I've bought and installed dynamo powered lights on my bike. A little bit about the setup - It's the bottle dynamo kind pressing against the tire system. The Dynamo produces current turning clockwise and counterclockwise. The setup completes the electrical circuit using the frame of the bike. The front and the back light are powered by the same dynamo but are wired through separate circuits.

I've set up and used the system successfully. However I was doing some work on the bike and had to dismantle and reattach the front light. From the moment i attached the front light back the system hasn't worked. If i disconnect electrical influx and the return loop from the front light the back light works. If I connect merely the influx connector the back light works again. In other words - the entire system stops working when the front light is connected fully and properly in the manner that the electrical loop / circuit is closed.

I've tried using a backup front light with same results. The bulb, even if it were out on both of the front lights i tried, can't be the culprit - the back light should still work as it is wired to the same dynamo but independently. I've checked the influx / hot / live cable of the front light, it isn't damaged - but again, even if it were and it were to touch the naked bike body somewhere, the back light system should still work.

This is the only situation in which the entire system fails: When the front light is connected properly with the electrical in and out cables attached.

Which would mean that the entire system fails when the flow of electricity through the front light is established, not impeded - which makes no sense! In all other situations the back light works fine. So what the heck is going on?

I apologize for using improvised terminology (influx and such), I'm not an expert on electrical systems, as such the answer could be glaring however I don't see it.

I've checked previous threads with similar issues like this one Diagnosing faulty circuit in vintage incandescent headlamp however I've found no matching issue or applicable solution.

  • It sounds to me like there's some sort of short in the front circuit. May 3, 2017 at 16:22
  • Note that all bicycle dynamos are AC. They are only rectified (if needed) within the light itself.
    – RoboKaren
    May 3, 2017 at 17:34
  • Karen: Thanks, makes sense Daniel: It has to be a short, but i don't see how or where. Would a short on the front light circuitry cause the back light circuitry to fail? Also, the short can only be located on the phase wire, if it were located on the neutral it wouldn't matter, it would be serving it's purpose. However when the phase is connected by itself the back light works - so the short can't be on the phase wire. (i worked on my terminology) May 3, 2017 at 22:15
  • 3
    Did anything with the headset change? Most conduct and some don't. (Obviously not relevant if the dynamo is in back. Also you should tell us where your light is mounted.) May 4, 2017 at 2:07

2 Answers 2


There are four things you need to know:

  1. Almost all (99%) of bicycle "dynamos" are in fact AC alternators and they produce alternating current (3~6 VAC) with a current limiter (usually 1A max). The frequency of the AC varies by speed.

  2. Most dynamos tie one of the output legs to frame ground (neutral). The dynamo manual will tell you which, but if you don't have the manual you have to figure this out. Some will only have one output and this is the 'hot' side with the frame serving as 'neutral'.

  3. Most dynamo lights take both AC or DC input. The LEDs internally rectify it to DC.

  4. Most dynamo lights have one of the input legs tied to frame ground (neutral). The manual will specify which, but you can also test it out yourself.

If any part of your bike's electrical circuit is accidentally shorted to ground, then the no lights will light. If the headlight draws more amps then the dynamo can produce, then the dynamo's current limiting / self protection circuit may shut it down.

In order to diagnose the problem you're having, you will need a multimeter with AC voltage detection as well as a continuity tester function.

a. Dynamo: First you should figure out which output of your dynamo is tied to ground/neutral and which is hot. If there's only one output, then it's 'hot' and the frame is ground. Use your AC tester to make sure the dynamo is producing around 5 VAC when the wheel is spinning.

b. Light: Use the continuity tester function of your multimeter to see which of the outputs is tied to frame ground/neutral. One of the two will be grounded. If there's only one input, then it's by definition 'hot' but check to see if it's actually shorted to ground. If it is, there's a faulty in your light.

c. Light Try powering the light with it removed from the frame and using a 5VDC power source. A cheap USB power pack can be used for this or 4 AA batteries. Both are rather current limited so it's a good test to see if the light is drawing too much current (but you could also use your multimeter's current function for this). Your light shouldn't drain more than 2A in most circumstances.

tl;dr: My first guess is that your light ties one of the inputs to ground and you've accidentally setup a short circuit when you wired it in. My second guess is that your front light draws more amps then your dynamo can produce and it's shutting itself down to protect itself.

  • You know more than me about dynamo systems but it works surprise me if (some, cheap) bottle dynamos didn't omit explicit current limiting and just relied on the resistance of the windings. This may affect the failure modes. (just a minor query, a very definite +1)
    – Chris H
    May 6, 2017 at 13:15

As previously mentioned, the most obvious answer is that the "live" wire from the dynamo i.e. the one not connected to the frame, is being shorted to the frame somehow. Even if you don't understand electronics/electrical circuitry, it's logical that if the system worked before and you made a change, this is what has caused the problem. There has to be a break or a short in the live wire feeding the headlight. The easiest thing is probably to replace that wire and see what happens. I'm a qualified electrician and have been using bike dynamos for decades.

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