Working out which wire does what for USB

Occasionally I want to solder the power wires for something 5V to USB (A) so I can power it from that (most recently a lovely little thermal printer, and previously an amp for Radiodan). The first time I tried it, my research suggested that particular wires were power that were actually data so it smoked a little when I plugged it in, and obviously didn’t work. There doesn’t seem to be a standard colour for the power and ground wires for USB so you need to work out which ones they are. Damian taught me this way. Posting so I don’t forget this again and in case it’s useful.

You’ll need:

  • a multimeter
  • wire with male USB A at one end
  • wire strippers
  • a match or something similar
  • jumper wire
  • a soldering iron

1. cut the end off the USB cable, leaving plenty of wire in case you make a mistake, and strip back the wires. There should be 4. Some are whispy things that look more like shielding. Twist them round into wire.

2. You may need to use the match to carefully burn off any plastic coating on the wire (not the bit you can strip off, but invisible or metallic-looking covering on the unshielded wire)

3. Turn the multimeter on and move it to the resistance section (the ohm bit). It doesn’t matter where you put it in this section. The display should say “1”.

4. Place the USB end so that the white part faces down. There are four contacts inside the housing, from left to right: 4 3 2 1 –

ground Data+ Data- power

(see Wikipedia on USB)

5. Put one multimeter contact on the “4” contact in the USB plug and test each bare wire at the other end in turn. When it fluctuates towards “0” then you have found the corresponding wire

6. Do this with the other three contacts as well so that you’ve identified each wire

7. Solder the ground and power to your jumper wires and ignore the data wires.

8. Done.