Do USB ports carry voltage and how much

By DCO57
Jan 21, 2006
  1. Fellow techs,
    I definately need to improve my electrical knowledge of PCs. Other then using a multimeter to test molex and ATX/mobo connector voltages, I really haven't focused on the electrical aspects of troubleshooting.
    I recently had two situations regarding USB ports and voltage.
    The first is a Dell Dimension 5150. The customer plugged in a Dell "All-in One" printer (A968..I think) and it would only work with the USB ports on the front of the case, not any of the 5 in the rear. A fellow tech mentioned that some USB ports have more voltage then others and maybe the rear USBs didn't have enough power to run a power hungry printer/scanner/fax/copier. This was the first I heard of USB ports having different power ratings. Has anyone heard of this?
    The other situation is one where a customer plugged in a cable modem USB line to a USB port and it sparked and fried the mobo.The mobem wasn't plugged in to power yet, just the coaxial cable. I thought the coaxial cable must be coming in contact with electric, however it works fine when he uses it on his laptop.This may or may not have anything to do with the power coming FROM the USB port, but if both the port and the cable had voltage...POW...which is what happened.
    Can anyone refer me to any links concerning USB voltage.
    Thank you in advance.
  2. ej0c

    ej0c TS Rookie

  3. Nodsu

    Nodsu TS Rookie Posts: 5,837   +6

    All USB posts carry the same voltage (~5V), what your (obviously ignorant) fellow tech was referring to is the fact that the amount of current you can draw from a given USB port is limited and depends on how many other devices you have on the same bus segment. And this is only a problem when you are using devices that draw their power from the USB port. Most printers don't.
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