1
\$\begingroup\$

I am looking at this USB connector. It connects to two of your computer's USB ports in order to provide more current than a single port would be able to.

From what I have heard, connecting two power lines directly is not advised as a slight voltage difference between the two can cause a current to flow between them and lead to problems.

What kids of problems would a direct connection cause?

And what are some ways this problem is remedied?

\$\endgroup\$
  • 3
    \$\begingroup\$ Both USB Ports are most likely supplied by the same 5V rail, so no Problem there. \$\endgroup\$ – Jogitech Apr 11 '18 at 7:10
  • \$\begingroup\$ Even if they were powered from different rails, they wouldn't be connected directly, as you said, bad idea. They would likely have diode protection \$\endgroup\$ – MCG Apr 11 '18 at 7:46
  • \$\begingroup\$ @MCG Next time I'll design a USB gadget, I'll let it accidentally apply +30V on VBAT because hey, they would likely have diode protection! \$\endgroup\$ – Dmitry Grigoryev Apr 11 '18 at 12:09
  • 1
    \$\begingroup\$ @Dmitry Grigoryev think you looked into that a bit too much. Hence why I said most likely and it was a comment, not an answer \$\endgroup\$ – MCG Apr 11 '18 at 12:22
2
\$\begingroup\$

What kids of problems would a direct connection cause?

Worst case: A user connects 2 computers (like laptops) directly together and damages the hardware. Some products (with a fruit logo) have known problems when a USB port is externally powered while the device was off.

\$\endgroup\$
  • \$\begingroup\$ Now I not sure whether you mean small, red-ish fruits, or larger, ranging from yellow to dark red \$\endgroup\$ – Michal Polovka Mar 22 at 12:51
2
\$\begingroup\$

The PC USB ports are rated to 500mA each. Actually they are also hardware limited to 500mA, if you sink more, the voltage will drop and/or the peripheral will be stopped by software and you will get also a warning on the screen. Using a Y cable allow up to 1A current for the peripheral, one USB will limit the supply to 500mA, the other will source the remaining current needed. Only one holds the data lines the other is just for power. Both USB connectors must be connected to the same PC where the supply comes from the same 5V line. Connecting to two different devices is not recommended. The cable is made only to be connected with both connectors on the same PC.

\$\endgroup\$
  • 1
    \$\begingroup\$ A single PC can still have several USB hubs internally, and different kinds of USB ports (USB2, USB3, USB-BC) with independent power supplies. \$\endgroup\$ – Dmitry Grigoryev Apr 11 '18 at 12:03
  • 1
    \$\begingroup\$ This is less likely for common PC-s with an usual power supply to have two 5V independent lines since 5V is less used for anything else. \$\endgroup\$ – Dorian Apr 11 '18 at 12:30
  • \$\begingroup\$ When you say "common PCs", do you mean desktops? The most common PC form factor (in terms of units sold) is a laptop. \$\endgroup\$ – Dmitry Grigoryev Apr 11 '18 at 12:45
  • 1
    \$\begingroup\$ Cool down, a laptop is less likely to spend precious space for that. Maybe some usb's placed far where it would be convenient to use a different 5V source. Even so, it's not so dangerous as it seems, there can be at most tens of millivolts between them. I would blindly short two USB-s on the same machine safely. In fact I did. \$\endgroup\$ – Dorian Apr 11 '18 at 13:11
  • 1
    \$\begingroup\$ It is not exactly what I meant to say. I did that many times on many machines, PC or laptops. Nothing happened and I doubt that something can happen. However, an Y cable on a PC and a wall adapter is really suicidal. Wall adapters have more or less higher voltages up to 5.4V to compensate the voltage drop on the contacts not speaking of what happens when powering off only the PC. Never did that. \$\endgroup\$ – Dorian Apr 11 '18 at 14:23
0
\$\begingroup\$

Most of the PC mother board can provide at least 1A, some can even supply 2A current. The manufacture usually use one fuse for several usb ports. So have a look at the user manual of mother board before you buy any Y cable.

\$\endgroup\$
0
\$\begingroup\$

You got it right: using such a cable can overload USB power supplies and damage the hardware in some circumstances. This is why Y-cables are explicitly forbidden by the spec:

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.

It's true that such damage is unlikely to happen in practice, but if it does happen, you will not be covered by warranty. E.g. I used to have a laptop which kept one USB port powered while hibernated, to let the user charge their phone or whatnot without having to start the laptop. With such a cable, you would risk to damage the USB ports if you forget to unplug the cable before hibernating.

The problem is remedied by refraining from using non-compliant hardware. If you bought this cable, return it to the seller for a refund. If the seller refuses, try to get chargeback on the grounds that you've been sold non-compliant hardware. There are plenty of external HDDs which either can work from a single USB port or have an external power supply, so there's no need for such a cable if you use those.

\$\endgroup\$
  • 2
    \$\begingroup\$ You're right, but Y cable wasn't always prohibited, just after 2010. \$\endgroup\$ – Dorian Apr 11 '18 at 12:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.