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I have a 36V ebike battery that I want to use to power my dynamo bike lights (6V 3W LED lights.) I put a switch and a buck converter in between.

The schematic is as follows:

enter image description here

  1. 36V 10A Li-ion battery
  2. Motorbike switch with a 12V LED in series
  3. 22uF 50V capacitor
  4. TSRM 1-2465 DC/DC buck converter with a fixed output voltage of 6.5V
  5. 10 ohm 5W current limiter resistor
  6. LED Lights: Frontal - Busch + Müller Lumotec Lyt B N plus, Rear - Spanniga Solo XDvS

The problem is, when I connect the circuit the voltage at the output of the buck converter collapses (around 2.5V shows at the output.) The rear lights of the bike are on but not the front one. I tried with several buck converters of different power settings but the result is the same.

The lights are working because when I connect a 5V battery both LED lights are on. I read about starting circuits and inrush current, so I tried putting an inductor in series with the LED but there is no apparent difference.

Could somebody give me a hint on where could be the problem? If for help there would be needed more information please let me know.


Thanks @vir, indeed the problem was the capacitive load. As the dymano lights have a large capacitor to store power from the dynamo, the buck converter was entering the short ciruit mode again and again.

The solution seems to make a "soft starter" circuit. I did some research but I don't know if there would be an easy but effective solution for it. Does anybody have some thoughts/ideas on this?

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    \$\begingroup\$ Could you post part numbers of everything involved? \$\endgroup\$
    – sbell
    Jan 26, 2022 at 17:44
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    \$\begingroup\$ Do you have datasheets for your buck converters and lights and schematics showing how you hooked them up? Inrush current shouldn't be an issue unless the lights are modules with big input capacitors. \$\endgroup\$
    – vir
    Jan 26, 2022 at 17:45
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    \$\begingroup\$ We don't know what LED lights you are using, how they should be powered and we don't know which buck converter you are using so we don't know if it is suitable for powering the LEDs. There is just too many unknowns to even guess what's wrong, so edit in the info. \$\endgroup\$
    – Justme
    Jan 26, 2022 at 17:46
  • \$\begingroup\$ If the output collapses to 2.5 , what about the input? Try connecting the Buck then with an extra switch on the LEDs to handle the surge. Unlikely you have a choke big enough to store energy needed. An ICL might work. \$\endgroup\$ Jan 26, 2022 at 17:59
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    \$\begingroup\$ Not all buck convertors or LEDs are alike. Are we meant to guess which one you have? \$\endgroup\$
    – user16324
    Jan 26, 2022 at 18:27

1 Answer 1

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Are you measuring the voltage before or after the 10 ohm resistor? That resistance is very high, considering the current you want to draw.

The LEDs, at 6V 3W, need about 0.5A. But 0.5A through a 10 ohm resistor will drop 5V. Are you sure it shouldn't be 1 ohm?

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  • \$\begingroup\$ I measure the voltage after the resistor, but there is not much difference if I meassure if before (It seems that the voltage drop in the resistor is very low). Removing the resistor or changing it for a 1 Ohm resistor does not solve the problem. \$\endgroup\$
    – Manuel SD
    Jan 26, 2022 at 18:51

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