I have a 3.5" HDD enclosure with external power supply. Inside I have a 2.5" HDD and there is space in there for the RPi. The problem is I don't want to have two power supplies so I was thinking to solder a connector from the 5V S-ATA power connector to the USB RPi power connector, both being 5V (parallel connection).

I'm no electronics technician but would the internal resistance of the devices interfere with one another?

I will also solder the usb hdd link to the one of the RPi's usb host port if this setup works.

  • Don't solder the USB- it is highly likely that it may cause data issues- Its not a good idea. Just use a normal, but short USB cable to connect the USB. – Piotr Kula Dec 2 '15 at 10:36

2.5" HDD usually require 500ma during spin up, and sometimes during seek - But during operation about 300ma- This is to conform with USB standards. Although, if inrush currents are supported (or not limited) then it may take up to 800ma on spin up to spin up quickly but will drop down to 500ma within seconds.

So if you putting your Pi inside the 3.5" enclosure, then leeching 5v for the Pi will be fine. since the 5v for 3.5" HDD should allow up to 2A.

The power supply is most probably 12volts (common jack), since 3.5" also requires 12V. The amps on the power supply don't mean anything in relation to the 5v line. It all depends on the regulator inside the 3.5" enclosure.

Unless the power supply is 12v/5v (4 Pin jack)- Then you will have Amps rated for each voltage, with no regulator in the enclosure. That should be 12v/1a 5v/2a

Generally speaking, the Pi will run from the 5v line without any problem when using a 3.5" enclosure, given that you are not using extra things like WiFi, Cameras, etc. LAN will be fine, since its always powered anyway.

If you are planning on putting a USB HUB inside the enclosure, then you are going to run into trouble with power.


If you do find that 5v wont be enough, you may as well use the 12v line and regulate it down to 5v for the Pi. You can use something like an UBEC, its cheap, small and reliable. (and will most probable enable you to use WiFi, Camera's etc with the extra power!)

  • 1A/12v gives you 12 Watts
  • 12 watts for 5v gives you ~2.4amp @ 100 efficiency. I wouldnt count on more than 2A from 1A/12v using a 2A UBEC anyway.
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    As the case is originally designed for a 3.5" drive, the 5V line on the supply that came with it might be underspecced just for the motor in the 2.5" drive. This is equally true if the 12V--5V conversion is done inside the drive case (though in that case a second/bigger regulator may be an option). To state explicitly what your answer clearly knows: 3.5" drive motors are powered off the 12V line, 2.5" drives off the 5V line. The motor draws considerably more current than the electronics. Testing is in order. – Chris H Dec 2 '15 at 9:47
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    You're probably right, but I had a 3.5" external drive with very little 5V indeed from its (external) PSU, and 2.5" drives modded into the case wouldn't spin up reliably. – Chris H Dec 2 '15 at 10:00
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    Yea.. it may be trouble as you say. So I just added an edit on how to leech from the 12volt instead, if required. – Piotr Kula Dec 2 '15 at 10:04
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    I'd give you another +1 for the UBEC. I'd never seen a nice packaged swicthing regulator like that (could have saved me some headaches on car stuff). – Chris H Dec 2 '15 at 10:43
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    Actually I did (and I'm surprised to see the first had no votes already) -- I'm thinking about building a Pi-based laptop and that would be an easy way to tick-off one requirement without building a board myself. – Chris H Dec 2 '15 at 10:53

the internal resistance does not matter much in this case, but the rating of your power supply does. RasPi requires about 0.7A on 5V and your HDD might easily require up to 1A, especially during startup.

you'll need a power adapter rated at least 2A to be used in this kind of setup, not every phone charger can do that.

  • The charger for the rack is no phone charger. It used to power a full size 3.5" 7200rpm hdd, and now it should power a notebook hdd (2.5" @7200rpm) and a RPi. I doubt the RPi+2.5hdd use more than a 3.5" old hdd. Also the s-ata power also has the 12v line and since the power adapter has 12v output, I'm guessing the 12v s-ata line is active to. – LucianMLI May 9 '13 at 8:24
  • @LucianMLI: of course you can try. It depends what other accessories you will decide to connect to PI (with powered USB HUB, keyboard, DMI to VGA adapter consumption goes over 1A). Unappropriate power supply is the usual culprit for most (or at least for a lot) of strange pi behaviour errors. – Rok Jarc May 10 '13 at 10:19
  • "Internal Resistance" is inversely proportional to current draw and thus power consumption (V*V/R). Granted, for neither a pi nor a disk drive is it constant (or likely even only real). Ultimately, it matters very much, but it may not be the most convenient way to express the power requirement issue for such variable loads. – Chris Stratton Dec 2 '15 at 1:48
  • He is putting a Pi INTO an old 3.5" enclosure, using the original power pack, be it 12v or 12v/5v and hooking up to the 5v "sata" line to leech power for the Pi. Sorry, but I had to down vote since you are not answering the question. – Piotr Kula Dec 2 '15 at 8:08

You might also try powering via the GPIO Pins -- 5v from the sata connector would do it. Sometimes, too, you can back-power from a powered hub or usb device, without plugging into anything else.

You may still run into power issues -- I've run into weirdness there on my Pi cluster. One thing which can help is to enable the USB high voltage. In /boot/config.txt add:


This allows a usb device to draw more power -- I've found it helpful with disks. That assumes, of course, that you're feeding the Pi 2A to be sure you don't have brownouts.

  • None of this addresses the specific question which was asked. – Chris Stratton Dec 2 '15 at 1:50
  • I beg to differ; I believe it does. Rather than bothering with the USB connector, use +5v and ground from the sata connector to power the Pi via the GPIO pins. Just watch out for brownouts if you have other devices plugged into the Pi. Additionally, even with powered hubs, there are some drives that I've had to set the max_usb_current in order for the Pi to be able to talk to a drive. – Matt Williams Dec 2 '15 at 13:53
  • Hi Matt. Welcome to the Pi Exchange. I understand what you saying but the original poster want to put a Pi into an old 3.5" enclosure, where he replaced the original 3.5" with a 2.5" HDD. He wants to use the original power pack, which is most likely 12volts. But the circuit inside the 3.5" transforms it into 5v... and he want to know if he can use that 5v to power the Pi and the 2.5" HDD at the same time. I didnt downvote you - you are a new comer and believe explaining is better than downvoting. Can you adapt your answer maybe? That will allow downvoters to upvote. – Piotr Kula Dec 2 '15 at 19:17

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