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A USB 2.0 cable should have 4 wires: VCC, D+, D- and Ground.

Will only disconnecting the VCC OR ground wires damage the connected device or the data wires?

I currently have a USB SD card reader that I like to disconnect and connect. The reader is mounted on the table, so to disconnect the reader, I would plug off the USB cable connecting the reader and the computer.

To solve this problem, I would like to place a switch in the power lines, but I wonder if this will break the device since the data wires are still connected but the power lines are disconnected.

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  • \$\begingroup\$ Depends on the device connected to it \$\endgroup\$
    – Marco
    Commented Jan 6, 2016 at 16:22
  • \$\begingroup\$ @d3l The device is an SD Card reader that is connected to a computer \$\endgroup\$
    – Bryan
    Commented Jan 6, 2016 at 16:24

2 Answers 2

Reset to default
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Put a disconnect switch in the power line, but leave the ground connected all the time. This works fine and does what you expect.

(Obviously if you try this with a self-powered USB hub or device it may ignore you turning the power off, but most are bus-powered devices.)

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put the switch on the data lines. USB is designed so that the data lines connect after the power connects. reversing that timing may have unexpected consequences,

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  • \$\begingroup\$ @Bryan, to (over-) clarify Jasen's point, if you look into a USB connector you should see the outer gold contacts are longer to ensure that power connects first. \$\endgroup\$
    – Transistor
    Commented Jan 7, 2016 at 0:09

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