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My question is quite simple. I'm asking if I can use the power cable you get along with it as a USB power supply. Instead of plugging it to a power socket, can I use the USB to connect directly to my computer? I'm asking this because I'm not sure about the voltage and I'm afraid I might fry it accidentally.

5

I do this most of the time using my Model B Raspi. It works fine, even when overclocked. However I don't know what long-term effects it might have, even if it's always worked fine for me.

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    very dangerous advice, that might work for you, but may burn the PC motherboard for someone else. – lenik Aug 4 '13 at 3:41
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    @lenik As I said, it might have long-term effects, but user1277207 doesn't seem to care because he ticked the answer. At his own risk though. – Matthew Aug 4 '13 at 14:18
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    @lenik : Matthew is stating a true objective fact, and not giving any advice. I'm glad I found this answer. – Abhishek Anand Oct 9 '13 at 1:21
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    Lenik it wont burn the motherboard. All USB chips have over current detection. Some motherboards have high power ports for up to 1000ma for super fast charging but that is not a standard just a feature. If the current goes off the USB port usually resets and if it resets a few times it will go into off mode to prevent short circuit damage if it may exist. The fact it works for Mathew if maybe because he does not plug anything into the Pi USB port or he uses another USB powered hub. It is not advised to use PC power because of reset problems on the port as B needs up to 1A – Piotr Kula Nov 9 '13 at 19:29
  • The answer below that was downvoted is more relevant than this one- the one with Y cable! -1 ! – Piotr Kula Nov 14 '13 at 16:45
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The most general answer would be: it depends.

I personally would never rely on the regular USB port's ability to provide such level of power as the one required by the Model B, but Model A's much lower consumption would be under the usually known 500 mA limit on the USB port.

In my experience, a number of USB hosts that I have tried in the past aren't even able to supply the old nominal 500 mA of the standard. I feel this value changed from older to newer USB specifications, last one I read was version 2.0.

Consider that "check your power supply" is by far the most common answer I give to a myriad of different issues I am asked about concerning the Raspberry Pi. Just use a good reliable power source that can provide 1 A, if you don't want to run into random problems (from reboots to dying network adapters to SD writing errors).

  • How do I check the power output from USB safely? I hava a digital multimeter and I am not sure which pins to touch the probe with – Abhishek Anand Oct 9 '13 at 1:22
  • What do you think about using two USB power supplies for the power of RB3b or RB2b? I am thinking if the device can receive power from many USBs at once. Now even 4 USBs in RB3b so I think such an power would be sufficient (2A) but not sure if all can receive power. – Léo Léopold Hertz 준영 Apr 10 '16 at 12:18
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You can use the Y usb cable to get 1A from 2 ports instead of one.

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    +1 I think this is an answer. Maybe not fully expanded but there is something in here that makes sense- especially that the OP wants to connect it to the computer USB ports. I thought using those Y cables is supposed to give you up to 1A form both ports? REF: raspberrypi.org/phpBB3/viewtopic.php?f=46&t=13580 – Piotr Kula Nov 14 '13 at 16:43
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USB, according to the spec, can only provide 500mA. But it depends on the motherboard you have in your PC. Most I've heard of happily provide more. If you draw more that the mobo is willing to give you, a fuse is tripped. Most mobos have polyfuses, so the fuse will reset after once everything is normal again. But there is a very small risk that you will permanently disable your usb port.

  • I would add that polyfuses work by heating a special compound which changes its electrical properties with temperature. This allows to draw current spikes (1A is not uncommon) for a couple of seconds before fuse gets hot enough to trip. Many portable HDDs count on this, as they require significant current during drive spin-up. – Dmitry Grigoryev Nov 18 '16 at 14:24
1

All USB ports work on 5V. That said, some ports may not be able to deliver enough current to the Raspberry.

I wouldn't rely on it for continuous operation. Worst case scenario the port's polyfuse on your motherboard will open the circuitry, and turn off the port for some amount of time. I doubt there would be any damages to the PC or the PI, but it depends on the motherboard (i.e. if it has said fuses incorporated into the design - some older, archaic ones may not have them). You'll probably be fine (at least for some time, since polyfuses have limited trip cycle, i.e. the number of times the fuse can be blown without a failure). In terms of doing things on Raspberry though - it's risky. Randomly turning off the device may corrupt the file system on your SD card.

Since laptop/PC USB ports are not suited to provide higher amperage, a better solution would be to use powered USB hub, or USBv3 high power port if you have one available on your PC.

If you draw too much current from USB port the voltage goes down by a significant amount, before the polyfuse kicks in. That's something to consider, since Raspberry PI's components doesn't work terribly well when on under-voltage.

Speaking of under-voltage...

Another thing to consider is the increased power consumption when the temperature goes up. The under-voltage scenario can lead to heating of internal components, both on Raspberry and PC - this leads to increased internal resistance, and even more power consumption. Due to this effect, the polyfuse openings can kick in randomly depending on temperature. The device may power on when cooled down, and then power off after temperature builds up. This may lead to random Raspberry shutdowns, and can be disastrous for SD card inside.

  • Comments are not for extended discussion; this conversation has been moved to chat. – Ghanima Nov 18 '16 at 15:47
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You can use (usually blue coloured) USB 3.x ports (at least those marked with a battery symbol) if your PC has one. They can provide more than the 500mA of a normal USB 2.x port.

I have an internal USB 3.x hub in my PC which suffices to power an Raspberry Pi 3 in most cases. (Occassionally under heavy load the symbol for not enough power showed up on the screen, but it never crashed or rebooted because of that.)

At least Thinkpads (maybe also other devices) also have yellow coloured USB ports (see https://superuser.com/questions/218053/are-usb-3-0-ports-yellow) which can also provide power if the laptop is not on. That's probably also to consider when choosing which USB socket to use to power an Raspberry Pi.

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Yes you can!😉 You can even connect your otg supported phone to raspberry pi using another micro USB data cable!! I do this most of the times on my Moto g4 plus 32GB variant

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As long as your power supply will supply enough power (700mA) for just the Raspberry Pi, no accessories, you will be fine. No need to worry about blowing anything up.

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    officially, there are no PC USB ports rated 700mA. – lenik Oct 9 '13 at 1:37