The setup is discussed
here.
Originally Posted by
SBinNYC
The big complication is that the dynamo output is AC not DC. That's where the 2nd zener in series with the first (cathode-to-cathode or anode-to-anode) comes in. A zener acts like a normal diode in the forward direction. It will conduct without limit (up to its forward power rating) and present a nominal fixed voltage drop of approximately 0.7 volts. In one half of the AC cycle, one zener will conduct in the forward direction and the other will act as a shunt regulator. In the other half cycle, the zeners' roles will be reversed.
The peak voltage across the "stable voltage ouput" terminals will be limited to the forward voltage drop of the conducting diode plus the zener voltage of the other. This will not be a simple sinusoid; it's a clipped sinusoid.
The question of where to set the zener voltage depends on how one interprets the longevity specs for incandescent bulbs. I interpret them to be DC volts because that's what's on the spec sheet. That's also the instantaneous voltage. Therefore, I'd opt to set that threshold at 6 volts. I've used a 5.6 volt zener and assumed a nominal 0.7 volt forward voltage drop. That comes to a clipping voltage of 6.3 volts. I'm slightly over my desired threshold.
The 5.6V Zener is well too low, leading to routine underpowering of the lamp. Without overvoltage, i.e 6V rms, the AC swings between -8.5V and 8.5V. Clipping it at 6.3V will reduce power to well below nominal, see in particular the above site. I have been using 8.2V Zeners, clipping at 8.8-8.9V (the knee is soft anyway). If you are cautious, you can go with 7.5V. In any case, the diodes are supposed to prevent the burnout under unusual circumstances, such as riding down a hill and not do much under usual circumstances. Correspondingly, the discussions of bulb life shortening under overvoltage are not particularly relevant.
Originally Posted by
SBinNYC
I used a 5 watt zener, like the 1N5339 because it comes in an axial package. I can solder the 2 zeners together and add leads to both ends. I can also put heat shrink around the diodes. The package looks like a wire. I attach it across my headlight because I use a bottom bracket generator and attachment there is not practical.
The diodes indeed should have 3-5W to prevent diode burnout when a bulb burns out and possibly to prevent tail lamp burn out if the front bulb burns out.