Joined: Aug 2010
Posts: 12,717
Likes: 2,104
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
It is my experience that a switch is not really needed. I ran a light and a USB charger in parallel. When the light switch was turned off, it drew no power and all power could go to the charger. When nothing was plugged into the USB port, the charger drew very little power and the light got all the power it needed.