How can you not fall into trouble, if the power requirements of your computer aren't sufficiently provided for by your existing SMPS (Switch Mode Power Supply)? You might be wondering "Why put an extra SMPS when I just installed one with 300 Watts of power?" Your answer lies therein.
Well, for most of you power users out there, plugging in say, more than 3 IDE devices, which might be a combination of anything, from Hard Disk drives to CD/DVD ROM / RW drives or the latest SATA drives is a common practice... but it can never go beyond 4 devices, isn't it? Oh, did I forget to mention that cooling fans need some power too. It is very likely that you might end up with a blown SMPS or an unstable PC in case the power requirements of your computer are way beyond what the SMPS can deliver.
Say, you got a combination of the above devices along with some fans and your friend has got over his HDD to copy some of the cool stuff to and from your drive. You would be in a mess trying to figure out which drive to disconnect in order to connect his drive.
In such a scenario, to get many devices connected inside your PC and running peacefully at the same time, the only viable option seems to be replacing your existing 300W SMPS with the more expensive 450W or higher variant. But there's one more thing you can do to solve this problem and save a few bucks. Simply add another SMPS to your existing rig and enjoy the ride!
Just buy another SMPS from the market, with a true rating of at least 350 Watts. In case you are one of those who own a power-hungry card like the Geforce 7800, better go in for something more heavy duty such as a true 400 Watts SMPS. Ideally, replace your existing SMPS with this one and use the older one to drive smaller and less power hungry items in your computer (assuming that your new SMPS has a higher power rating than the old one).
To use an additional SMPS, you need to simply attach it to the various devices that would draw power from it as regular devices do. The ATX power connector needs to get a jump signal to turn it on. This happens when the necessary pins are shorted. It receives such a signal through your motherboard when you press the power switch. As your motherboard would only have one slot to connect the SMPS connector, you cannot connect the second SMPS to it. So to turn it on, you need to short particular wires or pins on the ATX power connector.
Caution: Beware of short circuits and blown SMPS or fried motherboards/CPUs if done in an incorrect manner. Kindly take precautions while doing this exercise by disconnecting the power supplies and other hard disks or IDE drives. Check for a grounding or earthing connection prior to installing an additional SMPS for safety sake.