Pashley Dynamo Headlamp Upgrade
My Pashley came fitted with all the essentials of Civilised Cycling - mudguards, rack, chain case, and of course lights. The back light is a battery-powered LED affair, while the front is a dynamo-powered incandescent bulb.
The trouble is that at the beginning of this winter, I got a proper LED front light, and I’ve got accustomed to actually being able to see where I’m going. So for some time I’ve been wondering about how to upgrade the Pashley’s lights.
There are lots of pages of circuit diagrams that’ll tell you how to build your own from scratch, but they all looked very technical, and would involve some serious bodging of the lamp casing.
Then I came across this:
It’s a standard torch bulb, but with a 1W CREE LED in the nose! And, according to the site selling them:
- The bulb has function of “TACP” (Temperature Auto Control Power). That means the output power of bulb’s driver will be controlled automatically according to different temperatures. [So no need for complex driver circuits]
- The bulb has reverse polarity protection. The LED bulb has polarized contacts with the positive supply to the base (typical standard torch set up), however if the contacts are connected in reverse the LED will not light but no damge will be done and it will work when the polarity is corrected. [So no need to worry about the AC voltage coming out of the dynamo]
Basically, it looks like a plug-n-play replacement for the Pashley’s bulb!
Not quite though. At low speeds there was noticeable flicker from the bulb, and even at higher speeds, the light was more like the fast strobing of an electric arc. The reason for this is that you can feed these bulbs with AC, but they throw half of it away - they’re really designed for DC. An email conversation with the supplier also identified that running these bulbs on AC can significantly shorten their lives.
So now I’ve built myself a bridge rectifier, to convert the feed into DC:
This seems to work really well - there’s still some flicker at low speeds, but the light is nice and smooth at anything above a walking pace, and the bulb remains lit (though not as brightly) for more than 30 seconds after I stop.
Technical details in case you’re interested:
- Hub dynamo output: 6V AC, 2.4W
- Bridge rectifier made from 4 1N5817 Schottky barrier rectifiers. The Schottky diodes have a much lower forward voltage (<0.5V) compared with silicon diodes (typically 0.9V)
- The smoothing / power backup is a 1F Double Layer backup capacitor. This is rated at 5.5V, so should get along very nicely with the output from the bridge.
- The maximum voltage of the capacitor is 6.3V though. So I’ve wired a 6.2V zener diode across it by way of some belt and braces protection.
- The whole thing is about the size of two pound coins stacked one on top of the other.