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I'm using a 7 port usb hub found here: http://www.ebay.com/itm/AC-Power-Adapter-Cable-for-PC-Laptop-7-Port-USB-3-0-Hub-On-Off-Switches-/331707642029

The power supply it came with was very weak. It could not power on my external 2TB hard drive so I replaced it with a 5VCD 2.5A adapter of my own. It was able to power the hard drives now. Now the issue is that I connect a second 1TB hard drive on that Hub and things get unstable. It works fine if I connect it directly to my laptop. I'm using a raspberry pie which isn't able to power these hardrive on its own.

Hard drive details: 2TB seagate backup plus and 1TB WD blue both are 2.5" in size and 5400rpm. The 1TB WD drive uses 5VDC and 0.55A. I have no idea how much power specs the Seagate one uses.

I think I might need a better power supply to get rid of these issues but I don't know how much volts and amps it needs to be.

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  • This entirely depends on the voltage and amp requirements of the HDDs themselfs.
    – Ramhound
    Commented Nov 29, 2015 at 17:30
  • In any case, I'll just need to know how much power a typical usb 2.0 or 3.0 on a laptop supply to connected devices so that I can replace the hub's power supply accordingly. These drives work well on my laptop and they do not power themselves.
    – answerSeeker
    Commented Nov 29, 2015 at 17:54
  • Just for more info, when I connected the 7 port hub to my laptop, both drives (connected to the hub) worked fine with no issues. Also, I did not use the hubs power supply at the time. So all the power came from my laptop.
    – answerSeeker
    Commented Nov 29, 2015 at 18:05
  • @TatakaiWasumi - I don't understand your comment. If they work fine what exactly is the problem? There is only so much total power over a single hub, your laptop only has one, you can add 100 externally powered hubs and you would still be limited by the power of that single hub...
    – Ramhound
    Commented Nov 29, 2015 at 18:05
  • @Ramhound - I would like to know how much volts and amps does a typical USB port on a laptop supply to devices, either usb 2.0 or 3.0 port.
    – answerSeeker
    Commented Nov 29, 2015 at 18:07

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In a parallel circuit, the voltage across each of the components is the same, and the total current is the sum of the currents through each component. — Series and parallel circuits

So, you will need a charger that has the same voltage and sums up the currents (ampere). You can provide more current than the calculated sum, as the attached devices will only consume the current they need. However, DO NOT provide less current than the sum or you could damage the charger and possibly its environment.

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