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If the host computer for an USB device doesn't provide enough power for a device which has no dedicated power port, can I connect the power-only-plug of a Y-USB-cable to an other computer, an USB-charger or even a regular mobile charger? I'm afraid this could damage some of the devices.

Edit: The device in question is a USB Hard drive (Samsung S2 Portable 640GB (but on the back it's written to use 0.85A)) which is supposed to work whit just a single USB port. It did so well on a other laptop and whit some cables even on my actual net book back in Europe. But now in Malaysia it works only some times (I still try to figure out what the conditions are) and then it stops working after a while (maybe when it goes to power save mode). I heard from others their hard drives need one more power plug here in Malaysia to but can't imagine why...

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Afaik the Y-cables don’t comply with the USB standards. –  kinokijuf Jul 14 '12 at 8:04

6 Answers 6

up vote 2 down vote accepted

Yes. You can get USB power from another source but it still might not work...

USB power is only +5 volts DC. And as long as you don't connect the inner data lines (which can cause other problems) you should be just fine getting your +5VDC from somewhere else. But if this is an iPod or something then it probably still won't work. And the reason that it might fail is because some devices like iPods require a minimum amount of current as well as possibly even certain signals like a 1.5VDC level on a data line, for example. Just make sure you don't oversupply/overdrive your +5VDC with like a 20-amp source or something (highly unlikely from anything on a computer, but still not impossible).

Frankly, you'd probably be better off getting a powered USB hub. As long as you don't try to get an expensive hub, like for USB-3 or something, you probably can find one for under $20 or even under $10. So is it worth the potential damage to another computer over what might not even cost a saw buck?! (It's your decision...)

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Thanks for answering the basic question! It made me worry more about destroying any thing. Will try to find a small powered hub. As I'm traveling I try to minimize the amount of stuff to carry around and would like to use the same charger I use for my phone and batteries. But maybe better not. I add some info about the device in the question. –  Den Jul 14 '12 at 4:37

It should be safe electrically safe so long as your device complies with USB standards which state a max of 500ma can be drawn from a USB 2.0 port and a max of 900ma can be drawn from a USB 3.0 port. Powered USB devices have different standards (described in the provided link). With all that said I doubt your device would work correctly if both ends of the Y cable were connected to different computers as the computers would be fighting for control of the data. Of the two options you suggested the USB charger idea sounds more likely to work. Possibly an even better alternative would be the use of a USB power injector.

UPDATE:

Your device is USB 2.0 and the max load a USB 2.0 port allows is 500ma. You have stated your device can draw up to 850ma. The load your device puts on a single USB port exceeds the maximum allowed power consumption written out in the USB specs. A "Y" cable is a hack way around what the USB specification allows. Using two USB 2.0 ports each of which provides 500ma of power means 1000ma of power is available to your device, regardless if you plugged both ends of the "Y" cable into one computer, or plugged one end into one computer and the other end into a different computer. With all that said a USB power injector is designed to do exactly what you want to do without putting undue load on your laptop's USB power circuits or power supply. Do not let that dissuade you from using the "Y" cable though. Personally if I were in your position (wanting to travel with as little baggage as possible) and I already had the "Y" cable I would just use it and I would plug both ends into the same computer. I'm sure it will be fine, and I'm not just saying that :)

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Thanks. On a Y-Cable one end is only connected for power any way. There should be no problem whit the data. But that power injector looks interesting. –  Den Jul 14 '12 at 4:40
    
Is this USB power injector actually in any way different than my Y-Cable connected whit whit an adapter for this round bower plug? –  Den Jul 14 '12 at 4:54
    
I have updated my answer. –  ubiquibacon Jul 14 '12 at 5:18
    
In theory, without proper negotiation a USB 2.0 port may only provide one unit load, i.e. 100 mA. The power plug does not have data lines, so cannot be used for negotiation. Therefore, in theory, the maximum possible (total) current draw with one standard plug and one power only plug is 500 mA + 100 mA = 600 mA. –  Bob Jul 14 '12 at 12:33
    
@Bob Interesting point. I didn't think of that. –  Den Jul 14 '12 at 15:09

TL;DR: It depends on the power source. Assuming the power is well regulated (i.e. 5V with 5% tolerance either way, not too much fluctuation/rippling1), it's perfectly safe.


For the full explanation, first, we need to delve into how a USB connector/cable is wired.

Image of a USB connector

With a basic USB 2.0 cable, you have a ground (GND) and a +5V (VBUS) for power. You also have a D- and D+ for data. I'm not going to go into the data lines here, since the question is addressing power.

