Why does the dynamo in my bicycle generate DC instead of AC?

A dynamo is a generator that is designed to produce DC. This is usually done by the use of a commutator. A commutator operates like a switch alternating the connection to the armature so that the output voltage is always traveling in the same direction . Sometimes an Alternator will use diodes to rectify its output to DC. This is done in a car alternator.

An alternator is a generator that is designed to produce AC, instead of a commutator it will use slip rings.



If your unit has diodes you can remove them and you will have AC
If the generator uses a commutator you really have no practical option other than replacing the entire unit with an alternator.


Why do you need to generate AC?
 
It has to do with the commutators that collect the current. On the axle there is a copper piece that has a split between the two copper pieces and brushes that rub against them. In a DC the split causes the DC current to go one way by taking the top half of the sine wave generation and reversing the bottom half of the sine wave so that everything goes one direction only in output.

To make it DC you have to put two copper rings on the shaft, parallel to each other with a brush on each ring instead of the single split ring.

Like the other guy said..it might be easier just to buy an AC generator
 
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