I'm restoring a old motorcycle and the headlight is very dim. Upon inspection, there appears to be a shunt regulator. I infer that the part they call the 'regulator' in the diagrams is just a zener diode in parallel with the light bulb. I am basing this on my observation that the regulator has 2 leads which just go between the headlight + terminal and ground. My measurements seem to indicate the series resistance in the shunt regulator is about 1 Ohm.Anyway Vin comes from the magneto and is AC so we would expect the voltage to the headlight to be between Vz and about -.7V.
The light bulb is 6v and 25watts. If I just pull out the 'regulator' (what I think is simply a zener) the light gets bright and the voltage at idle is ~5v and it goes up much higher at higher rpms (>8). With the zener in there, voltage is limited to about 2V (should be 6).
I bought a new zener because it seemed obvious that the current one was permanently conducting too much current. The one I bought was 50W and 6.3V. I tried it and it is barely better! It limits the voltage to about 3V. How is that possible? How can the output voltage actually be lower than the zener voltage? I thought the zener wouldn't even start conducting until we reached the zener voltage, but it appears to be conducting even below the Vz. Is there anything I can do to fix this?