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I know that my USB powered Hard Drive doesn't work, but what is the minimum power requirement that the Raspberry Pi can fulfil for devices that connect via USB?

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2 Answers 2

up vote 68 down vote accepted

Recommended: 100mA

You should not draw more than about 100mA from the USB ports.

Source


In reality, it is more complicated than this.

I've taken two screenshots from the Device B schematic (released here):

Power in (fig A):
USB Power in

We can see that the 5 volt line (+5V0) is powered directly from the USB input power, through a 1.1A, 6V Polyfuse (miniSMD). I believe this means that the 5V input is current limited to 1.1A (maximum hold current, not peak).

Let's assume that you're powering the device from a regulated 5V supply capable of supplying at least 700mA.

USB Power out (fig B):
USB power out

As we can see from fig B, the USB ports are current-limited by polyfuses (miniSMDC014) to 140mA (constant draw).

Some users have discovered that their polyfuses at 100mA have approx 5 Ohm resistance, causing a voltage drop of V = iR = 0.1*5 = 0.5 V. This isn't ideal, as this means the output USB voltage would be 5 - 0.5 = 4.5 V. (This may cause some devices to not function if they expect 5.0 V)

As the current draw on the USB increases (ie to 150mA), voltage drops further - 0.15*5 = 0.75 V, resulting in an output voltage of 5 - 0.75 = 4.25 V, which is below the USB spec minimum voltage of 4.40V (source). This is assuming the resistance doesn't increase with current draw, although in reality the resistance/current draw plot will look something like this:
resistance/voltage (Source)

From a Q/A with Pete Lomas:

The fuses kick in hard around 280mA and fold back and limit to 140mA. If you remove them then all you have for protection is the 700mA inbound fuse. The tracking on the board is good for 500mA+ so you could if you really wanted to. What about a powered hub – to power the Pi and bigger USB devices.

To power higher power devices: (i.e. USB 2 devices)

You should use a powered USB hub, that can supply >= 0.5A per USB port. This means if your USB hub has 4 ports, it should use at least a 2A power supply.

Other options:

If you don't want to use a USB hub with a second power supply, you can do one of the following options:

These methods are not recommended, and may damage your board/devices.

  • Create/buy a split USB cable that uses a separate 5V source (such as your input source).

    Raspi USB out (pins TX, RX, GND) ___  
                                        \_____ device  
    5V supply     (pins 5V, GND)     ___/
    
  • Join the output of the two polyfuses (solder a jumper across). As the polyfuse current is being split, this will allow up to ~200mA for one USB device, or 50/150 across two.

    Soldering jumper across polyfuses

    You could also join the VCC of both USB ports: (untested)

    Joining VCC

  • Connect the input VCC to the output VCC (disconnect the polyfuses first?). This will allow you to draw as much (combined) as your input can supply.

    Input to output


Disclaimer: I am not involved in the design of the Raspberry Pi, and am not an expert on polyfuses. Modifying your Raspberry Pi is not recommended, will void your warranty, and may damage things. Please do not sue me.

Please feel free to correct any mistakes I have (probably) made!

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8  
Fantastic answer! –  Jivings Jun 15 '12 at 11:47
6  
This is one of the most comprehensive, best answers on this site I've yet to see. Thanks! –  berry120 Jun 16 '12 at 11:18
    
+1: That's the most complete answer I've ever seen on any SE site! –  Jon Egerton Jun 24 '12 at 21:50
    
@JonEgerton I don't know, I think Martin Thurau thinks differently stackoverflow.com/questions/3255/… –  Joe Jun 25 '12 at 12:40
2  
Needs updating for REV2 boards.. +1 –  ppumkin Oct 1 '12 at 14:55

Just noting that the rev 2.0 board no longer has the USB polyfuses, just the input 1.1A polyfuse. So the issues mentioned here should no longer be present in current generation Raspberry Pi boards.

source

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And REV2 Boards can handle a MAX of 2.5A if you bypass the poly fuses by supplying power via the GPIO +5V5 instead. –  ppumkin Oct 1 '12 at 14:54
    
Is the 2.5A figure definitely confirmed? It has been said that max 2.5A should be given as back fed power from the USB A connector, but can this same be given on the GPIO +5V5 pin? –  Nakedible Oct 1 '12 at 17:14
    
Also, I don't think anything says +5.5V, USB voltage limit is +5.25V. So please disregard the +5V5 in my comment above. –  Nakedible Oct 1 '12 at 17:20
    
Oh yea,Sorry its 5V0 - My bad- I was thinking of something else. Rev2 is confirmed! Rev1 not- Safe to say REV1 = MAX 1A , REV 2 = MAX 2.5A –  ppumkin Oct 2 '12 at 8:43
    
Here, it has been said that the GPIO pins would only normally be rated for 1A. raspberrypi.org/phpBB3/viewtopic.php?f=29&t=18928 Do you have a source for your information? It would be very nice to get to the bottom of this. –  Nakedible Oct 2 '12 at 9:03

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