Positive pole inside, negative pole outside. This USB to DC 3.5 x 1.35mm 5V Barrel Jack Power Charging Cable is designed to work with Fairywill FW917, FW508, FW507, FWP11, FWD1, FWD3, FWD7, FWD8, FWE11, and FWT9 electric toothbrushes, as well as Bitvae D2 and Seago 7AM2M electric toothbrushes, and other similar models.
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