I did a continuity check between all of the ports Vcc and gnd of a usb powered hub including the male end, all are shorted. If I want to make it an external powered hub, instead of connecting the usb female connector of the Vcc to an external supply, can I just connect an external supply on one of the usb female ports? By doing that, I would not have to do a surgery on my usb powered hub.
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4 Answers
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Some cheap usb hubs, are really really cheap, and do not follow appropriate standards. Some are so cheap, that they have all of four components. An all in one usb hub IC, a crystal clock, a decoupling capacitor, and an led. Input VCC is tied together to all the output VCC. These are not the best devices in the world.
That said, because VCC is tied together, you can have problems if you connect an external supply. You will have two competing supplies, and will put your computer at risk.
There are two solutions. The first, cut the VCC trace between the input usb port, and the output usb ports, but leave the usb hub ic connected to the input. By USB Standards, self-powered usb hubs and devices should still have the hub controller connected to the host usb, at the very least for signaling purposes. This option makes it so you always have to have the external power connected for anything to work.
The second option is to still cut the trace, but add a diode to protect the input usb host. This will allow the hub to still be used without the external supply, and to be used with it as well. Be sure to pick a diode that can handle at least 1 Amp with a low forward voltage drop, like a schottky diode (0.2v typical), to minimize any issues with low voltage.
Picture of said extremely cheap usb hub. Diodes are a cheap way to bring 5v down to ~3.3v for the hub ic. No components on the other side:
I am not responsible if you break your usb hub, your computer, your external power supply, or any of your usb devices. Good Luck.
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To be absolutely sure, you'll need to check the circuit diagram of the hub, but in general: No, you cannot supply power to the USB output.
USB specification allows to power on/off ports, implying that these ports are not directly connected to the hub power rails and in turn implying that you may stress internal components well beyond their limits.
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1\$\begingroup\$ Even after finding out that all the Vcc are shorted and ground are shorted? \$\endgroup\$– XegaraNov 3, 2013 at 10:11
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\$\begingroup\$ You can only tell for sure when checking the circuit diagram. \$\endgroup\$– jippieNov 3, 2013 at 10:18
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1\$\begingroup\$ If after checking the circuit diagram, it was found that Vcc and ground are shorted correspondingly. Then connecting an external supply to one of the usb ports is allowed? :) \$\endgroup\$– XegaraNov 3, 2013 at 10:26
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Yes I have done It just by removing +5V wire which came from (usb)PC to hub then connected external 5V supply to the Hub's common +5v rail. Please note that ground(-ve) must remain connected and both, computer and external supply to hub shares common ground.
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\$\begingroup\$ This doesn't answer the question, which was about supplying power from a downstream port (not the upstream host connection). \$\endgroup\$– DavidFeb 15, 2014 at 8:46
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\$\begingroup\$ @David fundamentally the same thing. The 5v rails are all directly connected. \$\endgroup\$– PasserbyFeb 15, 2014 at 22:15
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\$\begingroup\$ I agree with Passerby, all ports upstream or down port are connected in same power rail(+ve as well -ve). so you have to remove connection (+ve only) comming from upstream port and then connect +5v to rail and GND will be common for upstream as well as down stream. I used the same config and used 3A, 5V PS, then connected 4 nos. Portable HDD to hub all working well!!!!!. Now I am planning to use connector which can switch between self powered to bus powered just by plugging in or out the connector pin similar to our mobile phone charger(old type). \$\endgroup\$ Feb 16, 2014 at 8:28
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The expectation with a cheap hub is that VCC and GND are going to be directly connected together on all output ports plus connected directly to the VCC and GND of the Input Port ... this can be easily verified with an ohm meter without disassembling the hub
If you were to plug in a powered USB plug in to one of the output ports then yes in the above scenario the power would be available at all of the other ports ... the issue is that you would also be applying power to the input port.
In a perfect world you could hook two batteries of the same voltage in parallel ... this is NOT a perfect world so I would NOT recommend having 2 VCC sources hooked in parallel ... that is why the 2 solutions provided are:
- Just cut the red wire from the input port or
- Cut the red wire from the input port and insert a low voltage drop schottky diode ... anode to red wire and cathode to the input port VCC contact.
The second option will allow you to still use the HUB without having to power it.
