How good is your particular PSU?

Hoot · Jan 31, 2002

I see a lot of posts from people wanting to know is their particular make and model of PSU is any good. I have a simple "Gold Standard" for assessing a PSU. It involves looking at the specification sheet that came with it, is printed on the side, or is at the manufacturers Web Site. The specification that tells the most about your PSU is the Combined +3.3V & +5V maximum output power. First off, if your unit does not have that specification, it is instantly suspect. This is the equivalent to buying a car that does not have a specification of the Brake Horsepower for the engine.

So here's the magic numbers to compare to.

If your unit claims to be:

300W, the Combined 3.3 & 5V output should be 150-160W
350W, the Combined 3.3 & 5V output should be 180-185W
400W, the Combined 3.3 & 5V output should be 220-245W
500W, the Combined 3.3 & 5V output should be 270W

If your PSU falls short on this specification, you need to question what other specifications does it falls short on.

73, Hoot

CrystalMethod · Feb 1, 2002

It's been a while, what's the formula to get watts again? Is it "V*I" ? i'm usually working with just straight amperes, volts, and ohms, and rarely have to convert to watts. So, I still get the formulas mixed up.

Hoot · Feb 1, 2002

That's correct. P=IE. You can not simply do that math on the max values listed on the side of the PSU. If you did that for all the specs, you would see your total wattage way in excess of the PSU specification. Those four maximum current specifications are based upon not drawing much, if any current from the other three outputs.

Keep in mind that if each output had its own dedicated components, all the way back to the AC plug, then the power consumed by one output would not impact the other three and the PSU would cost considerably more and be physically much larger. As it stands, in PSUs, the +3.3V, +5V and +12V share many components.

Metaphorically, think of the plumbing in your house. All the loads (spigots) share some common paths the further upstream you look. Just about everyone can relate to what happens when you are taking a hot shower and someone in the house flushes a toilet, or starts the washing machine.

PSUs behave the same way. Better PSUs use "larger pipes", so when you turn on one "spigot" it has less impact on what is reaching the other "spigots".

Hoot

donny_paycheck · Feb 2, 2002

I know some about electronics and I've done quite a bit of work on switching power supplies (high voltage) before so I was confused when drawing a lot off of the 5 volt leg drops it down and raises the 12 volt leg. I thought they were both regulated, as well as being independent of each other all the way back to the power transformer. This obviously isn't the case or else this wouldn't be happening. Anybody know why this is?

Hoot · Feb 3, 2002

It keeps the cost down.

Hoot

TigerClaw · Feb 5, 2002

re

only to see if I have understand (my english is not very good)

My Power Supply:

3.3 -- 15A
5 -- 32A

Watts: (3.3 x 15) + (5 x 32) = 210.5?

Is this correct?

CrystalMethod · Feb 5, 2002

I just wanted to know as a refresher. The only real way I'll be able to calculate how close I am to maxing my PSU's is to rig an ameter (<-- I know the spelling is off, it just looks wrong), to the PSU's an see how much they draw at full/peak load. Would saying "I haven't had a problem, with them so far", suffice? in order of my sig, these are the PSU's I'm using.

1- 300W Enermax
2- 300w Aopen
3- 235W PowerMan

Hoot · Feb 5, 2002

Results speak louder than specifications. If they are not giving you problems, then don't worry about it. The thread is mainly to help people choose a PSU when shopping around, or to evaluate whether the one they have is on the "suspect list" when they are experiencing an otherwise unexplainable problem.

Hoot

Stick00 · Feb 5, 2002

well i just got a future power 400w.>>>> not sure on specs have to got to the site and chek....should be here tommorrow.I hope it works good . eheh

Hoot · Feb 7, 2002

I bought one as a backup. It's not very good. At core voltages above 1.95V the +5 in it starts to collapse. Not bad if you don't intend to overclock.

Hoot

customrig · Feb 7, 2002

mines pretty bad no name 350 watt

ButcherUK · Feb 8, 2002

Just got a quietpc.com 300W psu to replace my enermax (which is +0.6V on 5V and still at 4.78V), rated at 220W 3.3V&5V combined

donny_paycheck · Feb 8, 2002

The 550 watt Enermax (EG-651P-VE) I use keeps the +5v at 5.05 and +12v at 12.4-12.5 volts and it doesn't vary when I change the motherboard voltage settings either. Plus it puts out up to 24 amps on the 12v line so I can run an 80 watt TEC with it.

Monaco · Feb 9, 2002

According to the label on my Power Man 250watt PSU, it's combined 3.3 and 5v is 177watts.

Should I believe it? Seems pretty high for just a 250 watt PSU.

Hoot · Feb 9, 2002

You're right, that does seem odd. That leaves only 73 watts for the +12, -12 and -5. Either they are very conservative in their overall wattage rating or just didn't design that PSU to pump much power to the +12V. Bottom line is, if it works, don't sweat it.

Hoot

eobard · Feb 9, 2002

To paraphrase the #109th rule of Acquisition:

"A Cyberzone 300w power supply and an empty sack is worth the sack".

ButcherUK · Feb 10, 2002

BTW to people wondering.
Taking the 3.3V max amps and 5V max amps and multiplying then adding does not give 3.3V+5V combined power.
Here's an example, my enermax:
3.3V, 30A
5V, 32A
Gives 3.3*30+5*32 = 259W GREAT!
Or not so great, max is actually 160W.

This means at 32A on the 5V you can't actually use any 3.3V power and still be within spec.

Hoot · Feb 12, 2002

If your PSU falls short on this specification, you need to question what other specifications does it falls short on.

In case you have not gotten a PSU, or if you're planning on a new one, here is an excellent link to see the specs on many different brands of PSU.

http://www.pc-beratung.net/Download/Power.pdf

Hoot

Starfoxer · Feb 18, 2002

my 320W PSU is 3.5+5=160W peak output. this seems a bit lower
than i'd like.

Ozzman · Feb 19, 2002

Hoot said:
I bought one as a backup. It's not very good. At core voltages above 1.95V the +5 in it starts to collapse. Not bad if you don't intend to overclock.

Hoot

does this mean i should not even consider a vmod for my MOBO without a new PSU??

well that blows

o well.. no cpu i guess would be better..

How good is your particular PSU?

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