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Question about watts, volts, and amps (oh my)

Last response: in Components
March 18, 2003 8:19:47 PM

Hello all,

I just realized that I don't fully understand the whole watts, amps, volts thing. For example, my system is:

Athlon XP 2700+
Asus A7N8X Deluxe mobo
512MB PC3200 Corsair XMS
GeForce4 ti4600 128MB
Lite-on 52x24x52 CD-RW
Western Digital SE 80MB 7200RPM HDD
Samsung 1.44MB FDD
5 80mm quad-LED case fans
one cold cathode
350W Enermax PSU

Is this a good enough power supply? I have had no issues thus far (about a month), but I'm starting to worry. I see many posts on this board with people stating their specs and asking for a PSU recommendation and it seems as if everyone always recommends 430W or more.

Also, how do watts, amps, and volts relate to one another? I thought it was simply watts = volts * amps but, after reading the specs on my PSU, I don't know if that is right. My PSU claims it puts out 32A on the 3.3V rail, 32A on the 5V rail and 25A on the 12V rail....however (3.3 * 32) + (5 * 32) + (12 * 25) = a whole hell lot more than 350W. So what's the deal?

Any help is appreciated.

<P ID="edit"><FONT SIZE=-1><EM>Edited by mister_e on 03/18/03 05:21 PM.</EM></FONT></P>
March 18, 2003 8:35:02 PM

First if your system is up and stable don't sweat it your PSU is fine.

Most people tend to recommend over what's actually needed for later upgrades. Even then typically things are overstated (ie a good 300 watt PSU can power a pretty damn good system).

Actually you are correct amps x volts = watts. However on standard ATX PSU's there are 2 wattage caps you need to pay attention to. The most important is the +5v and +3.3v combined output (150-250 range typically). There should also be a combined +5v, +12v and +3.3v which should be higher (I don't recall seeing an instance where the two have been the same).
The first combined output is the really big one. Normally te +5v, and +3.3v share the same output transformer. This means if one is being taxed the other can't give all the power that the label states (ie if 150W is the peak output, and your +5V takes up 125W, that only leaves 25W for the 3.3V). Most people who have any idea of what they're talking about, will say to check this number more so then the total wattage of the power supply.

March 18, 2003 8:46:22 PM

Thanks for the help AntecRep. Do you really work for Antec? :) 

Also, does anyone know of a site that lists what parts use which PSU rail? For instance, I know fans typically use the 12V rail, but what about other parts such as CD-RWs and hard-drives?
Related resources
March 18, 2003 9:07:03 PM

Ok, looks like you got a good answer about the PSU question... If it's working, don't sweat it.

Now for your question of how Volts Amps and Watts relate... A bit of basic electronics.

Think of voltage as the pressure that drives the current... just like the pressure in a water pump... 0 pressure means nothing happens. Apply the right voltage and things begin to happen... current flows, work is done, a function can be performed.

Amps are a measure of how much current is flowing as a result of applied electomotive pressure (voltage), the electrical equivalent of gallons per minute in a water pump.

There is one more element: Resistance. This is a measure of how easily the voltage can push current (amperage) through a given circuit. 0 resistance amounts to a dead short so every working circuit that does anything but blow your powwer supply needs to have some resistance. Continuing the water analogy, this would relate to the effect of a tap used to regulate water flow.

In any electric circuit, applying Voltage causes amperage in proportion to resistance.

This is called "Ohm's Law":
E == I x R
R == E / I
I == E / R
Now whenever voltage is applied and current flows, work is done. This work is measured in Watts.
P == E x I
E == P / I
I == P / E

The symbols used above are standard electrical symbols:
E = electromotive pressure in volts
I = current flow in amps
R = resistance in Ohms
P = Power in watts

So... now you have the relationships... for future reference.

--->It ain't better if it don't work<---
March 18, 2003 9:25:58 PM

You might find this link interesting:

<A HREF="" target="_new">How Much Power Do You Need?</A>

And this <A HREF="" target="_new">one</A>, which illustrates the typical specifications for a 350W and 460W Enermax PSU.

