Bike Dynamos
Both of our bikes are fitted with schmitt dynohubs. Much has been writtten about them on the internet. There is a page here with some pictures. Peter White Cycles Initially I was unconvinced about whether the dynamos were worth the money. I think we had to pay and extra 180 pounds for each wheel fitted with the hub. However they have proved invaluable.
I found there was some good information available online, and there were a lot of people who had the idea to use the dynamos to provide power for charging batteries and electronic equipment like phones and mp3 players etc. However most peoples electronics knowledge is limited so there is a lot of misinformation too.
So if you want to use a dynamo to provide electrical power for your trip here is what you need to know.
The dynohub is not a dynamo, It is in fact an alternator so you need a rectifier to produce DC for battery charging.
It does not produce a constant voltage, in fact the voltage is proportional to the speed that you pedal. Instead it is designed to only produce 500mA maximum. So its the current not the voltage thats fixed.
Its very efficient > 99%
Despite the fact that it doesn’t produce DC and it doesn’t produce a constant voltage that doesn’t mean that you need to resort to complicated circuits.
Having said that here are two articles that I found useful that do have good complex circuits. One uses a transistor to match the impedance of the dynohub to produce more power and one uses a MOSFET rectifier to reduce voltage losses.
High Impedance Circuit
Low Voltage Loss
Anyway here is the circuit diagram of what we use. The only losses are in the diodes which drop 0.8 volts each. I actually don’t know at this time what that means in terms of power loss. I used 1n400x diodes from maplin. They cost next to nothing.
The diodes only let current flow one way. Ive used four of them to make a full wave or bridge rectifier.
bike charger circuit diagram
As there is no complicated electronics all the energy from pedalling should go into the batteries. It is important not to overcharge NimH batteries as it can reduce their life. The recommended current for charging NimH batteries is C/10 which means that if you have 2650maH batteries you can safely put 265mA into them and they will not overcharge. The dynohub is a bit more at 500mA, so I try not to leave them charging for too long. The formula to work out how long to charge them is the capacity in mAh divided by the charging current in mA times by 1.4 which is the inefficiency of the charging process, in other words if you charge the batteries for 1.4 hours, only one hours worth of electricity is actually being stored by the battery and the rest is being lost as heat.
So to fully charge 4xAA batteries takes 1.4 x 2650 / 500 = 7.42 hours.
As well as charging the batteries which comes in really useful for my GPS and debbies camera, we can also connect debbies eBook reader, my mp3 player and our sony ericsson phones. The advantage of using 4xAA batteries is that the combined voltage is around 5 volts which is perfect for a lot of portable equipment.
rectifier mounted inside audio plug
inside the rectifier plug
You can see the diodes mounted inside an phono plug. I used phono plugs for the electrical connections as they are easy to get hold of. After I took the picture of the inside of the plug, I filled it with araldite to make it waterproof. So far the rectifiers have remained working even in the rain.