Railing. Now pretend that the Amps are the amount of water pressure those lines have.
In the water world, the multiplying your water pressure times the size of your pipe will give you an idea of how much water travels a second. Wattage is mearly a measurement tool and really does not ease our decision making.
Let me re-illistrate again
(Pipe Size)*(Water Pressure) = (Water traveling per second in square feet)
(Voltage)*(amps) = Wattage.
Since now days components mainly run on 3.3v rail, 5v rail, and 12v railings we only need to worry about the amount of amps that each of these lines get allocated. We also really only need to worry about the positive railings as the negative railings usually will not apply to the average person.
Here are a few Examples.
Premeir 300w
3.3v * 15a = 49.5 WATTS
5v   *  29a = 145 WATTS
12v * 11.5a = 138 WATTS
TOTAL WATTS = (49.5 + 145 + 138) = 332.5
In this case the manufacturer rounded down and labled it a 300w. For that I praise Premeir.
CompUSA 250w
3.3v * 14a = 46.2 WATTS
5v * 15a = 75 WATTS
12v *  10a = 120 WATTS
TOTAL WATTS = (46.2 + 75 + 120) = 241.2
In this case the manufacturer decided to round up. Close enough I suppose...
The truth is many people are saying that an athlon 64 CPU will not post on anything less then 30A. That is very wrong and is a fallcy that people need not beleive. Others have said 18A is the extreme bare minimum however in many cases this also is not true.
You cannot simply say one device sucks all the amps out of a single line, other wise where would all your fans and other components draw power from? Since most of the stuff in your system all use the 12v rail we will analyze how much each device draws on that line. How much amps are needed for each specific device.
To simply post a motherboard, cpu, and Gcard your system doesnt NEED 30a // on a 12v.
My test lab consists of 5 of those CompUSA PSU's. We use those to pre-post customers hardware before they leave the store to ensure our hardware is good. 95% of all our sales are AMD, and at least 85% of those are Athlon 64's. I have yet to have any type of PSU problem. Even with a meir 10a to a 12v line.
In truth most athlon 64's range anywhere from a 5amps (Sempron 3100 not 64bit) to a 9amps draw (FX-55).
Thats right the FX-55 draws a meir 9a. Now I have never posted a FX-55( I would probably drool over it). No customers has forked out the cash for one of these yet. However We have sold a few 4000+, and all three of those worked fine on our 10a line.
Now remember this is simply a post. Meaning, only ram, CPU, and Gcard are included.
So lets add it all together.
Athlon 64 12v Requirements = 5a - 9a
Average Gcard Low to high .3a - 3.25a
Average MB draw              .3a
So to simply post a motherboard on the low end very minor 6 amps will do the job. On the flip side you may still need 13a. However that is still no where near our 18a assumptions. Now that is a simple post. The 4000+ cpu's I posted those on K8T Neo's using only TNT 64 Graphic cards which have a very low power consumption. The TNT 64 is our cheap test card, which is why we use them for testing!
So if your just doing testing for multiple posts on many different machines any 12a or higher amperage will be sufficient. From a troubleshooting aspect you may not necessarily need the heftiest PSU. From a home user workstation or system then I would invest in something above par for your system.
There is no point in investing hundreds of dollars to simply post boards. However when it comes to your home system or workstation your thought process should not be on saving money, but on investing it. Investing it in the reliability sense making sure your components are well handled.
You may ask why some of these low Amp PSU's have so many problems still, even with higher Amps?
Well there is still something that needs to be discussed when it comes to cheap power supplys. Also, simply because a power supply is expensive it does not mean it is not cheap!
Back to the ATX standards, any railing can go up or down at .5volts.
Again, ATX standards require PSU manufacturers to make sure their lines do not breach a .5 threshold. In some cases on cheaper PSU's I have seen the line be at the bear minimum of 11.5v or jump up to 12.5. Both cases are bad news.
That there, I beleive is inhirant from cheap PSU's. Lower voltage ,or to high, ends up bringing inconsistancy in the lines and an instable system. The system is getting to much power, or not enough.
That is why my 300w premeir PSU will post my K8T Neo board however my cheap 420w Enlight will not.
Now what about the average user? Well everyone is so different how much amperage do you need? And what about the other 3.3v railing and 5v railing?
It all depends on how many toys you have under that sweet case of yours!
Below is a spreadsheet of a very above average system. Comparing to yours I doubt you will be draining as much power as this!
Even with all of that he still barely breaches 18a on his 12v!
Now I would recomend for the average user to have at least 18a on their 12v railing. That is for 1 or 2 optical drives, 1 or 2 Harddrives, and a average graphics card on a 64bit system. For the enthuesiasts, better get 25a+. 30a is definately sufficient enough for any one whos not running 8 SCSI 320 15k rpm harddrives with a 6800 U, and a Athlon FX-55! Even with all that you wouldnt need much more Amps on a 12v railing.
Does that mean that if your system is unstable and you have less then 18a to rush out and spend MORE money on a new PSU? Absolutely not, but it is questionable. Your amperage level in comparison to the type of hardware you have needs to always be brough into questoin, or check your 12v railing and make sure its sitting at a consistant tolerable range at somewhere near, 12v. Anything above or below .5v I would start to worry. Most AMI bios's will show you your railings voltage in PC health section under Vcore.
On a side note, some PSU manufacturers have decided to put double 12v railings in an attempt to increase stability. I have yet to see stability improvments from these, in fact I have seen more problems with these PSU's. Often times the amps on them were much lower as well. However try it out at your own risk!
For more information I highly, HIGHLY, HIGHLY, recomend this article
http://www.bleedinedge.com/guides/psu_select/psu_select_01.html

Answer 1

Quote from: Tiresmoke on 13-February-05, 06:33:04
I did not make any refference to the 5v rail as simply put we do not use it as much as past systems. Nor did I state they did not use any. Please do not read stuff into what I said that was never intended.
All we have are words:
Quote from: Tiresmoke
Newer cards like the X800 and the 6800 series eat amps for a snack and they snack 24/7. in fact next to the CPU they take the most juice to operate. Boys and girls they use 12v period
As I demonstrated, the demand on the 5v rail has not significantly lessened in the last two video card generations.  The point I am trying to make is that demand on the 12v rail is not the be all and end all - just take a look at the loading table for efficiency measurements on the ATX spec.  In your effort to simplify a complex subject, you are (IMO) misleading those who aren't as educated on the subject.
Quote
Also note that I am not saying at any point that a system with just an A64 and say an X800 can't operate on 20a on that rail. I am saying that the greater the needs on that rail the heavier the PSU you will need. Count in Heat, efficiency and other factors and you may want to leave some overhead but I am not saying that 20a is insufficiant. So stop putting words in my mouth.
Huh?  Nobody mentioned the 20 amps figure before you did.   Similarly, the first mention of a X800 Pro in this thread is in your post.