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Assume I have a small bicycle generator which usually feeds the front and back LED/light bulbs. Powering a small gadget from this generator should well be possible.

The setup would involve a full-wave bridge rectifier followed by some sort of regulator. However, the rectifier output voltage does vary wildly depending on speed and load:

  • low speed and/or high load by lighting: low rectifier output voltage
  • high speed and/or lighting switched off or defective: high output voltage

The first case is non critical and simply means that I need some sort of battery backup and/or a boost converter for hold-up power while going slow or standing.

The second case is more critical since it dictates the voltage rating of regulator and capacitors. And I wonder: Is there a clever way to limit the voltage and or current of a bridge rectifier?

I considered zener diodes but these would dissipate quite a lot of power and I wonder wether there is a more clever way.

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    \$\begingroup\$ Chose any smps topology depending on the desired output voltage and input voltage range \$\endgroup\$ – PlasmaHH Apr 14 '16 at 13:53
  • \$\begingroup\$ buck-boost regulators sound right. \$\endgroup\$ – Andy aka Apr 14 '16 at 14:05
  • \$\begingroup\$ yes, but with rectifier output voltages up to 30V the available range of buck-boost regulators is very limited \$\endgroup\$ – Arne Apr 14 '16 at 14:09
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Bicycle dynamos use the inductance of the winding to regulate the voltage and, for the recommended load, keep it fairly constant across a wide range of frequencies.

See my answer to Non-LED simple bicycle dynamo light system.

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You will want to consider building a pair of regulators. One will have to be a boost style regulator to deal with the low voltage input condition. The other would be a wide voltage range buck type converter. There are plenty of devices available that can operate at the 'high' voltages that you specify. Certainly there are buck devices available for use up to 60 volts and some even to 80 volts.

Then you need to build a FET switch circuit in front of the low voltage boost regulator circuit that turns off the input to that portion of the circuit when the input voltage goes too high. Some careful work will be required to make sure the switch over point of the whole setup from boost to buck operation will not produce hiccups in the outputs.

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