USB Isolation

[Oleg] over at Circuits@Home has made a USB isolator for his hacking needs. This isolator separates the signal, ground, and power lines of a USB host device, such as a PC, from a USB device like a USB oscilloscope or logic analyzer. This might be useful for Keyboard sniffing, ECG, EEG or diagnosing the control system on the positive ground of your autonomous Ford 8N. What other applications can you come up with for this tool?

20 thoughts on “USB Isolation

  1. Seems like a great way to protect your computer from mishaps, too — say, accidentally shorting mains voltage on a project board to the USB lines. I’d imagine that would fry any PC USB port, and could potentially take out the whole machine.

    Looking at the data sheet for the ADuM4160 part used on the isolator, the isolating insulation is rated to withstand 5000 Vrms for 1 minute. So it should do a good job of protecting your PC.

  2. @hpux735

    Most host controllers disconnect the port if they sense a short or similar. I’ve created shorts on my Macbook’s(Running debian) ports a few times and there’s a nice message in dmesg saying that the host controller detected a short and disconnected the device.


    W(hy)TF would you have mains around your USB projects? I wouldn’t worry about blowing a port up, I’d worry about blowing yourself up! :P

  3. @cantido

    Home automation
    Lighting controls
    Relay or dimmer packs
    etc, etc, etc.

    There are lots of projects for using USB around mains voltage. And if you are rolling your own, then there is always the opportunity to cross a whire someplace, bridge a solder joint, or just plain user error that could drop mains voltage onto the logic side of board.

  4. My friend fried a $350 mother board and a atmel programmer when he was programming his 12 channel 120v dimming xmas light controller. Then I unfortunantly let his borrow my spare motherboard and the next night he took it out along with a another borrowed programmer.

    This was because he forgot to unplug the programmer and I guess the USB doesn’t like 120V. Oh and right after this he found the mistake on his PCB. But didn’t have a computer to reprogram it.

    Luckily asus didn’t know and replaced both motherboards.

    So yes this would be a great gift for him.

  5. This is a great reference design, but it does NOT provide mains isolation. The PCB layout clearance is only 1.63 mm, which means it can only isolate 150 volts DC at less than 1 watt of current.

    The board should be redone with at least a 5.5mm between the primary and secondary side of the isolation to provide the isolation specified in the datasheet.

    Here is a rule of thumb: If you are working with a component that provides isolation, do not create copper traces that are closer together than the footprints for the part.

  6. @Oleg Mazurov

    The device inserts the same timing delay as a USB hub, but it does not react like a hub. The high/low speed settings on the device takes care of the wait states between signals and the artificial delays required to keep the proper timing requirements. This is why both sides have a separate jumper to set the speeds.

  7. This seems like it could be useful for DIY “medical”/monitoring projects which require safe coupling with a human body, eg OpenEEG and the like. IIRC, they usually use an RS-232 serial port because it’s easier to use optoisolators on the lower speed signals.

  8. Certainly useful for something, but I’m not sure it’s quite up to stopping mains from doing damage, I don’t think that’s quite the idea, just because something isolates signals and 5v doesn’t mean it’ll stop everything you know.

  9. Oh one more thing (last one I promise), if you have issues with shorts you could simply use some diodes at the USB connector, cheap and quick, no need to go through lengths like this project does.

  10. THis is also very useful with connecting with PBX with usb ports for programming. Siemens HiRise 1100 series only allows a USB conection with a proprietary insulating dongle ( a ps/2 connector on the PBX side).

    With this you can just make a connector change-over, and use the insulator in the middle! Thx for the great hack!!!

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