Electronics > Power/Renewable Energy/EV's

Bike Dynamo to 5V USB Design for thoughts?

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solarbot:
Hi,

I'm designing a circuit to allow the safe and efficient charging (I hope) of USB devices from a DC 6V 3W bike dynamo with some protection and output buffering via a 5.4V 5F supercapacitor (PHV-5R4V505-R).

Thought I would pass the design as it stands by the good folks here for comment and help chosing the right varistor, assuming this is the best option for protecting the input side from spikes and overvoltage?

Attached is the schematic - any comments on how to improve the design of course equally welcome.

Cheers.

Ian.M:
Ordinary silicon diode bridge rectifiers have an excessive voltage drop.  You'd do much better with 4x 3A Schottky diodes - it would start charging at a significantly lower speed.

nali:
A couple of points..

If you're using this with lights off and no load, then the input voltage can get pretty high depending on the  wheel speed and model of generator / lights. There is usually a clamping diode somewhere but that's not guaranteed. I looked at it once, as recall I measured something like 40V @ 20MPH on my hub dynamo.

If using with the lights on - the generator won't supply much more current than the rated 0.5A, so the lights will dim.

(by the way, such products do already exist. I have a Busch & Muller front light with a USB charger port (only works with the lights off). 

solarbot:
Thanks Ian,

Something like this?

http://uk.farnell.com/on-semiconductor/mbrd340g/diode-schottky-3a-40v-d-pak/dp/1431096

solarbot:
Thanks Nali,

I won't be powering lights directly but charging either batteries for lights or devices such as GPS etc.  There would definately be times when there is no load at high wheel speeds so need to have protection like you say - I've never used varistors before but was thinking this would take care of high voltage spikes and output?  I've looked at commerical products but kinda fancied learning a little more electronics and having some output buffering via a capacitor which I'm not sure is generally implemented?

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