My idea is to build a electronic system for my bike to supply my phone and lights with energy. I want to use a Dynamo (6 V, 3 W) to charge a USB Powerbank. From its two USB ports I want to charge my phone and run the front and back light. As a front light I want to power a 3 W high-power LED with ~3,75 V @ 0.6 A. As a backlight I want to use ~3 red LEDs with 0.02 A at around 2 V.

The setup should be as energy-efficient as possible and easy to repair.

First problem: How to charge the powerbank via the USB-Port with 6 V? Is it sustainable to charge the powerbank with 6 V instead of 5 V? I could drop the voltage through resistors, but that would generate 0.5 W (???) of heat. What options do I have here?

Second problem: How to wire the lights the most efficient way? I used the high-power LED with two 1 Ω resistor to run it at around 0.6 A, however, that seems way to inefficient to me. What is a better way here? Maybe connect the high-power LED in some way with the red LEDs?

  • 1
    \$\begingroup\$ 600A? Aren't you missing a decimal point somewhere? \$\endgroup\$
    – Lior Bilia
    Jan 11 at 19:50

1 Answer 1


First, a typical 'dynamo' for a bike produces AC, not DC. It's not really a 'dynamo' then, but an alternator. Turns out we can use that to some advantage, see below.

Second, the alternator voltage will also vary with speed. 6V RMS will be its typical output, but could be greater or less depending. Measure it with a voltmeter to be sure.

This in mind, your system would look something like this:

  • AC-DC rectifier for raw voltage (6V RMS => 7 to 8V raw DC)
  • charge controller to Li-ion cell (1S), 3.7 ~ 4.2V (this can also be the raw feed to your lamps)
  • boost converter to make USB 5V

For the second and third pieces you might be able to find this as one IC, or pre-made as a 'powerbank' circuit board. Just Look for one that can accept a fairly wide input voltage range, say 3 to 15V.

Here's a piece I found about bike 'dynamos': https://www.bikeradar.com/advice/buyers-guides/bike-dynamos/

And, toward the end there's a link in there for a DIY dynamo charger: https://www.instructables.com/10-BICYCLE-USB-CHARGER/ They use a neat trick: voltage doubling the input AC so that it works at low speed.

Related: Converting and Stabilizing output voltage of a dynamo hub


I've been thinking about the problem of rectifying such a low voltage with less diode losses. I've developed a solution here: Why does this bridge rectifier claim to have no diode forward voltage drop?

And, combining this with voltage doubling I come up with something I've not seen before: an ‘ideal diode’ voltage doubler (simulate it here):

enter image description here

I think this will do low-loss voltage doubling down to less than 3VAC, even lower if the comparators have a local bias supply.


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