What you have to be aware of is that the +3.3v and the +5v rails <i>share</i>, which means, in layman's terms, that components that draw too much power from one can limit the amount of power available for the other. This is why these two rails are shown on the spec sheet combined, right above the TOTAL O/P.

Pay particular attention to the overvoltage protection and the efficiency level. Preferably, you need the rails to stay within 5% of the optimal, and the total load should be around 70-72% for the best performance, or the PSU might overheat.

Enermax makes an excellent PSU, and I don't forsee any real problems with your system, but be careful how much you draw on that +12 rail with all those LED fans and the cathode tube. You don't want to cause the hard drive damage due to a lack of power. I generally build gaming systems, and the last thing I want to do is run out to a site and replace a PSU because the user decided to add a few extra components without regard for the power plant that drives the system.

Personally, I just slap in the 550W models when I build a new system, and be done with it. Power supply problems can be a real headache to diagnose and troubleshoot. And it's no fun to replace components that were damaged due to a lack of clean power.

Take a look in your BIOS to check out your current voltages ... many mainboards offer this feature. If your +12v rail is out-of-spec, you might think about getting something more powerful.


<font color=red>First Rig:</font color=red> <A HREF="" target="_new"><font color=green>Toejam31's Devastating Dalek Destroyer</font color=green></A>
<font color=red>Second Rig:</font color=red> <A HREF="" target="_new"><font color=green>Toey's Dynamite DDR Duron</font color=green></A>

<A HREF="" target="_new"><b><font color=purple></font color=purple></b></A> - <i><font color=orange>A better place to be</font color=orange></i>. :wink:
March 18, 2003 10:55:22 PM

Thanks guys for all the info/help :) 

I think I'm OK. I've been watching the voltages via Motherboard Monitor 5 and they all operate within the 5% threshold. My only concern now is that the 12V and 5V rails seem to be performing "just good enough". The 5V rail averages around 5.75-5.8, and the 12V rail averages around 12.49-12.55. These numbers are just at or barely above the 5% threshold but they do appear to be stable. I may unplug the cathode until a (possible) PSU upgrade. And I definitely won't be getting any more lights (as I originally planned) any time soon. But for now, at least, all seems "good enough".

Thanks again for all the help.
March 18, 2003 11:05:31 PM

If your 5 volt rail is at 5.75 that's way over the Plus or Minus 5% spec!

In fact it's high enough that it may damage your system. To be within specs, your 5volt rail should be between 4.75 and 5.25

I <i>strongly</i> suggest you get that checked by a technician with a reliable voltmeter. If it's not a reading error in MBM, you need to put in a new power supply.

--->It ain't better if it don't work<---
March 18, 2003 11:32:54 PM

Oops. Major typo. The 5V rail is around 4.75-4.8 and the 12V rail is at around 11.49-11.55.
March 18, 2003 11:36:37 PM

Lucky you!

Had me worried for a second there :smile:

The corrected voltages are fine.

--->It ain't better if it don't work<---
March 19, 2003 12:26:26 AM

Heh. I didn't think 5.75 was too high, I just thought smoke was SUPPOSED to pour out of my it character and a nice smokey flavor :wink:

Thanks again for the help.
March 20, 2003 1:47:36 AM

Those are very good numbers. Anything under 6v on the 5v line and under 13.5v on the 12v line should be fine. You'll hear arguments to the contrary, but I've seen systems take a crap at slightly low voltages, but run for years on somewhat high voltages.

<font color=blue>Watts mean squat if you don't have quality!</font color=blue>
March 20, 2003 1:18:09 PM

I'm guessing you didn't see my next-to-last post, I explained that I fat-fingered the numbers and that my voltages were in fact 1 lower than the ones I posted. But thanks anyways :smile:
March 20, 2003 1:52:30 PM

You're right, I didn't see that one.

<font color=blue>Watts mean squat if you don't have quality!</font color=blue>
March 20, 2003 3:05:31 PM

It is nice to see someone else agree that a 300W PSU with amps nicely spread over the various lines is more than enough.

<font color=purple>* Bum_JCRules cheers</font color=purple>

<A HREF="" target="_new">THGC meeting? Would you go if there was one?</A>