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When someone pushes his bike beside him in a constant (low) speed the light will not be at constant (low) brightness, instead it will flash with it's full brightness.

Why does it work like that?

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This happens because the power from a hub dynamo (which technically is a magneto, not a true dynamo) is not clean sine-wave AC but consists of short pulses with alternating polarity. At high speeds these pulses follow each other fast enough that a LED can burn continuously with a small capacitor and a filament bulb does not have time to cool down between pulses.

The effect does not have anything to do with programming or the fact that LED is a diode, and it happens with filament bulbs too. A more complex LED setup could have a capacitor to enable continuous dim light and perhaps even be programmed to flash at low input power, but I doubt this is the case.

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  • Fillament lamps will glow with AC on either part of its wave. LEDs don't do that because they're diodes and only let power through in one direction. Thing I don't know - does the AC frequency change with wheel speed ?
    – Criggie ♦
    Dec 30, 2017 at 21:06
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    Dynamos have a fixed set of poles, so when speed changes, the AC frequency changes too. In at least Shimano hub dynamos you can actually feel the poles and see that flashes happen when you cross a pole.
    – ojs
    Dec 30, 2017 at 22:37
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    A LED should not really be connected to a dynamo without any driver circuitry. Voltage regulation is needed unless you want to destroy the LED and filtering capacitor will help its job. Once all that is in place, a full bridge rectifier doesn't add much complexity.
    – ojs
    Dec 30, 2017 at 22:44
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    @Criggie - I would assume that the dynamo contains a full-wave rectifier, so that both sides of the AC cycle get through.
    – Daniel R Hicks
    Jan 1, 2018 at 15:46
  • @DanielRHicks dynamos output AC. They were originally designed for filament bulbs which work fine with AC, and LED lights were later developed to work with existing dynamos. A simple driver circuit can be found here: electronics.stackexchange.com/a/161794.
    – ojs
    Jan 1, 2018 at 16:26
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If the light is LED and the dynamo has no AC to DC conversion circuit, then the LED should turn on when it gets the right polarity and turn off when it gets the reverse polarity. When the AC frequency is high (which is when the wheel is spinning very fast) the flickering is not noticeable.

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If there is a super-capacitor in the system it will need a full charge before permanent full brightness. The electronics may be programmed to switch to bright flashing mode at low voltage and low rpm/speed before the full charge at normal operating voltage and high rpm.

A bright flashing light it is more visible than low brightness permanent light.

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    Smells more like the super-cap is missing.
    – Mast
    Dec 31, 2017 at 4:19

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