There's really two and a half types of USB Y cables. Physically, there's:

  • One male connector, two female connectors

    There are two types of the one male two female cables (hence the two and a half). One is a basic charging cable, splitting one port's power across two devices. The other is a specialised cable that somehow splits the data lines - and it's only used in very specific applications.

  • Two male connectors, one (fe)male connector

    This is the one you are referring to. What happens is you have your USB cable, one side for the host and one for the device, but with an extra connector attached. This extra connector does not have any data lines (D-, D+); it only has the power lines (GND, VBUS). It's attached in parallel to the existing cable. In other words, VBUS is connected to VBUS and GND is connected to GND.

            Power connector
    
     VBUS -------\
                 |
       D-        |
                 |
       D+        |
                 |
      GND -------+-\
                 | |
                 | |
                 | |
     VBUS -------/ |
                   |
       D- ----     |
                   |
       D+ ----     |
                   |
      GND ---------/
    
            Host connector
    

    What happens is the voltage becomes the average of the two and the max current is the sum of what the host connector and power connector can provide. Assuming both are connected to standard USB power supplies, the combined VBUS line should be at +5V.


Now, what does this mean? Well, max current is the max available, you can draw up to this amount. In other words, the max current provided by the sum of the connectors should be at least what your device needs. As a rule of thumb, being able to provide more power is not a bad thing. The voltage, for USB, must be at 5V.

So, if you have a computer connected to the host connector, the USB 2.0 specifications limit a standard port to 500mA. This may not be enough for higher power devices, in which case you can connect the power connector to an additional source, often another port on the computer. Depending on the port2, you may get up to 500mA again. If you add them, you get 1000mA at 5V, providing more power for the device to draw on.

Now, your question asks about an external power source. Assuming a good USB charger, and no shorts anywhere, no damage will result: the voltage will be 5 V and the max current will add to whatever - but the device will only draw as much as it needs.

It gets tricky, however. There are cheap USB chargers out there with horrible regulation and a wide variance in voltage, especially depending when loaded. This may not be a major issue with some devices, even possibly including phones which may have their own internal regulation. However, other devices may be more sensitive. It depends on how bad the power source is, and how sensitive the device is; both are really a case-by-case thing, though some device categories will perform better than others.

Generally, if you have a reliable USB power source/charger, everything will be perfectly fine. A not-so-reliable one might work, but, well, won't be very reliable.


1The USB specifications specify 5% tolerance for USB 2.0, i.e. from 4.75V to 5.25V. The actual device you are using may be more lenient.

2A standard port should only provide up to 100mA without negotiation. Since a power connector has no data lines, it cannot request more power. So typically the host + power connectors can provide a max of 600mA. However, there are special charging ports in the specification that may provide 500mA or even more without negotiation, and those ports may actually be more common nowadays. This isn't really relevant to using an external power source, however.

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"There's really two and a half types of USB Y cables": I introduce you to yet another kind of USB Y cable: a triple male! superuser.com/questions/559589/triple-ended-usb-cables –  That Brazilian Guy Mar 23 '13 at 15:13
    
"A standard port should only provide up to 100mA without negotiation." Amen! If I could give this question +10 then it would get that just for this part alone. –  Hennes Mar 23 '13 at 15:15

I believe that the Y-cable method of increasing the available power to a USB device should be fine as long as the external 5 V DC supply is very well regulated (ie nice clean and constant 5 V DC supply). The Y-cable will create a common ground between the two supplies, and the USB device should be able to get the extra power it needs to function properly.

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Yes, but it is not recommended to use 2 different computers. A Y cable acts accordingly, it is both for power and data. You plug the 2 ends that are closer together into 2 usb ports on your computer, then plug it into your device. Unless your device is powered by an adapter, then you might just fry your external drive.

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This whole thing is a bad idea. You should never directly connect two power sources to any one point.

Let's say your USB port is +-5%, so the computer is running at 5.25V DC, but the USB power supply your plugging the other end of the Y into is at the other end of the 5%. i.e. it's voltage is 4.75V DC. That gives you a difference in potential of 0.5V DC. Power doesn't have to be connected to ground to flow; just a voltage lower than it.

That said properly designed supplies are supposed to have diodes on their output to prevent back feed into the supply. Who knows if your stuff has these. If you want to do this, then you have to sever the positive line on the end of the Y cable that will be plugged into the computer. Don't sever the ground line! The grounds of the two supplies need to be connected.

(This advice is given without warranty and is used at your own risk.)

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