I have often suspected that the 3rd wire on the higher end models corresponds to the availability of the FM transmitter. FM reception from my TT720 has always been VERY sensitive to the position of the power cord, and for power only with no data, there's no real reason for 3 wires to a USB power source. I still think it's an antenna, although running it in parallel with the ground wire can't help the transmission any.Not that I endorse this, but my cigarette lighter usb power cable wasn't long enough either. I simply cut the wire in half, stripped the ends and extended it using another wire. It works perfectly, and is alot cheaper than a usb extension cable.
That said, depending in your model, you'll either need a 2 conductor or a 3 conductor cable to etend the wire. On your 730, you'll need a 3 conductor cable. Red, black and white wires from the cigarette plug. XL's only have a red and black wire, iirc.
They expect SO much I2R voltage drop across their own short little USB wire that they have to use a sense circuit at the business end? That's one strange design philosophy. Also isn't universal across the model lines.The third wire is voltage sense from the plug which controls the regulator circuit in the ciggy plug body...
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