I am working on an adapter that will allow a device to be connected to both a PC and external power supply. I am wondering if I will need to isolate the 2 5V supplies with a DC/DC converter and some opt-couplers or will I likely get away with what I have below?

Am I completely off here or am I heading in a reasonable direction?

USB Suppa Power

EDIT: This is a computer talking to a tablet device.

  • \$\begingroup\$ Is there really an issue? Will it be connected to both PC and external power supply at the same time? No tablet requires isolation that I know of since it's either one of them, never both. \$\endgroup\$ – Gustavo Litovsky Mar 1 '13 at 22:55
  • \$\begingroup\$ Yes, it will be hooked up to both at the same time. \$\endgroup\$ – Hair_of_the_Dog Mar 1 '13 at 23:03
  • \$\begingroup\$ can't you just not connect the USB power from the computer? \$\endgroup\$ – Grady Player Mar 1 '13 at 23:06
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    \$\begingroup\$ I am saying you should use a properly sized external power supply and not use the computer's USB power at all. then the differential signal on the USB should still work... and you will only have one ground state. \$\endgroup\$ – Grady Player Mar 1 '13 at 23:16
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    \$\begingroup\$ @Hair_of_the_Dog electronics.stackexchange.com/a/57216/7036 The question deals with dual supply circuit where USB +5V is disconnected when the external supply is present. $0.02 worth. \$\endgroup\$ – Nick Alexeev Mar 1 '13 at 23:58


USB is a bidirectional differential bus, and will be hard to brute-force isolate with optocouplers. I'd recommend purpose-built IC's to do the job for you. I've been happy with the ADUM line, though I haven't done USB isolation.

As to whether you need to isolate or not, we'd have to know much more about your application. There's no reason per se why you would need to isolate just because you're using an external supply in addition to Vusb. That said, I've used unisolated USB audio interfaces that I sure wished were isolated.

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  • \$\begingroup\$ This is a computer talking to a tablet device. \$\endgroup\$ – Hair_of_the_Dog Mar 1 '13 at 22:36
  • \$\begingroup\$ You shouldn't need an isolator for that, but you also can't tie your supply rails directly together. Try something like at electronics.stackexchange.com/questions/57209/… \$\endgroup\$ – Scott Seidman Mar 1 '13 at 23:17
  • \$\begingroup\$ The grounds in my schematic are tied, but the 5V rails are not. I plan on connecting both the computer and the external power supply at the same time. You'll notice that the power coming from the computer is connected only to an LED, which is completely optional. The external power supply is connected to the LM317, which is producing 5V that is output to external tablet device. \$\endgroup\$ – Hair_of_the_Dog Mar 1 '13 at 23:49

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