Electronic – AC overvoltage protection for bike dynamo (alternator)


I built my own MOSFET active rectifier and DC/DC stepdown for a bike dynamo hub and wanted to use it for charging a mobile phone.

The circuit is working, but there is one thing which needs to be solved better.

When USB output (5V) is not connected to the load and I'm riding too fast (downhill 70km/h) the dynamo (alternator) produces overvoltage, e.g. 100 V. That overvoltage is transformed to heat in the transil. It is too much heat and the SMD transil was desoldered from the PCB, then all my electrolytic capacitors exploded.

Does anybody have a better solution for overvoltage protection on AC side (sine wave) voltage? It can disconnect circuit from dynamo if necessary, or just limit the voltage.

Overvoltage protection with 30V varistor and 30V transil

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Best Answer

There are two approaches I would suggest.

Simplest approach

Permanently connect a power resistor in parallel with D1 to provide a minimum load on the alternator. e.g. 220 or 470 ohm. This will prevent the output voltage from rising too high but has the disadvantage that it will 'steal' some power from the load. Nevertheless, you may be able to find a compromise in the resistor value, i.e. a value that prevents an overvoltage yet has negligible losses.

More complex

Connect the following circuit across the load terminals (after the rectified output of the alternator). If the voltage rises above 10.1 + Vgs_threshold (the threshold voltage of the MOSFET), the Mosfet will turn on, effectively connecting the 100R resistor across the load. This will then reduce the load voltage, protecting your voltage-sensitive components.

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