This afternoon, I went to my local computer store to pick my harddrive. I’m happy. The moment I reached home, I installed the drive in my PC. And whaddya know, It didn’t work.

Now, having just returned from the store, the possibility of the harddrive to be broken is very small, so I started to be suspicious with the power supply. I unplugged the power to my DVD-RW drive and my the other two harddrives installed in my PC. So the only thing left now are the problematic drive and some case fans. And when this didn’t solve the problem, I started to be worried.

I tried to calm myself, and browse around the net about this. Not finding anything useful, I tried shotgunning it. I unplugged everything, and turned on the PC with only motherboard, processor, ram, and the graphics card. It worked. I plugged the problematic drive. Worked. Plugging the other two harddrives didn’t change the condition, and so did adding the DVD-RW drive. But when I plugged the case fans, the problem returns.

The culprits are those fans.

The three fans are ordinary LED case fans. each of them has red, green, yellow, and blue LEDs on. I didn’t think this should be enough to tax the PSU. I was confused.

But then I remembered that my PSU has two 12v rails. one of them, connected to the DVD-RW and a harddrive, is rated at 13A, while the other, connected to two harddrives and three fans, is rated at 10A. Which roughly translated as 120 Watts

Now, at worst case scenario, a harddrive should only consume 25 watts. Two of them means roughly 50 watts. A fan with four LEDs won’t reach 5 watts, but we’ll use that to make things simple. The total power consumed shouldn’t reach more than 65 watts.

Then why didn’t My PSU cope? The answer is because every PSU, especially cheap and dodgy PSUs, can’t deliver their maximum rated capacity on one rail, especially if the other rails were demanding quite an amount of power. So, 13A on one rail and 10A on the other won’t add to as 23A on both, more like 14A, tops.

This is one of the problem that made people confused like hell. I actually considered buying a new PSU this afternoon before realising the two rails thing.

So, moral of the story is: Take your PSU’s rated wattage with a large grain of salt and overbuy it. I’m serious.