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hamrack-
Comes on and stays on. I presume that the light has some system for allowing the capacitor to power the light only when the voltage from the dynamo drops below a certain level.
@mespilus there's only one (COB) LED for both running and standlight functions. No daylight sensor, it's a fairly simple light. Here are pictures of the board. The wires to the LED are the red and black ones at the top and bottom of the board. The pair of red and blue go to a 0.1F capacitor, but the big 1F coin-shaped capacitor under the board is the one that, I think, actually powers the standlight function.
ffm
Just got a dynamo light from a friend that has a weird fault (the light, not the friend). When turned on the light if off when the wheel (with the dynamo hub) is turning, but comes on when the wheel stops. My interpretation is that the standlight capacitor is charging properly and correctly powering the LED when there is no power from the dynamo, but, for some some reason, power is not getting to the LED directly from the hub. Unfortunately that's where my electronics knowledge ends. Any suggestions of what might be broken in the light to cause this to happen?