My set up has only one Molex cable which is hooked into a 12v fan which is working fine but now I want to plug in one IDE harddrive to the machine. Can one Molex power cable power a 12v fan and one IDE hard?
Fan 12v 0.15A
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Sure. That's actually a very standard way to add a fan to a system, with many add-on fans directly wired up to a "pass-through" style pair of Molex connectors.
Your system probably has many Molex connectors on the same rails for a given voltage anyway, so it shouldn't matter how/where the fan is connected.
A quick Google search entirely unscientifically suggests that a fan uses under 0.5 A, and a hard drive uses under 2 A. I suspect you ought to get 6-20 A on a 12 V rail, but you can trivially get that from your PSU.
I think you'd be best off looking at your power supply's rail rating (on your power supply itself), fan, and HDD to decide if its "safe"—but chances are it should be entirely within specifications.
Should do - I use molex splitters all the time, and did back when I had a stack of IDE drives.
They are very cheap to buy and can split the 1 power socket into 2 or more.
Give it a try, if there is not enough power it wont work - is the system overloaded in some other way? How powerful is your power supply? What else is plugged into that power?
Maybe a safe way to test it would be to plug just the IDE drive in first - if it works ok, then you could try it and the fan at the same time.
I used to use them to power both IDE drives, slave and master when they were on the same cable, and for neatness and ease of working out what was what, I split the power into 2 - so both drives came from the one molex into a 2 way splitter.
I would not trust the Trial and error method as its not recommended. Some have mistakenly figured that one splitters worked fine for this component so, "I can just add 4 more splitters and hard drive disks on the same wire." Main reason for saying this is that I do not know exactly what your system specifications are. You could have a PICO PSU that is already to its max and 1 additional 1 watt component could be enough to shorten the life of the PSU or other capacitors on your system.
There are many factors that would determine if there is enough power for additional computer components. But fans genneraly use a surprisingly small amount of power. So, the simple method may be enough.
Power Supply Rating: this can be found on the side of the power-supply somewhere on a sticker.
Adding up all the components that use power.
System board, CPU, PCI cards Drives, external peripherals, fans
Thankfully there are many calculators online where you can just plug in all of your components and see what the average current draw is and compare that to what your Power-supply is rated to give to the system
Here are just a few Power-Supply calculators:
Note: Without really knowing how stable your PSU is and if you want to get good life out of the system, I would stay under the PSU rated output by 10%.