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Does RPi B+ have high enough power output?

Former Member

I have just bought a RPi B+ and found it cannot power my WD My Passport 1TB USB3 drive. The drive just clicks rather than spins up.

 

I'm hoping that it is something I ma doing wrong. Do all the USB ports on the B+ provide the same power?

  • 0

    There are a few considerations.

    The harddrive takes about 1A, or about 1.5 times more than the raspberry pi itself. So your powersupply must be able to provide 1.7A total. If you have a "2A" chinese powersupply, chances are it is powerful enough for a raspberry pi, but not for the pi plus the WD.

     

    Secondly, if you power the pi the normal way, there is a polyfuse that is supposed to prevent fires in case of overcurrent. This should limit the current of the 'pi plus --whatever it has on the USB ports-- to about 1A, or maybe 1.4A.

     

    The trick to using external (usb-powered) harddrives with the 'pi is to connect them to the pi through a powered hub.

  • 0 in reply to rew

    Roger,

     

    Thanks for the swift response. Are you talking about the B or the B+? Everything I've read about the B+ indicates that it should be able to power a disk. I do have a new 2A supply, so presumed it would all work.

     

    Sadly, not at the moment - even though, as you say, it does work through a power hub.

     

    Charles

  • 0 in reply to Former Member

    Just out of curiosity, do you have the double usb plug? Or a single? USB (technically) is only supposed to supply 500 ma, much of time on the powered usb hubs they don't limit the current to the individual ports, so you can get the full current at any of the ports. They are non-compliant, but it is very common.

     

    Mike

  • 0 in reply to mconners

    Mike,

     

    I'm using a double USB cable. Looks like USB2 (ie it's not blue).

     

    Charles

  • 0

    A less expensive alternative to the powered USB hub is to buy a 5V 1A (or greater) USB wall charger and use it for the "power" USB plug from the drive. This is

    what I do on a hard drive attached to an older PC that does not have enough external connectors available for both of the drives USB connectors.

  • 0 in reply to gdstew

    I went for a B+ to avoid having extra bits like powered hubs and extra USB chargers. 4 USB with a promise of enough power....

  • 0

    The 'pi has, just as the Michael says all the ports wired in parallel.

     

    The revision one had two extra polyfuses in the 5V lines going to the USB ports. Those were supposed to currentlimit them to about 100mA. But that was a long time ago. The current "B" is very similar to the B+, except that it has a polyfuse that is one notch smaller.

     

    I've done some measurements on a "classic B". When the 5V drops too low the first thing that fails is the LAN9512. On the 'B this happened when the voltage on the 5V dropped below 4.3V. It is entirely possible that the raspberry pi B+ will continue to run down to about 3.0V: The LAN chip (which is also USB HUB) is the main consumer of the 3.3V. The lan chip will tolerate 3.3V +/- 10%, so at  3.0V it will continue to work. But below that it fails. The CPU also runs off the 5V line... But it too tolerates voltages down to at least 3.0V. So apart from the CPU and the 3.3V regulator is there anything else that uses 5V? Well, yes: The HDMI provides 5V for self-powered converters. And USB.

     

    So, assuming the polyfuse is creating your trouble, you can try to power your raspberry pi "behind" the fuse. i.e. on the GPIO header or on the USB connectors. So what usually works is to have a powered hub and let he lead to the hub power the 'pi too. But that still has the bulky hub in your system...

     

    One other thing you can try: Plug the power-plug of your harddrive into that 2A powersupply..... See what happens.

    (First I'd like to see the disk spin up. Then when you plug the dataplug into the raspberry pi, I'd like to see the 'pi power up as well... )

  • 0

    Buy a bigger power supply for the RPi.  The B+ should be able to power your HDD.

  • 0 in reply to Former Member

    Well, as I said in my first reply: You're going to need a nominal 1.7A for a short while during startup. It is not at all evident that a powersupply marked as 2A is actually able to deliver that...

     

     

    I have a test-rig that allows me to test the current capabilities of power supplies. Most 500mA powersupplies are comfortable at 350mA, and above that the voltage sags too much. Similarly the 2A power supplies that I tested often start to drop out around 1.5A (and sometimes even at 350mA!).

     

     

    On the other hand, there might be a voltage drop across the RPI that the WD drive is not happy about. I'd be able to measure such things if it interests me. But in practise it is more important to get things working. So getting  a slightly larger powersupply might solve things for you.

     

     

    If you're into DIY, you could wire up something that "steals" the 5V from inside your big PC. That's a stable and accurate 5V capable of delivering at least around 10A. If it works on there, a better power supply might help. If it doesn't the voltage drop across the 'pi is too much anyway. This is just a suggestion that you might try to verify if buying a bigger power supply might help. I'd only do it if you have everything in house already....