I have a Shutter Precision PD8 hub, and a B&M headlight. I happen to have a spare B&M headlight of a different model in my parts bin. I'm thinking about putting the second light on just to have extra light, and potentially to aim it lower so I can see close-up better. Can I just patch the second light in? And if I do, should I put it electrically parallel or series? And will it be any brighter at all, or will each light just be proportionally dimmer?

1 Answer 1


Most bike dynamos produce roughly constant current, so if you ride fast enough, doubling the resistance by wiring two lights in series gives 12v, neatly split between the lights. The classic setup in old days was two lights in series and switch to short circuit one when riding slowly, so that the other would get the full voltage.

Some new LED lights designed for dynamo can take advantage of this feature even with single light, so it would make sense to not add any current limiting to new models. The only way to know if it works for any specific model is to try.

  • Since dynamos produce speed dependant tension, they have an inbuilt constant voltage regulator. Although some systems have the regulator built into the lights. All lights hooked to a dynamo are always wired in parallel. If you add two headlights the drawn current might exceed the capacity of the dynamo.
    – Carel
    Oct 11, 2020 at 13:47
  • Two lights/lamps wired in series will only work properly if each of them draws exactly the same current, which due to manufacturing tolerances is rather unlikely.
    – Carel
    Oct 11, 2020 at 13:53
  • @Carel do you have any sources for the claim about inbuilt constant voltage regulator? If all dynamos have one, how do you explain the two lights in series setup? Why would using two lamps with slightly different resistance, either in series or parallel, lead to dramatically different power to the lamps?
    – ojs
    Oct 11, 2020 at 14:51
  • Conc: voltage regulator, LED lighting is highly voltage sensitive, so either the lighting unit or the generator must carry a regulator and a rectifier to protect the LED from over-tension. For lights in series and current flowing through, check Ohm's law, the resistance of both devices adds up, the max. current flowing being determined by the highest resistance: A 6V2W lamp in series with a 6V3W will burn brighter connected to a 6V source and both dimmer.. In // they shine at their standard brightness
    – Carel
    Oct 12, 2020 at 15:30
  • @Carel you seem to believe that dynamos produce constant voltage rather than roughly constant current, and because of that, the rest is nonsense. It is true that LEDs require a driver circuit, but it regulates current, not voltage and is in the light, not dynamo.
    – ojs
    Oct 12, 2020 at 16:32

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