krishnathelord said:
why use 400W smps over 350W or any other?
who can u check hte wattage of a smps?
i know that this a foolish quetion but could not help myself
I don't think these are foolish questions at all.
The first one has been adequately answered. The answer to the second one is more complex than many people would think. Pradeep's answer tells you how to measure the power drawn from the mains, but the issue here is how much the PSU can supply reliably.
The ratings chart pasted on a PSU (SMPS) states how many amperes of current can be drawn from each voltage rail. But in reality, many, if not most, PSU's would soon fail if these full-load currents were actually drawn by the load. Many manufacturers overrate their products as a marketing ploy.
One way to test the actual capacity of PSUs is to draw the full-load current from all the voltage rails and measure the voltage with an accurate voltmeter. The voltage output should not change by more than an acceptable limit from no load to full load. E.g., the 5V rail should be not more than about 5.1V at no load, and not less than about 4.8V at full load. (The 5.1V and 4.8V values should be taken as guides and not as strict standards).
The problem here is to how to provide the proper loads. A 300W PSU usually has a 5V rail rated at around 30A. To test this at full load, one would need a 0.17-ohm resistor rated for 200 watts or more, and it will be next to impossible to find such a resistor in an electronics parts shop.
To test the 12V rail rated at 15A, you'd need a 0.8-ohm, 250W resistor.
Such resistors can be made up by combining available parts, but that's hardly practicable except for a testing lab or a keen professional.
Even if the PSU can deliver the rated currents, there's the question of long-term reliability. I've seen PSUs that rate the 5V rail at 30A, but use a 10A or 15A rectifier to supply it. Such a PSU is very likely to fail after some time if you draw a continuous load of, say, 20A or more from the 5V rail.
These manufacturers hope to get away with such practices because the full rated load is seldom used. The truth is a 300W PSU by one manufacturer may actually be better than a so-called 400W unit by another company.
BTW, right now I'm testing a newly assembled computer with an A64 3000+, two 120GB hard disks, 1GB RAM, DVD-R/W, CD-R/W, FX5200, video capture card with TV tuner. I'm using a 300W PSU that I tweaked to deliver the full capacity.
Sorry I can't give a simple answer to your question, but I hope I have at least managed to present a broad overview of the issues involved.
