I use a hub-generator powered LED lighting system in my bicycle, made by myself using rectifier, capacitor, and Zener. It's working very well for many years, the Zener does not get hot, and the high-speed voltage is very stable:
Now I want to modify the system, by introducing the possibility to turn the system load off. My first idea was to add a switch before system load, but preliminary tests show that if I disconnect the load and spin the generator, an excess current flows through the zener, as expected, and it gets very hot very fast.
I suppose that, with an always-on system load, there is no excess current through the zener, since the power from the generator (not much more than 3W / 0.5A) is well balanced with the load), and I can use the Zener as a current sink (that's why I chose a 5W zener in the first place).
But with a load that can be switched off, I think I should redesign the circuit in a way that the zener acts as a reference, and another type of "turn off switching" occurs.
I imagine there could be some configuration using perhaps a mosfet, but my knowledge is not enough to devise a safe, energy-efficient and effective configuration.
My goal is to have a simple-but-effective circuit that powers the system load, when present, with a clamped voltage, while preventing excessive, shunted current flow if the system load is switched off.
Some important considerations:
- It is not enough to move the switch to the hub output, since my goal is, quite soon, to add a battery and a micro-controller to the system, in a way that no actual, physical switch will be present - I want the system to be "smart", in an "always on" sense, and this would be an initial step towards a DIY, poor-man's power-path management;
- I'm not sure about this, but I think a 7806 regulator is not a good fit, since I don't have enough voltage drop (RMS voltage under load is quite close do 6V). Correct me if I'm wrong;
- The hub generates relatively large voltage spikes when running without load (more than 5V), and the capacitor I am using (6800 uF) is rated at 16V. So of course any solution should prevent the large voltage spikes to raise the capacitor voltage above its limit.
UPDATE (in response to Peufeu's answer):
Your hub is a current source;
Your description fits perfectly with the manufacturer's, a lot of tests I have already seen online made by other technically-oriented bicyclists, and with my own experience as a bike and DIY electronics enthusiast. The voltages and waveforms vary a lot as a function of speed and load, but the current has a characteristic plateau around 500mA.
You want to use the dynamo to charge a battery or power a load, but when the battery is fully charged (or the load is off) then what to do?
Then I would like the rest of the circuit to behave as it was open, or with a high resistance/impedance, so that no significant current is drawn from the hub.
(...) you can safely short it. (...) it won't waste your muscle power.
Now this contradicts my practical experience. I happen to have wheel in a test stand beside my desk right now. If I spin it connected to some circuit, I can see the wheel decelerating. If I spin it without load, it runs much longer, almost as if it wasn't a generator hub at all. But if I spin it short-circuited, the deceleration is very pronounced, so I prefer not to consider this option.
it can actually output scary voltages open circuit (...) which will destroy any (...) kind of series regulator, LDO, switching regulator, etc.
I agree, and irrespective of any other solution I eventually adopt, I will always put some 5W zener in the circuit to shunt these voltage spikes. I suppose, though, that by then I could choose a larger value (16V or 25V, depending on the capacitor rating, seems to be fine) just "for the emergency".
The idea is to add a comparator (...) Make sure you add lots of hysteresis in the comparator.
That's what I want to do: use a voltage comparator to somehow detect "states" in my system, and switch the proper power paths on and off. I only would prefer to leave the hub circuit open when no load is present, instead of short-circuit.