Although the figure below is for a Zener I'm posting it because it's interesting in its own right as well as being a preview of a magneto-related instrument I'm working on that I hope to have done in a few months.
I purchased a used Lucas
"12 V" Zener on eBay
to use in an instrument I'm designing and needed to test it to be sure it actually works. The curve is the current-voltage characteristic of this Zener with a greatly offset x-axis showing the important region where the Zener begins to function to regulate the charging voltage for the battery
. The voltage at which the current begins to sharply increase is the "Zener voltage" and here it's seen to be ~15 Volts.
Although at the scale plotted here it looks like the current below ~14.8 Volts is zero a tiny current actually is flowing. However, at 14 Volts this Zener is passing only 16 microAmps (R = 0.88 MegOhms). Why a Zener is useful a voltage regulator is illustrated by the fact that at 15.25 Volts the differential resistance has dropped by over a million to only 1/4 of an Ohm. That is, the harder the stator tries to generate more voltage than the battery
wants to see, the better the Zener works to clamp the voltage at ~15 Volts by conducting excess current straight to ground. In essence, it's a nearly perfect insulator for voltages below ~15 V (~ 1 MOhm), and switches to being nearly a dead short for voltages above that (~1/4 Ohm).
I limited the current from my power supply to a little over 1 Amp for this measurement so the maximum power dissipated in the Zener was limited to ~15 Watts (Current x Voltage = Power). However, extrapolating the curve to 2 Amps (30 Watts) shows the voltage would only increase to ~15.5 Volts at that point (~15.65 @ 45 Watts, etc.)
I should add that the Zener voltage is temperature dependent to some extent (increasing with increasing temperature) so if I wanted to thoroughly test its characteristics I would need to instrument the Zener with a thermocouple (because its internal temperature, not ambient, is what matters) and make the measurements after reaching thermal equilibrium at several current/power levels. Although I could do this, the results aren't important for how I will use this Zener.
doesn't care what internal temperature the Zener has, it only cares about its own temperature. Since the optimum charging voltage for a lead-acid battery
that's at 20 oC is 14.6-15.2 Volts this Zener seems like it would be an excellent voltage regulator. However, on a warm day (with the battery
at, say, 30 oC) the required charging voltage required drops to 14.2-14.8 while an even warmer diode will have an higher Zener voltage.
Of course, if you're riding 120 mph with your lights off through Death Valley in summer the poor Zener will be dealing with nearly all the power your stator is generating so the voltage will rise and the battery
will suffer. Still, even with its intrinsic limitations a Zener is not a bad solution at all for most riding conditions, especially when you compare what you get with it vs. an MCR1 electro-mechanical voltage regulator.
Anyway, the above figure shows this Zener passes the test and so it will take its place as one of the many components in the instrument I'm building. More details in due time.