Alternatively go hook it up to a 6V battery and see if it works normally. Then you'd know it's the dynamo and not the light!
No.. Hub dynamos are alternators they put out AC not DC. the light would have a Bridge Rectifier in it
to internally convert to DC (if it needs to be so), ... a converter to USB is both rectifier and voltage regulator ..
You (RHM) emailed Planet bike yet, their import office is in Wisconsin. .. It work Steady On?