Computing stuff tied to the physical world

Hard disk power #2

In Hardware on May 31, 2011 at 00:01

Yesterday, I described a design to control the power of hard disks. Now let’s see what happens when two hard disks are hooked up this way:

Screen Shot 2011 05 29 at 19.24.25

I’m including the USB cables to the server now, because that’s what causes a major problem: ground loops!

USB is a 4-wire (+ shield) connection between computer and devices. It is not isolated, i.e. all devices get hooked up galvanically. This in turns means that the ground potential of all inter-connected devices will be the same.

With a disk powered from a 12V power supply, ground is the negative wire. It’s also the ground of the USB connection to the server, so the “–” side of the power supply is connected directly to ground on the USB cable.

When two or more disks are plugged into one computer via USB, all ground pins will be connected together.

So far, no problem. Most devices have their own power supply, and these are all “floating”, i.e. they will adopt any ground voltage potential imposed on them (if any). You can connect any number of power supplies together through their “–” pins, and they’ll happily each feed their own devices and share a common ground.

Yesterday’s design is different – it uses a MOSFET to connect or disconnect the GND level from the disk drive:

Screen Shot 2011 05 29 at 19.36.05

The MOSFETs act as switches, connecting or disconnecting the wires on top to the JeeNode (and power supply) ground. So what one disk sees as “ground” is not the same as what the other sees (unless both are powered up). But… with the USB cables inserted, we’ve in effect created a direct connection between “GND1” and “GND2” through the USB’s common ground wires.

The result will be that no matter which MOSFET turns power on, it will end up powering BOTH disk drives!

And what’s (much) worse: the power supply current for the other drive will go through both USB cables and the server, which isn’t designed to handle the 1..2A peak currents flowing on startup.

Phew! I’m glad I didn’t start building this circuit :)

Tomorrow, I’ll describe a better design.

  1. Hi!

    Why you don’t use high-side mosfet switch? (p-channel instead of n-channel) the grounds can be tied all the time together and you can switch the +12V supply.


  2. I would just throw in a bunch of relays and switch the mains on and off and keep the small PSUs. The problem with one beefy PSU is that it will still take a certain VA from the net to generate electromagnetic fields and such. Therefore, the less current you draw, the lower your powerfactor will be and thus you will still have more current flowing than you actually need! It’s the same effect that you’ll get when you leave your phone charger plugged in without your phone connected. The charger will still consume power.

    You will not get billed for those currents, because what you don’t use will be “returned” to the net, but they will have to be generated somewhere. So if you’re looking for going green, you should probably stick with “plugging things in and out”.

    If you are using those disks not too often (like you suggested) the excess power the “more beefy” power supply draws could actually put more strain on your home’s grid and/or cost you more on your electricity bill. It also solves the ground loop by the way…

    It’s up to you to determine the point where one approach overtakes the other.

    • And I forgot to mention the fact that you probably already have all the small PSUs you’ll need for this…

  3. Good points. True, I do have all those small PSU’s… but it’s not just energy, also clutter and larger power strips.

    Indeed, this would only save energy if the single PSU draw less. Right now, I’ve got a 12v @ 4A PSU “brick” which appears to draw 0.41W when idling. It might just be enough for 3 drives, if their spin-ups are staggered.

    • 0.41 Watts when idle for a 48w supply is on the high side of normal though.

      If you’re all about decluttering, go for it. Otherwise, hook up those 3 small ones when idle and see what they draw when combined they are on one by one. Because cheap PSUs can also be on the high side of normal. If this is the case, the financial part of the energy consumption is blown off the table, but this still leaves the powerfactor and VAs to consider.

      However, I’m curious to see what you brew up!

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