Measuring Power Usage for H/W Components

[ianmeister]

Hi,

I have just started taking classes on computers, and one lab that we have to do is determine the power usage of all the hardware components, add them up, and determine if the current power supply is adequate for the system. I am unsure how to accurately do this.

I have looked at the manufacturers' websites to see their data sheets for the components, but not all of them are clear (at least to me) as to how much power the components actually draw. Also, I have looked at various Power Calculating websites that give general power consumption info on particular components, but they can vary widely from site to site.

I understand that power is measured in Watts and is determined by Voltage x Amperes. But when I looked at one data sheet, I was unsure how it should be calculated. For example, an ATi graphics card listed its power as follows:

+5V +/-5% @ .4 A typical
+/-3.3V +/-5% @ 2.0 A typical
+12V +/-5% @ .1 A typical.

How do I calculate the watts for this device?? :(

Any and all help with this subject would be great. Thanks in advance!

[saphalline]

In general we don't outright answer homework problems here on the forums. However, it's clear you've already tried to find the answer and got mired in the cosmos of PSU's. In addition, I'm always suspicious of PC classes and their ability to teach this area of PC's to my level of satisfaction. Just so you know, I consider the two most important components of any computer to be the mobo and the PSU. This problem in particular is not to be taken lightly and you may just find some info here that a PC class can't teach you. If you do learn anything important that is not taught in your PC class, please enlighten them for us. We get so many PSU-related problems here on the forums that could easily have been avoided by a little knowledge of the power subsystem and quality concerns with various PSU's.


PC Power & Cooling makes some of the best PSU's in existence, and they have decades of PSU experience specifically with computers. So I trust their power usage table (http://www.pcpowercooling.com/technology/power_usage/) above all others. They also give more realistic instructions for calculating how much wattage a PSU should have to support a given system. They mention that PSU's perform their best when loaded to no more than 70% of the max load. If you add up a system and determine that it requires 350W, that should be the 70% mark, so the actual PSU wattage you need is 500W. The reason is most PSU's are not only more efficient at 70%, but they also can't handle the full 100% load. 99 out of every 100 PSU's rated at 500W will never be able to handle 500W of system power requirements. They'll just crash.

In terms of that specific vid card, it works like this:

5V * 0.4A = 2W
12V * 1A = 12W

The next part gets tricky. +/-3.3V means that it will use both rails. While these voltages cancel eachother out, there is still amperage being used. My guess is that +3.3V is used together with -3.3V to acheive a differential of 6.6V. So:

6.6V * 2A = 13.2W

Now just add them up:

2W + 12W + 13.2W = 27.2W total (+/- 5%)

Depending on which ATI VPU is being used here, this could either be the full load power consumption or the idle power consumption. Difficult to tell by the term "typical".

When it comes to the quality of a PSU, they are not all made equal!! I can't stress this enough because, like I said, many people come here to the forums with problems that are directly related to them trying to "save money" by buying a cr@ppy PSU. Poor quality PSU's can output "dirty power" which is characterized by large voltage or amperage swings. This can cause stability problems with the system (very difficult to diagnose) and in extreme cases can even damage hardware. Poor quality PSU's can also have issues with efficiency (thereby dropping their max wattage output) and overheating and unbalanced voltage rails (I've seen some 500W PSU's that don't have enough amps on the +12V rail to power a Pentium4! :eek: ).

Quality PSU's are made by certain manufacturers that I personally trust, such as Antec and Thermaltake and of course PC Power & Cooling. If you check my sticky (http://www.pcguide.com/vb/showthread.php?t=39018) you'll see which PSU's I hand-picked that qualified to be on the list. PSU choice is very important when configuring or building or upgrading a computer.

[Whyzman]

I'll add this to saph's handiwork!

http://www.firingsquad.com/guides/power_supply/


Also, Welcome tohttp://www.pcguide.com/ubb/pcgubb.gif Forums!

[ianmeister]

Thanks very much! :D I appreciate the detailed answer. I will check out the links that have been provided, and I probably will have more questions. For one, a couple books I have mention nothing about the "rails." I hope to find the info on the provided web sites. Many people in my class didn't bother to spend the time looking their stuff up and instead just fudged their numbers. I don't know how they expect to be computer techs if they don't understand basic concepts like power usage.

Thanks again.

[ianmeister]

Just FYI: I need to correct one of the pieces of info I gave you.
Instead of +/-3.3V +/-5% @ 2.0 A typical, it should have read as just +3.3V +/-5% @ 2.0 A typical. Thanks again.

[saphalline]

Ahh, that makes more sense. So now that vid card is at:

5V * 0.4A = 2W
12V * 1A = 12W
3.3V * 2A = 6.6W

2W + 12W + 6.6W = 20.6W total (+/- 5%)

That must be an old vid card, eh?