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I'm trying to use Raspberry Pi to power a floppy drive with the 5V GPIO pin. I know that the floppy drive requires a voltage of 5V and a current of 1A and Raspberry Pi gives the output as following:

Maximum permitted current draw from the 5 V pin is the USB input current (usually 1 A) 
minus any current draw from the rest of the board.[14]

Model A: 1000 mA - 500 mA -> max current draw: 500 mA
Model B: 1000 mA - 700 mA -> max current draw: 300 mA

So am I right that when using a power supply with the voltage of 5V and current of 2A to power the Raspberry Pi, 5V GPIO pin will give the output current of 1.3A? Is it safe to draw that much current from the pin or is there a maximum that you can't exceed? Also, when I use the 5V GPIO pin as an input for the floppy drive, then what GPIO pin I should use for the output that comes from it?

I'm not sure if that's possible, so I hope someone could answer these questions.

  • Do you want to use one of the logic I/O pins or the power 5V pin that happens to be on the same connector for the job? – Marco Poli Jul 8 '14 at 13:54
  • I meant using the 2nd, 5V pin. – danrodi Jul 8 '14 at 13:58
  • There are no 5V gpio pins. All the Pi's gpios are 3.3V. If you power the Pi via the microUSB you'll only be able draw an additional 0.5 amps from the 5V rail, as the polyfuse limits the current. If you backpower the Pi through the USB or power the Pi via the 5V rail you will be able to draw 2 amps from the 5V rail. – joan Jul 8 '14 at 14:32
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Short answers: 1-No. 2-There is a 1.1A polyfuse after the microUSB power connector, so anything trying to pull more power than this (includung the 700mA for the Pi itself) will cause the board to stop functioning. 3-The return pin should be GND.

The long version:

First things first: there are at least 2 electrical motors in a floppy drive. And if I am not mistaken, they are 12V motors. So, I don't think you will go far using only the 5V power pins as for powering up a floppy drive.

The Raspberry Pi, unlike other electronic hobby-targeting platform, has no protection whatsoever in the GPIO or power lines. That alone should drive every project thought to make use of these signals to think about some sort of protection, at least a transistor. You are talking power, motors. You should always use some sort of protection and never connect it directly to the Raspberry Pi.

You can use the Pi as a controller, but shouldn't use it as Power Source. Use it to control some power circuit that will source the power you need.

That said, according to the the Raspberry Pi Hardware guide:

The microUSB input port also has a 1.1 A polyfuse (700mA "hold current") which may also have enough resistance (although much smaller than the 140mA fuses) to cause a significant voltage drop on the board, even below its 1.1 A total current.

Deeper information can be found at the Polyfuses article on that wiki.

The I/O pins also have maximum source/sink currents, and these are very low, signal-only.

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