Computer power supply questions?

Question

Can someone explain to me how the psu works?

For example, a psu specifications listed are: +3.3V@17A, +5V@25A, +12V1@16A, +12V2@18A, -5V@0.5A, -12V@0.8A, +5VSB@3.0A

How do watts, voltage and amps fit in?

I'm trying to see if I need to buy a new psu for a new graphics card I'm buying, but I'd also like to understand how this works.

Are there specific rails that power specific parts of the computer? Like do I have to make sure there is enough power going to case fans, motherboard, etc. seperately when buying a new psu?

Answer 1

Watts = Volts * Amps.

So you have:
+3.3V@17A, 56 W
+5V@25A, 125 W
+12V1@16A, 192 W
+12V2@18A, 216 W
-5V@0.5A, 2.5 W
-12V@0.8A, 9.6 W
+5VSB@3.0A 15 W

Those are the maximums for each line individually. Your total power may well be less than that.

Typically a video card will use the 12 V off the flying leads and the 5V rail off the system bus. They use the 12V because they can not pull enough power though the bus due to track size and bus connector limitations. High end GPUs are pulling more power now than most CPUs.

Usually they will say that you need a XXX W power supply, but what they really mean is that you need a 12 V rail that can supply YY W. They take that YY value and then scale that up based on what the typical difference between the 12V rail power and the total PSU power to get XXX.

With about 200 W on each of two 12V rails I doubt that you will run into a problem unless you have a lot of very power hungry hard drives in your system.

Answer 2

For example, a psu specifications listed are: +3.3V@17A, +5V@25A, +12V1@16A, +12V2@18A, -5V@0.5A, -12V@0.8A, +5VSB@3.0A

The section to the LEFT of the @ sign is the voltage and the part to the right is the current in Amperes.

To find the wattage of a rail multiply the voltage by the current.
(formula P=V x I)


How do watts, voltage and amps fit in?

This is a highly simplified version, but should give you enough of an understanding to be able to figure out a powersupply.

Firstly a Watt is a unit of work, that is power. In electronics a voltage of 1V at a current of 1A produces a resistance of 1 Ohm and a power of 1Watt.

In an electronics device such as a computer various voltages are required by various areas on the computer, for example a +5V like for certain components, a +12V line for others and a -5V line for other parts. Thus your powersupply has seperate output voltages each with it's own current rating.

The PSU will also have an overall rating like 500W. This number should tally with the sum of the wattages calculated from the invididual outputs of the PSU.

Answer 3

The simple answer is YES, it all matters. In these days the more power the better, by power I mean Watts. All PS's are rated in Watts, 300-400-500-and more. Generally the more the better although you may be paying more for something you don't use. As far as graphics cards go you need to see the specs. Is this a PCI Express card, AGP or what. The good news is most higher powered PS's have connectors for just about everything you'll need. I say for general use and some gaming applications go with at least a 500Watt PS.