Originally posted by: rocketPack
OK, I'm back and armed with pictures (well, A picture...)
This should give a better idea of how to calculate what you are doing to your PSU and its wiring. Examine the following picture and read the details below.
http://www.rocketpack.net/stuff/psu_currents.jpg
In the picture you will notice there are three variables and three "zones". Do not mistake that the variables refer directly to the zones. I is representative of "current" (measured in amperes, or amps)- why I most people ask; well, because "C" is the constant for the speed of light, for one thing, but it has other meanings I don't want to get into - just take my word for it. Since this is pure (and basic) DC, there are only 3 (technically 4) things we need to be concerned with: VOLTAGE (V or E, measured in VOLTS), the force "pushing" electronics through the wire (known also by E for the alternate definition "electro-motive force"); CURRENT (I, measured in AMPS or AMPERES), the number of electrons moving through the wire (kind of like volume when talking about liquid); and RESISTANCE (R, measured in OHMS); and lastly POWER (P, measured in WATTS [among other things]), which is a combination of VOLTAGE and CURRENT to give an overall idea of the amount of energy being consumed. OK, that's the background.
As previously stated we can have about 72W flowing through each line of the power supply. This is because each wire has a small amount of resistance - which causes heat (this is energy being consumed because it is pushing back against the voltage- think of it as electrical friction), even though its intended to conduct, this world is not perfect. At lower current levels this resistance is quite nominal, but it determines the maximum amount of current a piece of wire can handle before it heats to the point where its insulation and eventually the wire itself melts which poses (hopefully) obvious risks and begins to break down.
Now, each of the Is (I1, I2, I3) in the picture represent the current flowing through each plug on the line (for the purpose of this post the entire span of wire will be refered to as "the line") - aka the current being demanded by the device attached to the plug. So, let's say for example that the plug on the very end (the floppy connector) is connected to a floppy drive which consumed a fairly nominal amount of power, maybe a couple of watts tops (2 or 3). Because the wires (as you can see) from that plug are attached to the next plug, the current from that wire is now flowing past the connector corresponding to I2. These two currents are
additive as they share a wire. Now say that your second plug (at I2) is connected to a hard drive. They consume (give or take) about 10W. Now we're up to about 12W. All 12W of that power is being drawn through the greenish/yellow zone of the cable now. Say that at last we attached a splitter to the connector at I3, and hooked up another hard drive (10W) and then our very hungry video card's power adapter, say 60W (let's pretend). Each of our wattages seem quite harmless, 60W, 10W, 10W, 2W... but wait a second, they're all
additive. The red zone is now forced to carry the entire 92W load - well above its specifications allow. Adding more splitters to this scenario can easily make this into a catastrophic situation, causing power supplies to overheat and either catch fire or fail (often putting out unpredictable currents and voltages to sensitive electronic devices as it fails), or the wiring to overheat and melt, causing a short which would again lead to fire or power supply failure. In this example it didn't take much!
Also note that any electrical noise created at the connector corresponding to I3 will be spread on to the other devices on that rail, and if any one one of the devices changes its voltage or current demand drastically it can cause a power dip or spike in all other devices on the same line, so make sure you chose carefully when you plug your devices in. Things that use motors (like fans) often create electrical noise (only a small amount because their power demand is so little, but noise is noise). Personally I would never put important things like video cards and hard drives on the same line as a fan or similar device, so as to make the possability of interference a little as I can.
I hope that the picture and scenario explanation can shed some light onto why using splitters and adapters is such a risky and sometimes dangerous practice. Please chose carefully when you buy a power supply, and make sure it has what you need - buy a power supply that will require as few "adapters" and "splitters" as you possibly can (while, obviously, still meeting the demands for output rating and quality assurance).
Good luck, be careful, and feel free to ask me if you have any questions - I'm not claiming to be a pro or an expert (nor will I take on that kind of responsibility
), but if you're in doubt it's always good to ask around! I'd rather take a moment to give someome advice than to listen to them whine about a wrecked rig!
-Scott
Facebook
Twitter