Characterising a Bicycle Dynamo Hub

During Lockdown, and for want of something stimulating to do, I set about characterising the output of my bicycle dynamo hub.

USB charger/converters that run off dynamo hubs are typically very expensive (£100+) and also tend to break (I have written off 2 whilst touring) and so I wondered how difficult it would be to make my own. When looking online, many hobbyists treat the hub as a constant current device, This logic would suggest that you could double the power drawn by doubling the resistance, following an I^2R relationship, then wonder why they have no light at low levels, aren’t able to achieve a consistent supply without dropouts or blowouts, and why there are random voltage surges. Therefore, the first step to doing this is to try and characterise the output of the dynamo hub, how the voltage and frequency changes with velocity and applied load – essentially to qualify and quantify an equivalent circuit that would suit modelling, dusting off some 2nd year electrical engineering

Test Setup

Being stuck at home, away from a workshop of any sort, and locked down due to Covid, i was rather limited in what i could achieve.

I bought a cheap scooter motor and motor controller, bolted them to a bicycle fork orphaned from a cracked titanium frame and drove the hub with an elastic band.

For measurements, I bought a cheap ‘solder your own’ oscilloscope kit off ebay and spent best part of a wet march day putting it together, and wrote a simple script to pull the measured data off realtime to my PC for analysis.