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I have been advised to use a USB Y cable to power a portable hard drive from 2 ports since it could not draw enough power from a single port. Is there any danger of overloading the drive this way?

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What do you mean? Why would it be dangerous to do what the drive is designed to do? – Scott Chamberlain Jan 6 at 21:55
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no, power is Drawn into a system. it is not normally pushed into it, unless something is very very wrong (like lightening). – Frank Thomas Jan 6 at 23:01
    
This is not an answer to "is there any danger..." but solves the powering problem: get some USB phone charger, and connect the power side of the Y cable to the charger. – Mindwin Jan 7 at 15:05
    
Connecting the power side to a phone charger and the data side to a computer is a recipe for disaster if the power lines from each USB-A port aren't diode-protected. A cheap Chinese knock-off charger (and even some brand name ones) could inject noisy power, excess voltage, etc into a computer if not well regulated. Conversely, an under-voltage charger could get backfed from a computer and lose its magic smoke if it's not well-protected (which I can almost guarantee it won't be). – Doktor J Jan 8 at 16:43
    
I have a drive which usually needs a Y cable to spin up and then works without it. It does seem to get disconnected occasionally when using one plug, so I try not to do it when I have a choice. The key here as mentioned in several answers is that USB ports (are supposed to) provide a fixed voltage. As long as they do that, then the device will "determine" how much current it needs and will draw (at most) that much. This is because of basic physics and is not dependent on anything "fancy" in the USB port or in the device. Adding a second port just makes more current available. – Joe Jan 12 at 1:19
up vote 68 down vote accepted

The USB specification prohibits the use of Y-cables:

Use of a 'Y' cable (a cable with two A-plugs) is prohibited on any USB peripheral. If a USB peripheral requires more power than allowed by the USB specification to which it is designed, then it must be self-powered.

But the real-world said "to hell with your silly rules" and uses Y-cables all the time.

Technically, there is pretty much zero risk of anything bad happening. Just don't use Y-cables via an unpowered USB Hub. Plug the connectors straight into the computer's USB ports.

"is there any danger of overloading the drive this way?"

Using a Y-cable doesn't increase the Voltage, it only makes more current available, so it's all good. The drive will take as much current as it needs.

You are far more likely to damage (the contents) of a drive by running it under-powered. Nothing worse than write operations failing mid-way. So I implore you, use the Y-cable ;-)

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I bet something bad could happen if you were silly enough to try to connect two ends of the Y cable to different computers or something else unusual. – Zoredache Jan 6 at 21:59
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@Zoredache, there's a reason the extra bit is so short - you could connect to 2 machines but it would be tricky. – Chris H Jan 7 at 9:37
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@ChrisH use some old USB phone charger to plug the power side of the Y cable. Never had a problem with my drive. – Mindwin Jan 7 at 15:07
    
@Zoredache If you plug it into two computers then which computer would would be allowed to read/write to the drive? – MonkeyZeus Jan 7 at 15:18
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@MonkeyZeus Typically, the cord to the second connector (it's not really a "Y" in the conventional sense, the extra one is actually on a separate wire off of the first USB-A connector) only has the power wires, no data wires. There are even USB 3 versions where only the main one is a USB 3 plug. – Random832 Jan 7 at 16:01

The Y cable is forbidden by the USB specification for a reason. Connecting the A-plugs of an Y cable into two different host ports (e.g. two different computers, a computer and a hub etc.) can lead to multiple electrical problems:

  1. One of the hosts may be unpowered while the other one is powered on. This will result in powered host pushing current to the unpowered one in the wrong direction, possibly resulting in damage.

  2. Even if both hosts are powered, one will provide a slightly higher voltage than the other, resulting in a problem similar to #1, perhaps less severe.

  3. If you're extremely "lucky", you may get two PCs with poorly isolated PSUs connected to different Mains (120V or 240V) networks. Connecting those with a common-ground cable (which is Y-shaped USB) will zap both computers instantly. Though in this case I would be more concerned about electrical safety in general rather than about Y-cable use.

If you make sure to connect both A-plugs to the same USB host, using an Y-cable is fine (I have one myself). Don't worry about over-powering the drive, it will take just as much current as it needs.

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+1 for explaining why the standard says this shouldn't be done by explaining the cases where it can go seriously wrong. – Dan Neely Jan 7 at 15:01
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Couldn't the cable incorporate diodes to prevent backfeed? – Random832 Jan 7 at 16:04
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@Random832 Diodes have a typical 0.6V voltage drop, which is 12% of the voltage USB provides. Add some losses in the connectors and the cable itself, and you'll quickly go below 4V. Many HDDs simply won't spin up at that voltage. – Dmitry Grigoryev Jan 7 at 16:07
    
@DmitryGrigoryev two words: Schottky diode. A quick search turned up a SMT diode on Mouser that has a Vf of 0.35V and If of 2000mA. Still not the greatest, but even with losses it should still be able to get ~4.17V@1.5A to the drive on a 3ft 28AWG cable. If they're smart enough to use 24AWG cable for the power pairs, that goes up to about 4.52V. – Doktor J Jan 8 at 16:55
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@fredsbend Remember the last time you got a device with an unknown connector. Did you read the manual before connecting it, or did you just plug it in because the connector fits? Also, USB is limited to 4 ports, so any laptop which has more is bound to have a powered hub inside. – Dmitry Grigoryev Jan 11 at 11:06

The usual problem with trying to use two DC power supplies in parallel is that one will supply much more current than the other due to slight differences in voltage. This isn't usually a problem with USB as the ports have a maximum rated current draw after which they will not supply any more current. So, while one port may produce the bulk of the current required, it won't produce "to much" current.

This, of course, assumes the USB devices supplying the power are working to spec. There are devices out there that are poorly made and could have issues like burning out. These problem devices are easier to find in the phone charger market.

The cable used for these usually only has the power lines hooked up. If data is possible, it's usually only available on one connection and the Y-adapter should indicate which that is.

As others have mentioned, Y-adapter cables that enable this are not technically USB cables.

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Slight differences in voltage could also cause one to backfeed the other, in theory. – user20574 Jan 7 at 5:00
    
@user20574 This is true. The backfeed voltage would be the difference between the USB voltages. If you've plugged into the same USB power device, the difference should be negligible and the power regulation semiconductors should be able to block it. If you plug into two different devices for power, the backfeed voltage could be (based on the specs) as high as 1V and this could be enough to blow those same semiconductors. – Ouroborus Jan 7 at 5:18
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"ports have a maximum rated current draw after which they will not supply any more current. " Dangerous assumption to make in practice, some hubs etc might just have a self-resetting fuse instead of a proper current limiting circuit. As long as we are not powering from two different devices (see Dmitry's answer below), it should not matter much though. – rackandboneman Jan 7 at 9:03

It would be more dangerous to use the incorrect non Y cable and have the drive be underpowered.

Many laptop sized portable drives have no AC power adapter, to make up for that they use a USB cable that has two on one side so it can draw more power than one USB port can provide.

enter image description here

Only one of the two on the double side will have its data pins connected, the 2nd connector only has the power pins connected.

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In practice "not enough power" is so common as to not constitute a failure mode. Most desktops aren't too fussy about the USB specs and will happily provide more current than specced. A user used to that, plugging in to a laptop (which is more likely to be fussy) Is quite likely to underpower the drive. At least one drive I've had like this specifally says to only use the 2nd plug if necessary, and there's only one way to find out if it's necessary. – Chris H Jan 7 at 9:35

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