FAQ:  PSU FOR ECS K7S5A MOTHERBOARD (2)
Jiggs 22 Dec 2001

1.  Introduction:    

This FAQ is intended for the purpose of establishing a reference guide on what Power Supply Units (PSU) are compatible for the ECS KT7S5A motherboard.  It’s NOT a comprehensive discussion on all the types and brand names of PSU but a discussion on what ratings you will be looking for in a specific PSU.  This FAQ does not in anyway or form makes any recommendations or in opposite thereof to any specific type or brand of PSU.  Information provided is based solely on personal experience and also from other experiments conducted by forum members. 

2.  PSU Rating Defined:     

PSU are rated by the total amount of power they can handle or provide without causing damage to its internal circuits.  Power is measure in the unit of Watt.  Represented by the letter “W”.  Watt is defined by the formula: W= I X E; where in the letter “I” represents the current (measured in Ampere or Amps) in the circuit while the letter “E” represents the total Voltage (measured in Volts) in the circuit. 

There are 5 voltages output on a typical PSU:  +3.3V, +5.0V, +12.0V, -5.0V and the –12V.  The most common used voltages are the +3.3V, +5.0V and the +12V outputs.  Outputs are also referred to as “rails”.  The outputs are provided on more than one wire.  Actually, all of the outputs are provided in sets of wires.  But we will discuss that later.  The total watt rating of a PSU is taken by adding all the product of each output rail multiplied by the rail current (amperes) rating.  For example: 300W X-brand PSU has the following output rating:

 +3.3V=14A (46.2w)

             +5.0V=22A (110w)

+12V=9A (108w)

-5.0V=0.5a (2.5w)

-12V=1A (12w)

+5.5Vsb=1.5A (7.5W)

***+5.0Vsb is the standby power provided by the PSU to the motherboard.  This is only applicable on ATX type PSU’s. 

Adding all the wattages, the total comes up to 286.2Watts.  The other 13.8 watts is the power loss due to impedance or total resistance of the circuit.  There will be no such thing as 0 watt loss.

There is another important thing to know about PSU rating with regards to their individual rail output.  On other PSU the +3.3V rail could have two numbers on it like 10.6/28 and the +5.0V will have 40/28.5.  These two numbers represents the total of power you can load the rails at the same time.  Meaning if the +3.3V rail is loaded at 10.6A or less then the +5.0V rail can have a maximum load of 40A or if the +3.3V is loaded at 28A then the +5.0V rail is rated at a maximum of 28.5A.  The table below will show this relationship: 

Note:  The total combine power of the +3.3V and +5.0V is 235W.

When +3.3V is loaded to 10.6A then +5.0V output limit is 40A

When +3.3V is loaded to 28A then +5.0V output limit is 28.5A

ATX 2.01 specs require 5% regulation on the 3.3 and 5.0 volt rails.

One important thing to note on the above table is the Total Combine Output (TCO) of the two rails, which is equal to 235W.  Most quality made PSU will have this note.  This is an indication a comprehensive testing was conducted in the design of the PSU.  If the TCO is not written on the rating label of the PSU skip on it.  Some PSU will not list the two number limits on the total amps that can be carried by each of the rails instead they will just place the lower numbers from the +3.3V and +5.5V rail outputs.  So it’s not unusual to see the above example to have the 10.6A and 40A as the respective maximum output rating of the +3.3V and +5.0V rails with an included note on the total combine output rating of the two rails.  But new marketing gimmick uses the high ratings of the 3.3V and 5.0V rails.  You can do the math on this one and most likely the total will exceed the total combine rating of the rails.

Remember the multiple wires mentioned earlier for outputs?  Well, this is the trick used to be able to harness all the power you can get from the PSU without the use of gigantic wires.  Most PSU’s uses the 16AWG and 18AWG wires which are rated at 11A and 9A respectively.  With the use of multiple wires you can reduce overloading a single wire thus prevent overheating and reducing power losses.

Efficiency of PSU’s ranges from the low 50% to the high of 75%.  Efficiency is the ratio of the power taken by the PSU from the wall outlet and the power taken on its load side.

The information provided above is not even a tip of the iceberg on the topic of PSU’s but the information should be enough to armed you with the knowledge to buy a decent or quality PSU.  So continue on with the FAQ’s. 

3.  What is the best brand PSU? 

I wish I could just answer this question in one word.  Unfortunately, since the ECS K7S5A is used worldwide and each locality makes his or her own brand and make of PSU, no one brand will fit the description of being the best PSU.  Instead of focusing entirely on the brand you will be better off if you concentrate more on the ratings of the PSU available in your local store.  Ratings you will be looking for depends entirely on your budget and also to what system you will be using if for.  The following is a general rule of thumb.

If cost is not a concern:

Buy the biggest PSU you can buy while at the same time noting the total combine output of the 3.3V and 5.0V.  Minimum will be 24A on the +3.3V rail and 28A on the +5.0V rail.  A typical quality 400W PSU will usually be rated at 220W on the TCO of the +3.3V and +5.0V rails.  You will also want to get an ATX 2.03 spec’ed PSU.  This is basically the PSU that was designed for the P4- system.  P4-system PSU’s are backwards compatible with ATX 2.01 which the  standard PSU for the Athlon and Intel P-3 systems.  Why the biggest?  This is to make sure the PSU you buy today will last you at least for another couple of system upgrade in the future.

If cost is a concern: 

Look into the AMD Recommended/Approved list of PSU and choose the brand that is available in your locality.  Choose at least three different brands, and also if possible for two different watt ratings. The minimum is 300W but the desirable is 350W. When shopping always check for the total combine output (TCO) of the +3.3V and +5.0V rails. If you are using a 1.2 Ghz or less CPU you should be looking for a minimum of 180W TCO on the +3.3V and +5.0V rails. Or you should be looking for a minimum of 220W TCO if your CPU is 1.33 Ghz or higher. A note on Athlon XP CPU is that they consume less power compare to the old Athlon Thunderbirds by an average of 15%- 20%. Lower rated Athlon XP 1500+ and 1600+ and sometimes 1700+ have been reported to operate properly with 300W PSU with total TCO of 180W. Why look for two different watt rating? This is to make a comparison of the price. Usually, a 350W PSU of one brand can be purchased at the same price as a 300W of another brand. Or maybe with the price difference between the 300W and 350W  

4.  My PSU is on the AMD Recommended/Approved list; do I still need a new PSU?   

PSU’s on the AMD Recommended/Approved List is was made to make sure the CPU’s required power is met but not necessarily for the mobo.  Each mobo has each own design when it comes to its voltage regulator.  One of the best designer of mobo voltage regulator is probably Abit.  Abit uses the 3-phase voltage regulator while most of the other brands uses the 2 phase regulators.  2 phase regulators requires less components but also requires a lot of power to maintain acceptable power to the mobo. 

5.  Why is the +3.3V and +5.0V rail output so important?  

Most mobo’s are designed to take most power out of the 5.0V rail and used it for the 3.3V and the 2.5V for the dimms.  But other mobo’s uses the 3.3V rail for the dimms and also for the rest of the mobo circuits.  A mobo who takes 3.3V and 2.5V power out of 5.0V will be more stable and easier to power with compare to those who uses the 3.3V rails.  

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