I had a problem with my front bulb light at my bicycle. So, I bought a new one (LED) for the front, but somehow, the back bulb does not work anymore. I have checked the wires for loose connection, so I want to rule that out. Also it (the back bulb) was working in the past, so I want to rule that out as well. I am thinking now about the following to things:

  1. I think, the front and the back lights are connected in a parallel connection. So, both the GND and the + from both the front and the back light should be directly connected. Is that true? I could only measure GND here.
  2. Parallel connection also implies, the mix up of dynamo and back light at the connection ports of the front light should not matter. Right?
  3. Could the difference of a bulb light at the back and a LED front light be an issue?

Thanks for your help here, guys.

  • \$\begingroup\$ Dynamo by any chance? If the front bulb was holding the voltage down and the LED takes much less power, guess what happens to the voltage. Then guess what that does to the other bulb. \$\endgroup\$
    – user16324
    Commented Mar 15, 2020 at 23:13
  • \$\begingroup\$ have you tried unplugging the front light? \$\endgroup\$
    – jsotola
    Commented Mar 15, 2020 at 23:40
  • \$\begingroup\$ @BrianDrummond: I understand your point of view. I thought all lights (independent of LED / bulb) do have 6 V in bicycles? But I could easily measure that with the front light connected or not (the voltage difference). \$\endgroup\$
    – Ernte1893
    Commented Mar 17, 2020 at 21:39
  • \$\begingroup\$ @jsotola: Well it was working with a the former lamp (bulb). So, when the bulb broke, only the back light was shining and I bought a new case. Or do you mean only unplug the front LED itself? Good idea to exclude the voltage problem. Thanks \$\endgroup\$
    – Ernte1893
    Commented Mar 17, 2020 at 21:44

1 Answer 1


You haven't linked to the datasheet for the front lamp so we can't be precise here but there are a couple of potential problems.

Bike 'dynamos' are actually an alternators. They output alternating current. They also carefully design the coils so their inductance causes an increasing voltage drop (in the alternator) with increasing frequency. The result is, that when the intended incandescent lamps are installed, that the brightness is pretty good at quite low speed but you don't burn the lamps out at high speed. That is, the alternator output voltage is reasonably constant over a wide range of speeds when the correct load is applied.

Parallel connection of the lamps is correct. Both should be paralleled off the alternator. My LED tail-lamp is connected to terminals on the headlamp. I wouldn't connect an incandescent bulb to those terminals. Try connecting directly to the alternator.

If that doesn't work your LED headlamp may be pulling down the voltage enough that the rear lamp isn't working. Best bet would be to buy the matching LED tail lamp.

  • \$\begingroup\$ Thanks for the use- and helpful information. You are right with the datasheet, I will try to get that and also that one from the tail lamp. Regarding the parallel connection: My back lamp is also connected to the terminals from the headlamp. What is the difference between the terminals of the headlamp and the alternator directly? Since they are parallel connected..? I will try to measure the voltage once with the front lamp and once without it. Then I should be able to see it. \$\endgroup\$
    – Ernte1893
    Commented Mar 17, 2020 at 22:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.