I have a home-built NAS where I currently have ten 3.5'' SATA HDDs. The PSU I use there is rated for 140W on 5V rail and 480W on 12V rail. Since the 3.5'' HDDs use both 12V and 5V voltages, I would like to know what is the exact balance between the HDD consumption from the 12V and 5V rails?

For example, let's say my HDD is rated for 13W under idle. What percentage of these 13W is coming from 5V and 12V, respectively? I realize that it will most likely be model dependent, but even a very approximate number would be helpful since I assume it should be relatively similar if not identical for most HDDs. Unfortunately, I did not find this information at all in the datasheets provided by manufacturers.

What I am trying to determine is if my 5V rail is enough for that many hard drives. Unfortunately, modern PSUs are heavily oriented towards 12V rails. Likewise, many of the protections are located there rather than on the 5V rail which is why a potential overload on that rail is a concern for me.

So what is the load balance between the 5V and 12V rails for HDD? The only information I found is that 12V spins the motor while 5V powers the electronics. This is not very informative for my problem. In the case almost everything comes from the 5V rail, I might have a problem at some point.

I would appreciate your ideas for that possibility as well.

The solutions I see right now if I start overloading the 5V rail:

  1. New, stronger PSU - this is obvious suggestion but rather problematic since stronger PSUs I looked up are actually stronger only on the 12V rail. But my search was not exhaustive so maybe there is some high powered PSU which has juicier 5V rail as well.

  2. Get a second PSU and spread the load - not ideal, but my preferred solution if nothing better comes to mind.

Thanks for your suggestions!

Note: I realize there is 3.3V cable on SATA connectors as well. Since it's not used by any HDD I ever saw, I am going to ignore it for the purpose of my question. Wish they never put it there in the first place. Just one more thing that can cause a short circuit. :(

  • Using 2 current meters (aka ammeters) you could measure the actual current/power drawn on both supply voltages by a HDD. Your alternative is to use the max current rating quoted for each voltage. BTW your solution #3 makes no sense; a transformer cannot convert DC voltages. Alternate solution is to use single-voltage power supplies (i.e. forget ATX), sized to your requirement.
    – sawdust
    Jul 23 '19 at 21:18
  • 7
    "let's say my HDD is rated for 13W under idle" -- Do not use power used at idle for power calculations. You need to be concerned with the maximums. The PSU is overloaded when maximum current is drawn, not when devices are at idle.
    – sawdust
    Jul 23 '19 at 21:25
  • You'll need average ratings. Could be anywhere from 40% to 90% of maximum, hard to say without more information.
    – Mast
    Jul 24 '19 at 8:33

The thing that is going to sting you is the spin-up current on the hard disk. WD supplies the following Specification, which indicates that the blue 7200RPM drives draw 2.5A (or 30W) peak, but average 6.8W under load. Peak is associated with spinning-up the hard disk. You are close to the limit on number of hard disks you can support without staggering their spin-ups. Maybe a couple more? If you can stagger the spinups, you could put 4 or 5 dozen hard disks in this system.

With regards to the ratio, lets assume that anything not the motor runs off 5V. The specifications say the standby wattage is 1.2W. That must all be coming from the controller because the disk is not spinning. It also says that, between idle and active, the power difference is 0.7W. Lets also assume that is all controller related. I conjecture that at worst the 12V:5V ratio is 5.9W:1.9W or ~3:1.

The 5V rail is fed from the 12V rail via a rather beefy power regulator in basically every modern power supply. So, differentials equations yada yada yada, you could run 60 hard disks in this system. 114 watts on the 5V rail, 114+354=468W on the 12V. You'll have to remove a few hard drives to make wattage and overhead for the rest of your system, but the end results is not to worry about your 5V rail. It is definitely not your bottleneck.


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