power supply question

[EEVIAC]

Say you have a power supply with a 12 Volt rail with 15 amps max., rated at 300 W
and the hardware in your computer has used all the 15 amps available on that rail.  So you go get yourself another power supply that is 350 W, but the 12 Volt rail still only offers 15 volts max.  Will the power supply work?   

In other words, should you consider the individual voltage rails when buying a new PSU or the total wattage of the supply?

[Calum]

Individual amperage on each rail is very important.  In your example, the 350W unit offers more power on other rails, and will be no better than the 300W unit if what you need is more power on the 12V rail.
It's a common situation, cheap PSUs often supply more voltage on the 3.3V and 5V  rails which aren't used as much nowadays so that the total wattage is larger, which fools people into buying them.
When buying a new PSU, the most important thing is the reliability and quality.  This can be determined by first checking the manufacturer and OEM, and by reading reliable reviews of the unit.  Customer reviews don't count for much.

Also, welcome to the forums.

[EEVIAC]

Thanks that helps

[EEVIAC]

Actually that brings me to another question....  When isolating a problem to a potentially bad power supply, why is it often recommended to swap the current power supply with another of "equal or greater" wattage...  Shouldn't it be "equal or greater" amperage, putting emphasis on the individual rails? 

[Calum]

That's a fair question, it could be because the average user would not know what their PSU's amperage is or how much they would need.
However, if the user is swapping their PSU anyway, you could assume they would know those facts.
Perhaps the answer is because it's easier to say?  Also, it could be assumed that the PSU being swapped in is of equal quality to the one it is temporarily replacing, and would therefore be able to power the same components.
"Equal or greater amperage on all rails and an equivalent quality OEM" would be the most correct thing to say but is cumbersome and possible not necessary.

[EEVIAC]

"Equal or greater amperage on all rails and an equivalent quality OEM" would be the most correct thing to say but is cumbersome and possible not necessary.

yah I see what you mean....  thanks again

[patio]

Shouldn't it be "equal or greater" amperage, putting emphasis on the individual rails? 

Plus if you state it this way you will be answering endless questions such as

How do i find out he amperage on my PSU ? ?

What is a rail and where are they ? ?

[Computer_Commando]

Actually that brings me to another question....  When isolating a problem to a potentially bad power supply, why is it often recommended to swap the current power supply with another of "equal or greater" wattage...  Shouldn't it be "equal or greater" amperage, putting emphasis on the individual rails? 

Since most don't understand watts, volts or amps, why explain 3, when 1 will do?  And of course, more watts is better.  But is more amps better?
BTW, using the phrase "current power..." together really confuses things (current=amps, power=watts)
Better to say "existing power...".

[EEVIAC]

BTW, using the phrase "current power..." together really confuses things (current=amps, power=watts)
Better to say "existing power...".

good point

[Gizmologist]

The term"rail"  refers to the power supply buss connections for the 12 volt, 5 volt,and  3.3 volt. Each buss (connection point) from the power supply is referred to as a rail.

Just think of the "third rail" on subway trains. That is where the 600 volts DC is supplied to the motors.

[EEVIAC]

Another question just came to mind...   I've got this PSU, "300W Output" ....

+5 Volts,      1.5 amps min.    30 amps max.

+12 volts,   .8 amps min.     15 amps  max.

+3.3 volts,      .5 amps min.    20 amps

-5 volts,           0 amps min.    .5 amps max.

-12 volts,         0 amps. min.    .5 amps max.

+5 volts SB       0 amps min.     2 amps max.


How to they derive 300 Watts from this?

[Computer_Commando]

Watts=Volts * Amps

But...
No PSU can produce maximum amps on all voltages simultaneously.
For example, for your PSU rated at 300W:
+5 * 20 = 100
+12 * 10 = 120
+3.3 * 24 = 79
100 + 120 + 79 = 299W
Or...any combination that totals 300W or less.

[EEVIAC]

ahhhh, illumination 

thx, sir