Son hubdynamo
The SON hubdynamo is a AC dynamo with a rotating magnet, of the same type as found in bottle dynamo's. However dimensioning is such that at low speeds sufficient power is produced. Small size hasn't been pursued (there is plenty of room between the forklegs) but every effort has been taken to keep the weight down. The characteristic of the dynamo is such that under load the voltage rises with speed upto a certain limit (eg 7.0v) An electronic controler, eg with current eating Zehner diodes is superfluous. 

If you rotate the axle by hand, you will feel the magnet poles attracting the stator, which gives the impression of a maladjusted bearing. As long as no current flows, the losses are very small due to the hysteresis characteristics of the materials chosen. First when the circuit is completed and current flows, the windings will act as a electromagnet and mechanical energy will be transformed. Another typical characteristic of the SON is the flashing of the lights at low speeds (upto ca 5 mph, 8 km/h). This is caused by the then low frequency of the current, at higher speeds the eye will see a continuous beam. In practice this isn't bothersome, and cosmetic solutions to get rid of it will hurt efficiency. And it makes the cyclist more noticable, at times when there isn't much light to start with. 

The hub runs on two sealed industrial bearings. One bearing is permitted to move axialy, so that it will never bind. This is a common practice in enginering design, but one which never seems to make it to bicycle parts! Another thoughtfull detail is the very stiff axle and the extra gapseal. In the improbable event of a bearing needing replacement, the complete unit can be unscrewed from the hub with a special tool. (with the previous model SON the spokes needed to be removed first).

The hollow axle is made from high quality stainless steel. Standard the hub is supplied with a 5mm inhex axle, but a normal quickrelease can be substituted if required.

Electrical installation 
As the dynamo always turns the lights can be switched on electrically. The standard solution is to use the Lumotec c.q. Lumotec oval plus headlights modified by SON with a rocker switch and heavy duty coax wiring and fitted plugs. This is very reliable and easy to fit. Coax wiring is used, as it is neat and strong. Two 'real' 4.8mm plugs complete the connection to the dynamo.
For a rear light we recommend an efficient and reliable LED light, as the 4D-lite plus or Dtoplight plus with 'afterburner' from B&M. The rear light is connected to the headlight with a 2.8mm plug. (note in Germany it is illegal to have a separate switch for the rearlight, which prevents the other excellent solution of a LEDlight with a battery (use with a 3watt bulb) in that market).
A Lumotec Oval senso plus is also available, which switches on automatically in the dark.
The power and current characteristics of the SON are -as with any other dynamo- defined by the type-approval regulations. The losses however are about a factor two smaller, which makes the switching on of the lights hardly detectable. (we do recommend to turn off the lights in daylight as bulblife is limited, and to stop fellow cyclist from warning you all the time ;-) ).
Schmidt original?
Direct drive hubdynamo's are pretty old (eg Sturmey Archer built one in the fifties, which had 2.5v output, and weighted 1.1 kg) Only in 1992 Wilfried Schmidt managed to design with modern materials a hubdynamo which could and deliver 3 watts, and wasn't much heavier than a conventional hub plus dynamo. In 1995 Wilfried Schmidt started his own company to produce dynamo's, to ensure that his high quality standards would be kept. Several other hubdynamo's are now available, all cheaper, but also of lower performance. And SON's are continuously being improved, the latest version is smaller, lighter, and the unloaded losses were halved!