How much power can be provided through USB?


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.

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

How much power can be provided through USBSome 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).

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.

How much power can be provided through USB Schematic

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.

 

For more detail: How much power can be provided through USB?




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