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This question seems like it would have a logical answer, but I thought I’d ask to be safe.

As shown in the photo below, there exist externally-powered USB hubs (A) and USB power-adapters—like the kind that come with some GPS and other USB devices—(B).

Connecting devices like keyboards, mice, flash-drives, MP3 players, etc. to the powered hub (A) is not a problem, as expected. The devices simply turn on (eg light up) or charge, or even do nothing as the case may be.

But, what happens when you plug a device into a USB power-adapter like (B)? I’ve plugged MP3 players (and of course the GPS device it came with) into it and they simply charged. But what would happen if I plugged a mouse or flash-drive into it?

Logic says that they would not be harmed and act exactly like they would if plugged into a powered-hub, but is that indeed the case? (unusual, proprietary adapters notwithstanding) USB connectors only have four pins, +5V, GND, +DATA, -DATA; so presumably only the power pins would be connected, and thus it would be harmless (as plugging it into any other live port would be). Again, this is excluding unusual, non-standard devices/adapters since they would not be compatible anyway (you can’t use the pins for whatever you want).

Am I right?

Two different kinds of powered USB ports

Update

Okay, so I took the chance and tried plugging a flash-drive into the adapter. My idea was to use the orange LED in the drive, plugged into the adapter as a cute little, low-power night-light. Unfortunately I over-estimated the luminosity of the LED (I could have sworn it was a lot brighter). The first shot is of the drive with some ambient light (a 12-ultra-bright-white-LED torch with 3xAAA), and the second is of the hallway with just the drive’s light (at night of course).

I tried a different drive and it didn’t even light up (because there’s no data communication). I then checked both drives with my computer and they were both fine.

Oh well, at least I learned something. :-)

Flash-drive LED light with ambient lighting Flash-drive LED light without ambient lighting; sooo dark…

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  • It's my theory that you would create a short circuit since there won't be any load or drop in voltage. I don't think that you would cause catastrophic damage per se, but you would most likely bust a couple of capacitors. Also, I think you are doubling the voltage as well. I haven't quite wrapped my (novice) electrical knowledge around it yet.
    – skub
    Commented Jul 28, 2011 at 4:02
  • @skub, I think Collin is on the right track about how it would be like if the device were connected to the hub by itself or to the computer while it’s off (my last system had powered front USB ports, even while off, so I could charge without turning it on).
    – Synetech
    Commented Jul 28, 2011 at 7:10
  • I totally mis-read the post. I thought he was going to plug A into B. HAHA!
    – skub
    Commented Aug 1, 2011 at 2:13

3 Answers 3

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Your logic is accurate; there is no inherent reason to expect harm. The device will just see a +5V source. Various power adapters have the data lines open, shorted, or connected through a resistor network to the power lines, depending on which of the standard or non-standard “this is a charger” schemes they use; but I believe the USB specification requires devices' data transceivers to be robust against this.

Possible hazards:

  • Some power adapters might be poorly regulated, or have higher voltage for faster charging of the device they are meant for. At a minimum, check the label on the power adapter to make sure it is specified to output 5V. (Higher current rating, i.e. current capacity, is not a problem unless it is a “linear” power supply, which is unlikely in any modern portable gadget.)

  • An especially poorly-designed peripheral might be damaged, not by the power adapter directly, but by being connected to a power source for a long time without being communicated with (staying in initial power-on state). This is unlikely.

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  • Yes, the adapter (at least the on in the photo) is indeed +5V, and adapter like (B) aren’t just for charging, they are also for powering the device from the mains instead of the battery, so I would think/hope that it can handle being plugged in.
    – Synetech
    Commented Jul 28, 2011 at 7:12
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But, what happens when you plug a device into a USB power-adapter like (B)? I’ve plugged MP3 players (and of course the GPS device it came with) into it and they simply charged. But what would happen if I plugged a mouse or flash-drive into it?

Nothing. The USB device will power up, and attempt to communicate over the D+/- lines. Without any communication, the USB device will either go into standby, or keep polling the data bus (depending on how the hardware is implemented).

Logic says that they would not be harmed and act exactly like they would if plugged into a powered-hub, but is that indeed the case?

Yes. You can't force more current into something, the device itself draws as much current as it needs to drive the load at the supply voltage. Since the device "sees" a normal USB hub, there is no condition that would cause the device to be damaged.

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  • > Since the device "sees" a normal USB hub, there is no condition that would cause the device to be damaged. The USB charger/power adpater isn’t a hub.
    – Synetech
    Commented Aug 2, 2011 at 2:55
  • Yes, that is why I said it "sees" a normal (powered) hub. It has the same electrical characteristics on the power and ground rails.
    – Breakthrough
    Commented Aug 2, 2011 at 10:26
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    Hmm, interesting. So USB devices consider any powered port to be indistinguishable? A USB charger is no different than a hub that is not sending data? It certainly makes sense since there’s only the four, clearly defined wires; however I’ve seen things use (other) things differently from the spec, so I keep worrying about what something may be actually doing or interpreting (eg, there are 5, 6, 7+-pin devices that have USB adapters, which just doesn’t make sense).
    – Synetech
    Commented Aug 3, 2011 at 1:34
  • @Synetech some of those devices make use of powered USB ports, some could be USB 3.0, some mix eSATA with USB connectors, and some just aren't up to USB spec. It's up to the device to determine how much power it draws, since the device has to signal the computer with the power it is going to draw before it actually turns on. With a charger, it has all the resources it wants (as well as no communication method to let the charger know the power it is going to draw).
    – Breakthrough
    Commented Aug 3, 2011 at 1:39
  • I mean that while it all makes sense because there are only four connectors in USB 2-, I don’t know how things may actually use them because there are some devices that normally have more than 4 pins, yet have USB adapters. That makes me worry that they may be using a pin for something other than what the spec says (like how some sound cards use the same port for both mic and speakers and use fancy detection to determine what is plugged in and how to use it). I’m just relieved to get confirmation that the pins should be used for what the spec says.
    – Synetech
    Commented Aug 3, 2011 at 1:49
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I believe you're right.

In particular, the wall adapter should be just the +5v and GND lines wired up like you mentioned, so a mouse or flash drive should just get power and nothing else. It'd be like they're connected to a computer in standby.

A USB hub would also have the two data lines connected, but it's just to the hub circuitry which is meant to talk to devices anyway.

I can't see any of this causing problems. If it does, the adapter isn't conforming to the USB spec and should not have a USB port to begin with.

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  • I like the part about specifications and being connected to a computer in standby. My old system had powered front ports (an iPod would start charing since there is no communication). In fact, my TV-top DVD-player also has a powered port; the light on the flash-drive goes on when plugged in and stays on unless I turn the player on and then time things right to turn it off right when the light goes out (the drive’s light flashes or fades on and off when the player is on).
    – Synetech
    Commented Jul 28, 2011 at 7:30

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