I'm trying to build an USB 3.0 extension cable that provides power for the connected device. Similar to this cable.

I opened up an USB3 Type A male to female (extension) cable and cut the Vcc connection. Then I connected the positive side of my 5 V power supply (usual phone charger 5 V, 3 A) to the Vcc of the female connector and the negative side to the GND wire of the cable.

This setup is working fine if I connect my USB2 storage drive. However my USB3 hard drive fails to spin up its disk. Using an ammeter I can see the current going up to 0.9 A (maximum according to USB 3.0 spec) then falling back down to 0.3 A accompanied by a click sound of the drive and slowing down of the disk. It will continue this until disconnection.

Removing the power supply and reconnecting the Vcc with my ammeter inbetween I can confirm that the drive will use 0.9 A at maximum load.

What am I doing wrong here?

Wiring Diagram:

Wiring Diagram

Note: It's actually a USB Type A plug.

  • \$\begingroup\$ a USB3 device negotiates the power parameters with the power supply \$\endgroup\$
    – jsotola
    Commented Nov 2, 2022 at 20:01
  • \$\begingroup\$ @jsotola okay, that explains the problem. What if I use a normal power supply instead of a charger then? Is there some easy way to make the device negotiate the power with the charger (without using a controller)? \$\endgroup\$
    – CellarMonk
    Commented Nov 2, 2022 at 20:24
  • \$\begingroup\$ this may be an XY question ... what is the actual problem that you are trying to solve by building a powered extension cable? \$\endgroup\$
    – jsotola
    Commented Nov 2, 2022 at 20:39
  • \$\begingroup\$ @jsotola Im trying to get around connecting my USB HDDs using a powered hub for a NAS setup. I have read in some Forums that using a Hub might result in bad performance of the drives. However reading more of the USB spec myself, I can not see why that should be the case. Superspeed is way faster then the HDDs anyways. On the other hand I know from experience that for example connecting multiple extension cables can result in some I/O errors occurring. \$\endgroup\$
    – CellarMonk
    Commented Nov 2, 2022 at 20:49
  • 1
    \$\begingroup\$ @CellarMonk Those Y cables should not even exist because they violate the USB specs and are in fact prohibited. If the HDD requires more power than available from a standard USB source, it should be self-powered by the specs. So if the RockPi is not designed to provide enough current then it isn't a standard USB source. \$\endgroup\$
    – Justme
    Commented Nov 2, 2022 at 22:49

1 Answer 1


To get the gigabit speeds that USB 3.x provides requires finely tuned cables and cutting one open to solder in a separate power supply is likely the source of your failed USB 3.x connection, the shielding has been compromised and thrown off the delicate balance of the transmission line in the cable. USB 2.0 is a slower connection and so is going to be more tolerant of noise or impedance mismatch.

Any passive extension cable is a violation of the USB specification to begin with so you are rolling dice on if it even works. Not only is USB 3.x sensitive to noise and impedance but also timing, and adding even a few inches of extra copper can cause enough delay for the connection to fail.

An "active" cable is more than something that provides extra power, it is a signal repeater. A signal repeater will take the signal in, process out the bits, reconstruct those bits with the noise removed as best it can, then send that on. Since timing is so important to these high speed communications the repeater may have to manipulate the data going through so the data is still considered valid given the added delay.

According to the USB 3.x spec the limit on power is 900 mA but high power devices like drives with motors in them often use USB-PD or USB-BC extensions to the specification for 1.5 A (perhaps more) of power. The old version 1.0 spec of USB-PD uses the Vcc and Gnd lines for power negotiation, cutting those lines makes the negotiation for more power impossible. By cutting the Vcc line and splicing in a power supply that doesn't use USB-PD but instead the USB-BC or USB 2.0 protocol for negotiation, negotiation that happens over the D+/- lines, then you can add a beefier power supply but the device is still asking the host for power that it can't provide so the negotiation fails.

I see a number of possible means of failure here. One, you may have disturbed the USB 3.x communication by cutting the shielding and/or added what is effectively an antenna for noise to seep in. Two, by cutting the Vcc line the means for USB-PD negotiation is lost, without a power contract the device will remain in a low power mode. If low power mode means it will not spin up the hard drive then it's a dead device. Three, power negotiation and data may in fact be uninterrupted but the host doesn't know that there is an external power source, so when power negotiation happens the host says that's all there is and there is no more, so the device stays in low power mode and remains effectively nonfunctional. Four, extension cables violate the USB spec, and they violate the spec because they create discontinuities in the communications that add noise and reflections. Extension cables are often functional because there's enough error correction and such in the protocol to deal with it but if there's too much delay, voltage loss, noise, or whatever then the connection fails.

As much as you want to avoid use of a powered hub for this that's likely the path of least resistance here. Fixing the power negotiation issues would likely involve just building a hub anyway. If power is a concern then look out for USB-C hubs as they are allowed more power out than USB-A. I use a USB-C/Thunderbolt hub on the MacBook Pro I'm typing on right now. It was not exactly cheap but worth every dollar as each port is capable of 15 watts output.

  • \$\begingroup\$ thanks for the in-depth response. However this does not explain how a powered cable works. I would give you a up vote anyways, if I could :) \$\endgroup\$
    – CellarMonk
    Commented Nov 5, 2022 at 10:46
  • \$\begingroup\$ A powered USB cable is just a powered USB hub with one downstream port. You'd get the same effect by using a powered hub and using only one of the ports, the other ports won't diminish the available bandwidth to a device if nothing is plugged into them. I've seen powered cables that use a second USB-A connector for power but they violate the USB specification. If the people making the cable were willing to violate one part of the specification then there is a good chance they violated it in other ways, ways that could be damaging to your hardware, so it is best to avoid them. \$\endgroup\$
    – MacGuffin
    Commented Nov 5, 2022 at 19:20

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