FAQ:  PSU FOR ECS K7S5A MOTHERBOARD (3)
Jiggs 22 Dec 2001

6.  What about the +12V amp rating?  Isn’t this important too?

The +12V rail unlike the +3.3V and +5.0V rails is more tolerant when it comes to regulation.  The +3.3V and +5.0V rails are required to be regulated with +/- 5% while the +12V rail is /-10%.  The +12V rail output is used for motor driver components of the System like HDD, CD drives and fans.  

7.  My system worked perfectly for almost a week but now it’s showing symptoms of a low           powered PSU, could it be possible my PSU is underpowered?

PSU’s depending on the quality it’s made could degrade while in used.  A marginally satisfactory PSU can show symptoms of an underpowered system as early as the next boot after installation or as long as one year.  When PSU’s are placed into use, a minute change occurs with respect to their capacitance and inductance.  This change results in increase to the total impedance of the PSU, which will affect its total power output.  This can be noted in the change in the +3.3V and +5.0V rail outputs.

8.  How can I tell my PSU is underpowered?  

 

The most obvious indications will be the output readings of the +3.3V and +5.0V rails. ATX 2.01 specification requires this rails to be within +/- 5%.  At the low end of the spectrum, the minimum reading for the +3.3V rails should be no less than +3.135V while the +5.0V rails is +4.75V.  The fool proof to find out if the PSU is underrated is to bare the system.  Bare means to disconnect all the peripherals with the exception of the video car, and a stick of ram,  Boot the system and it should at least post.  If the system post then add a HDD.  If the system post with the addition of the hdd continue adding other peripherals while booting along each step.  Anytime, the system failed to boot after an addition of another peripheral this is a good indication you are overloading the PSU.

9.  But my bios or MBM readings on the +3.3V and +5.0V rails are within the ATX spec of      +/-5%, yet my system fails to boot when all peripherals are connected.  I can only boot after pushing the reset button.  Why?   

Your PSU is marginally operating within the power requirement of the system.  When a system boots, there is this so called “Peak Power Load” wherein the highest power requirement of the system is exhibited.  Usually, this happens during a cold boot.  During the cold boot process, all systems especially the HDD and CD Drives require a lot of power to get started.  In addition, the AGP and also the case fans are at their maximum power requirement state.  This peak power load puts a lot of strain on the PSU.  If the PSU is just marginally meeting the power requirement of the system in its normal state, during the peak power load it’s voltage output will sag (droop) thus starving the CPU and probably the rams of power for a few milliseconds or maybe longer depending on the quality of the PSU.  If the PSU output will sag long enough, it will trigger a safe cut off of the protective logic circuit in the mobo.  This logic will shut off the power to the mobo components until the power is restored to an acceptable level.  If this shut off stays long enough, it could actually cause the system not to boot properly.  When you reset the system, the hdd and cd drives and fans are still running and will not require a peak power load which will put less strain on the PSU.  In this case, the mobo will receive all its required power and boot properly.  The voltage sag of the PSU during boot up cannot be read with a regular multi-meter since it’s just a transient change but it can be seen with the use of an Oscilloscope if measured at the right point in the circuit on the mobo.

10.  My PSU has an input power rating of 115V/60Hz; can I use it with a 220V/50Hz power?

Most PSU’s have a selector switch on the back to select the proper voltage of the localities. Usually the selector is either 115V or 220V. All PSU’s are rated to operate with the 60Hz and 50 Hz power. If your PSU does not have a voltage selector switch, then you either use a transformer to change the voltage from the wall outlet or exchange it with one that has a selector. Beware, other PSU’s do not have selector switches but they can operate automatically with all types of power source from 100V to 240V both in 60hz and 50hz. This type of PSU has the so called auto voltage feature in them. This is explicitly written on the label of the unit.

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