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I am currently working on a Bluetooth project which needs micro USB for charging a battery. My original plan was to use a female surface-mount micro USB socket and connect its +5v and ground pins to my Bluetooth module's battery charger. However, after seeing how tiny the pins were on the sockets available at my local electronics store, I decided it would be tedious to work with in a non-SMD application. I instead opted to use a Micro USB female to male extension lead, which I would strip, connecting the relevant +5v and ground wires to the module.

After stripping the cable, I connected it to a USB power-bank used a voltmeter to ensure I was going to solder the correct wires to the module. I ran into an unexpected problem: There was no voltage across the Red and Black wires which diagrams I found on the internet said were for +5v and ground respectively. This same issue occurred when I used other power sources.

Voltmeter connected to +5V and GND wires of USB cable

That image might be a little hard to see, but I have the +5v and ground wires connected to the voltmeter and it reads an insignificant voltage.

Interestingly, after some experimentation, I found that connecting the shield wire (instead of the black wire) to the negative lead of the volt meter gave 5 volts.

Voltmeter connected to +5v and shield wires of USB cable

I had the same result no matter the device I connected the USB cable to. Should I go ahead and solder the shield and red wires to the module because they work, or is there something I'm missing which could result in ruining the board? Also, If I decide to connect the green and white data wires to the module (which can act as a USB slave) how should I wire it then?

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  • \$\begingroup\$ Sounds to me like the cable is defective. Might not ruin anything, but it is not supposed to be that way for sure. You should have 5V across red to black. \$\endgroup\$ – mkeith Apr 21 at 6:41
  • \$\begingroup\$ Hey mkeith, thanks for your quick reply. That's also what I thought, but since I have no experience with USB wiring, I thought it would be good to ask. \$\endgroup\$ – QWERTYUIOPYOZO Apr 21 at 6:46
  • \$\begingroup\$ I will mention that sometimes it can be kind of hard to hold the wires against the probes like you are doing in this picture. You have to try repeatedly before you can be sure that it really doesn't have 5V. Or use some type of clips to grab the wire ends more reliably. But it sounds like you have measured several times, so I assume that 5V really is not present. \$\endgroup\$ – mkeith Apr 21 at 6:51
  • \$\begingroup\$ Yes, I consistently got 0 volts between red and black, hence my reason for finding an alternative connection. I also bought a spare cable as a back up, although I doubt I will see different results given it is the same product. Any thoughts? \$\endgroup\$ – QWERTYUIOPYOZO Apr 21 at 7:26
  • \$\begingroup\$ As far as USB jacks go, the shell (shield) is usually grounded in some fashion. Maybe through a ferrite or something. So using the shield as GND might work much of the time, but it might not work totally reliably, and it might not work on all systems. \$\endgroup\$ – mkeith Apr 21 at 7:30
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In that case your cable is most likely defective. I have seen products where the shielding wires are Ground but then there was no black wire. I tested myself on my USB from Samsung and I get 5V between Red and Black and 0V between the shielding.

Having Shield as Ground has the risk that if something shorts to the Shield due to any defect that it is shortened to Ground.

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