Hi!

New here.

I’m installing a busch & muller led headlight on my old bike, powered by an old bottle dynamo.
Did a quick test after blindly guessing about the connectors on the dynamo and the light is working just fine. But I’m not certain I have the wiring right and I don’t want to fry the headlight, the connectors on the dynamo are marked only B and S. A quick search told me that it stands for headlight (B) and taillight (S). There was no indication about polarity or if the chassi and mount act as ”ground”.

Like I mentioned the light is working as I have it set up, I connected B to plus and S to minus I think for a quick test. (before googling..)

Can someone help me illuminate (no pun intended) the matter? The headlight has no ground connection at all as tested. Why would the dynamo have one connection for the headlight and one for the taillight?

Thank you
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B is probably Borden, German for ground.
S is probably Strom, German for current.

I'd use my ohmmeter to check for continuity between each terminal and any metal mounting hardware, then connect + to S and - to B assuming I didn't find things were reversed.

Separate head/tail lamp connections make no sense.

Usually the headlamp has a switch which controls both lights, so you connect the dynamo to its input and taillamp to its output.

My Schmidt Edelux II has a two conductor cable going in from the dynamo, a single output for the tail lamp, and has its case grounded for the second tail lamp connection.

Every bottle dynamo I have seen produces alternating current, no polarity. But they often are grounded to the fork, thus if the light is also grounded they might not work if wired wrong.

There are LED headlights for bottle dynamos without switches. To turn off the light, you move the generator off of the tire so it physically stops spinning. I bought a couple such lights off of a clearance table, put one on my errand bike.

A bottle dynamo doesn't need an electrical switch, because you would be a fool to engage the dynamo when you don't want lighting.

How do you wire up a bottle dynamo? - Bike Tinker

With the exception of modern bottle dynamos, older ones have a single outlet and are grounded to the frame.
LED lights use a double cable and have a separate cable/connector coming from the light itself for the rear light.

Some bottle generator LED lights will work with a single conductor wire if they are grounded to the dynamo or to the fork and if the bottle dynamo is also grounded.

I mentioned above that I bought some clearance priced bottle dynamo LED lights years ago. I put one on my errand bike when I bought a cheap dynohub wheel from a bike charity. Both the dynohub and the light are grounded to the fork, thus only one single conductor wire needed to power the light, no taillight on this bike so the wiring is amazingly simple, a piece of single conductor wire less than a foot long is all the wiring I used. No switch on light, it is on at all times when rolling. In my case, the threaded rod that I used as the light bracket is how the light is grounded to the fork.



If I ran a taillight, the tabs are on the light for that purpose, but did not have a spare taillight at that time to use.

I installed one of those Sanyo under-BB dynamos on my wife's bike. It grounds to the frame. I connected a ground wire between the lights and a water bottle bolt, and it works fine. (And of course, there is a hot wire between the dynamo and the lights.)

A lot of them are grounded to the frame. Some may not be grounded to the frame, but use two wires.

All bicycle dynamos I have seen, produce AC. For those not grounded to the frame, polarity does not matter. They work the same both ways.