In answering your question.
If you take a PSU that outputs a total of 30 Amps.
And you use subtraction for each device that consumes that amount, until a final result is found.
Here is an example.
A motherboard demands 10 Amps.
The total of cooling fans consumes 10 Amps.
A DVD burner drive consumes 10 Amps
The USB hardware consumes 10 Amps
The Video card demands 10 Amps
Now be advised my numbers are NOT exact, they are not being taken from actual figures by any means, because I seriously can not tell you which hardware is demanding what, without actually checking them with an Am meter, so my numbers are NOT accurate here in this example.
The best way I can create an exact formula is by placing a VOM meter in line with the device so its current flows through the meter, it will give me a reading in how many Amps (current flow) is flowing through the meter to the device being tested.
If lets say my numbers above are true, we have a total demand of 50 Amperes as the result, the PSU is only safely capable of supplying and satisfying a total of 30 Amperes maximum, that means we have a deficit of 20 Amperes showing in our formula above. In order for each device to work at all, each would only be allowed 6 Amperes a piece. That is 4 Amperes short of the demand for each device. The result is, some devices will function quite well, while others may fail to function with stability or may fail entirely.
Because Video cards have a clock signal and have to scan and redraw a certain number of clocked frames per second, any disruption in this clock signal can cause artifacts to show up in the signal, or a total loss of signal all together.
There are many examples of this deficit in real life. Take the electricity that comes to your house from your electric company.
Have you ever noticed a fluctuation in your house lighting, when for a few brief moments, your lights will dim down momentarily?
Have you ever let the batteries run down on a portable tape player and realize the tape starts playing slower and slower?
This happens because the demand is greater than the supply. Things may still work, but they won't function like they should be functioning until the demand is satisfied.
Sorry to be long winded here and although I realize the way I worded this post is not up to the professional way I was taught to explain electronics to someone, I worded it in a manner I feel most anyone could understand, without all the technical jibber jabber only my fellow electronically inclined associates would understand.
Edited by MrBruce1959, 10 August 2010 - 10:54 AM.