Which dynamo do I need?

Question

Hello,

For our 10th-grade project, we're building a bike station that should be able to charge a cell phone in a realistic amount of time. Logically, this would require a dynamo to convert the kinetic energy. The question now is which one we would need and which factors would need to be taken into account.

Thank you

jmc01 · Answer

Conventional dynamos deliver 6V/3W and cost about 10-12 euros. Hub dynamos cost around 50 euros. In addition, there are the costs for a circuit which regulates the voltage and smoothes and a charging automation.

d82twf · Answer

I'm just gonna have a few thoughts, even if it's late in the evening in Tokyo…

If you want to load a mobile phone, I'll set a voltage of 5V and a current of 1A, so most models can load.

An ordinary bike dynamo has a voltage of 6V and a power of 3W. That's what you're seeing pretty fast, that's just 0.5A. This means that at least two dynamos would have to be connected in parallel, because a little mechanical craft work is necessary. However, the 6V, 3W are only achieved under ideal conditions. But let's leave it now.

Now comes a crucial point: A dynamo is an AC generator – but a cell phone wants to have direct current. So you need a bridge rectifier, because you want to use both sine half waves and not blur half the power… Now comes the next trouble, a bridge rectifier made of silicon diodes has a voltage drop of 2×0.7V – that is too much. So Schottky diodes are announced – they are a little more expensive, here you have a voltage drop of 2×0.4V. With the computational 5.2V you can live. So that you can buffer the sine half-waves and compensate for fluctuations in synchronism, you already need some larger electrolytic capacitors. Experimenting is under load and an oscilloscope, because you don't always stand evenly. Possibly a small capacitor in the nF range for blocking the possible high-frequency disturbances, the dynamo will certainly not run clean.

And then that should work with charging the phone…