Originally Posted by
Road Fan
For the 6 volt slicer advocates, please tell me: how do you plan to deliver 3 watts to a lightbulb using a 6 volt sine wave, if you are going to re-direct some of that power to heat a pair of zener diodes rather than a tungsten filament? That's what designing a slicer such as you suggest will result in, when the voltage is such that the bulb would NOT be over driven.
What is needed is for the limiter to absorb zero power for any effective generator voltage up to say 1.03*6.0 volts (3% overdrive), or 6.18 volts. This implies that we don't want a limiter to act until the instantaneous voltage exceeds 8.74 volts on the positive half cycle and -8.74 volts on the negative half-cycle. Ideally the concept needs to be an 8.74 volt slicer. Practically, well, it's time to head off to the ON Semi website and see what I can get in a power zener diode these days, and it a small power Schottky diode. I might not be able to get a zener that will so precisely protect the bulb.
This link describes what diodes and zeners do.
http://www.reuk.co.uk/What-is-a-Zener-Diode.htm
The basic shunt regulator circuit is shown below the "Zener Diode Voltage Regulator Circuit" heading. Not shown is the load across the "stable voltage" terminals. The load has a resistance r.
The Zener conducts only when the the voltage at its terminals exceed its zener voltage. If the voltage at its terminals is less than this threshold, then it does not conduct and is out of the circuit. Thus, all the current passing through series resistor R, will pass through load resistor r.
If the voltage at the zener's terminals exceed its zener voltage threshold, then the zener will conduct without limit (up to its power ratings). This added current will pass through the zeries resistor, R, and increase the voltage drop across it. This will guarantee that the zener voltage appears at the load. The current flowing through the load will be the zener voltage divided by the load resistance, r.
The series resistance, R, is the winding in the dynamo. It also contains a fairly high inductive component. This helps voltage regulation because going faster not only increases the dynamo's voltage output but also increases its frequency. Thus, dynamo output will not increase linearly with speed when there is a resistive load.
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.
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.