Hi,

The power supply vendor usually indicate what current per output voltage it is capable of. Sometimes it is expressed in watts which can be converted dirctly to current by dividing the watts by the voltage, ie: 100W / 3.3v = 30.3 Amps.

If you look at the power supply specification tab on the website that was supplied, it can easily be seen that the current supplied to each voltage vary widely and do not scale with the overall advertised wattage of the supply. Some are targeted towards motherboard/RAM/Video voltages (3.3 & 5V) and some are targeted towards drives and other items. This was what I was referring to as matching the supply to your system needs (including future upgrade needs).

What I was trying to say is that each voltage may not be loaded if say the +12V has the capacity of 50 Amps. If that was properly loaded at 20% that would be 50 X 0.2 = 10 Amps. I doubt that many 3 disk drive (sets), fans, etc, would draw 10 Amps. A load resistor on that voltage might insure minimum load regulation.

The 20% load on the full 750W is not necessary, only on those that do not meet the minimum load that you suspect may cause problems. This can be located by observing the output ripple with an oscilloscope and a spectrum analyzer (with proper set-up (line LISNs, attenuators, etc)) - definitely not common place instrumentation, but, useful for locating these types of problems.

The advantage to a larger capacity supply is that if you wanted to provide minimum load resistors, they could be removed when that additional power is required in the future as you upgrade.

If you do provide minimum load resistors, be sure that you size them at double their wattage rating on the resistor. For example: if you need 20% of a 50 Amp (10 Amps) load on +12V:

1) Subtract out the existing current of the devices already in the system (say 6 Amps)
2) Subtract this from the 20% current minimum load (10A - 6A = 4A)
3) Divide 12V by the 4A = 3 Ohms
4) Calculate the wattage of the resistor Power = 4A X 12V = 48Watts. For safe operating area of the resistor, double the wattage (48W X 2 = ~100Watts) - it will draw only the 48Watts not the 100 Watts.
5) Obtain a 3 Ohm 100 Watt resistor and mount it so that it will be properly cooled (either conduction through the chassis or convection via the fan). Make sure the resistor mounting will not create a fire risk. Twist the wires together to reduce EMI. Make sure that it does not cause too much of a thermal rise in the system as well.

When you add resistors like these, make sure that it is needed to solve a problem, as it will burn constant wasteful power.

Ross