有時，當技術規範“禁止”使用某個特定的配件時，可能會讓人感到困惑，然而每個人似乎都認為使用有問題的配件是完全可以的。到底誰是對的？今天的超級使用者問答帖子回答了一位憂心忡忡的讀者的問題。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

照片由丹尼斯S.赫德（Flickr）提供。

問題

超級使用者讀者muttley91想知道在USB外圍裝置上使用Y形電纜是否有任何風險：

I have been advised to use a Y-cable to power a portable USB hard drive from two ports since it is unable to draw enough power from a single port. Are there any risks or dangers of overloading or damaging the hard drive by using a Y-cable this way?

將Y形電纜與USB外圍裝置一起使用是否存在任何風險？

答案

超級使用者貢獻者misha256和Dmitry Grigoryev為我們提供了答案。首先，misha256：

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 “forget your silly rules” and uses Y-cables all the time. Technically, there is pretty much zero risk of anything bad happening, just do not use Y-cables via an unpowered USB hub. Plug the connectors straight into the computer’s USB ports.

“Are there any risks or dangers of overloading or damaging the hard drive by using a Y-cable this way?”

Using a Y-cable does not increase the voltage, it only makes more current available, so it is all good. The hard drive will take as much current as it needs.

You are far more likely to damage the contents of a hard drive by running it under-powered. There is nothing worse than write operati*** failing midway through. So I implore you, use the Y-cable.

接著是德米特里·格里戈裡耶夫的回答：

Y-cables are forbidden by the USB specification for a reason. Connecting the A-plugs of a Y-cable into two different host ports (i.e. 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 the 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 problem #1 above (perhaps less severe).
3. If you are extremely unlucky, you may get two computers with poorly isolated PSUs connected to different mains (120V or 240V) networks. Connecting those with a common ground cable (which is a 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 usage.

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

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