Cannabalizing a hand crank radio for Bike dynamo switched mode power supply?

Thread Starter


Joined Feb 21, 2019
I recently got a good deal on a bicycle dynamo hub (6V, 2.4W), and ultimately want to set up a system for USB charging for long distance bike trips.

My circuitry knowledge is limited to 1 course in university 15 years ago, and frustratingly fiddling with a bread board during one of our labs, so please bear with me. As I understand it, the hub will produce a variable AC voltage based on speed, and able to supply current based on torque. To that end, what torque/current will the front wheel experience, since it's more or less just "along for the ride"?

Since a phone requires a pretty steady 5V DC signal, we want to rectify the messy variable one that comes from the hub using the magic of circuits. My first thought was a diode bridge with a voltage regulator, but my reading says these are notoriously inefficient, which is a big consideration since all the energy is coming from me. I read what I need is called a 'switch mode power supply' which apparently uses inductors which kind of acts like both a voltage rectifier/regulator, and a transformer to conserve energy.

There is this:

I've found which does the job, but unfortunately costs almost as much as my bike, and I want to do this on the cheap.

I saw another website which suggested using a hand crank radio dynamo with a friction drive, and that got me thinking, can I just solder the wires from the hub dynamo to where the power input from the crank is and call it a day? I have no idea if this is a switched mode power supply though, but given that it came from a hand crank, thought that maybe it is since efficiency would also be important there for human power? Or maybe since human hands probably have a pretty narrow power curve they could design around it? I really don't know much about circuits beyond being able to identify resistors and capacitors and ohms laws. Here a pic of the circuit if it helps:



Joined Dec 29, 2008
... seems like you could reduce the problem to charging a 6 volt light weight lithium battery, and then putting a 0.65 volt diode between the diode cathode (negative terminal) and the 5 volt load ... The load voltage coming in close at about 5.35 volts.
... The AC output voltage of the hub generator kind of necessitates the use of a diode bridge and capacitor and maybe a voltage regulator to get down to the 6 volt battery requirement. ... It seems like you will be required to perform at least some trial experiments to see how the hub generator actually performs.
... don't know about the power supply approach ... sounds excessive.

... using standard 18650 lithium batteries, two batteries in series, fully charged, would yield about 7.2 volts, so three 0.65 volt diodes, chained together would give about 5.25 volts at the load ... fully charged. For added storage, it should be possible to parallel the lithium batteries.

... as mentioned previously, it will be necessary to provide some more facts regarding the hub specifications. Is that 6 volt number the peak value of the AC generator output, or is it what is called an RMS value ... kind of a DC voltage equivalent? ... or is that 6 volt number actually DC output of the device? What is the maximum voltage ... peak or RMS that can be continually generated?
... and what is the purpose of that sidebar adjustment?
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