bicycle electricals info,interesting site!



For people interested in bicycle generators,LED lights,and light circuits check out this site:
Useful info about generator performance based on actual test measurements!. Makes it clear why generator output not proportional to rpm.(dirty secret is internal inductance of the devices,not inefficiency per se).Hub generators are a much better than the bottle variety,reason is lower ac. output frequency,combined with lower inductance.Can get 15W into right kind of load.Quite informative.JJ
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Nice website man with good/practical ideas....thanks for the look.


Active Member
Jan 11, 2008
wait a second - that site clearly shows that generator output IS proportional (roughly) to RPM of the generator (which is proportional to the speed of the bike)

A dynamo that puts out 2.8 W at 15 KPH puts out more than 6 W at 40 KPH...

(A generator's output power is rated in watts.)
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For hub generators this is seems to be true,as far as it goes, their internal inductance is rather small,but to be frank these power output measurements are really a bit misleading.They show that the generator is CAPABLE of delivering a power output proportional to frequency (ie. speed) PROVIDED the load resistance is ALSO proportional to frequency,and of the right initial value for max. power(For that matter 12 ohms is not necessarily the optimum value either).Lets say for example we have at 10 mph, 6 V at 0.5 Amp for a 3 W output and a 12 ohm load.
At 20 mph,twice the frequency that is,now 12V ,still at 0.5 A into 24 !! Ohms,for a 6W output,if you had kept your load at 12 ohms the output would be less than 6W,probably 4/4.5 W
,depending on the internal inductance.Bottle generators run at much higher frequencies than the hub variety,on top of this,they also have higher internal inductance,sort of a double whammy.If you are running lights only, this is all to the good,it keeps the output voltage within limits at high speeds so that your light bulbs won't burn out,but if you are charging a battery and have other loads in addition, it is not so great,you could have gotten more.I can't go into the more arcane aspects of what amounts to conjugate load matching or power factor correction.This situation is quite unusual from a power transmission perspective.Normally ac. power sources are constant frequency,here they are anything but that !
It may be possible to devise a charging circuit to take these constraints into account so that the generator sees about the right load at a particular speed,and thus can be made to deliver more power.The hub generators are obviously better candidates for generating max.power at higher speeds,that much is clear,JJ