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I would recommend to use a higher volt-rated zener diode and to couple it with a heat sink. When the hub dynamo puts out moren than 6.2 volt the zener diode shortens to prevent overvoltage → loss → heat

Wadim Dz

Nothing new...


Capacitor have a tendancie to loose voltage rapidly when discharging ... the effect here being the light intensity going down even though there's still some juice in the capacitor !
To prevent that and, thus enhance this hack, you can probably adapt this little cricuit :

@Wadim Dz : impressive link, bookmarked :)

John h

I don't understand all the fascination with dynamos. Bike technology has been about making the bikes lighter, faster, easier to ride fast. So lets put a miniature generator on it... it makes no sense to me. It makes it harder to roll. And the diode bridge just burns off the extra energy.


Thanks for a great hack. I have the same Pashley set-up, it will be fun to build this and install it as the original halogen bulb is not so good.

Karl McCracken (twitter: @KarlOnSea)

@Patrick - The capacitor is rated at 5.5V, which it should get once the schottky diodes have taken their 0.5V forward voltage drop in rectifying the 6V AC input.

The maximum voltage for the capacitor is 6.3V, and to overload this I'd need the dynamo to be generating 6.8V. But in reality, I don't know the exact maximum voltage supplied by the dynamo is. It says 6V, but who knows? Maybe it does go ~15% over this?

So the zener is there just as a bit of belt and braces backup to prevent me blowing up the capacitor. Assuming the dynamo doesn't generate more than 6.7V, it'll never be used.

@John h - A dynamo with lights fixed to the bike means never having to worry about batteries, and far less worry about them being stolen. The CREE bulb includes reverse polarity protection, but that doesn't mean it's too happy to be fed with AC. This is why you need the rectifier. It's built with schottky diodes to minimise the forward voltage drop and associated power loss. This isn't about making the light brighter though, it's to make it run better at lower speeds.


That's about the simplest way to do it. For those wanting more info, check out Pilom's page on the subject:


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