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Aufa generator

Cheap plastic "bottle" generator easily obtainable in bike shops. Nominal output 6 V~ / 3 W. The larger rubber wheel pictured on my thumb is a speed reducer - a separately sold accessory that reduces rolling resistance, wear and slippage. Useful thing if you ride fast and your lights are not very power-hungry.

Warning: this article is old and obsolete. The measurements are very inaccurate because I used a school-class comparation voltmeter (error of about one volt) and the test leads were attached with crocodile clamps (lots of parasitic resistance). Don't take the results too seriously. If you want sound data to build on, you'd better jump to this newer article about a very similar dynamo.


Everything is measured without the speed reducer. The output is independent on wheel size, the tyre always travels at the speed of the road.




V-A characteristics

Comparison of short-circuit current with and without the speed reducer:

Internal resistance and inductance of the winding limits the maximum short-circuit current to cca 540 mA. Looking at the shape of the graphs, I guess the output power maxes out at about 35 km/h and further acceleration just warms up the coil. I haven't reached the nominal three watts, but that might be caused by bad contacts in the test circuit. So to be sure, I'd recommend to design the lights for input of 8 V and 400 mA. It should correspond to an equivalent resistance of 25 Ω (marked by thick line in the graphs), where the generator output power is the highest.

Source data to download (XLS, Excel 97).


The weakest spot of the dynamo is the spring-loaded contact holding the output wires. It catches dirt, oxidizes and tends to lose conductivity over time. Its hole should be regularly cleaned by a round needle file.

The rotor axle may start whirring and whizzing after several hundreds of kilometres of use, usually only in a limited range of speeds. It is caused by insufficient lubrication, the axle rolls around the bushing walls instead of spinning. Fortunately, it can be fixed. After removing one Phillips screw from the bottom of the bottle, you can pul the bottom out, together with the stator coil and pole heads. Be careful with the engagement spring, it is quite strong and likes to leap out. The drive wheel is pressed onto the splined end of the axle, remove it by a strong pull (don't screw, don't wiggle). The rotor then falls out and its axle and bushing can be cleaned and greased.

Putting the generator back together is sort of a puzzle: eight pole heads and one disobedient spring must be inserted simultaneously. I haven't found any miraculously simple procedure, so be patient. Carefully with the grounding wire, it is just loosely inserted in a notch where a metal spring holder touches it.