Bike Dynamo to 5V USB Design for thoughts?

[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?

[nali]

It's not really a case of voltage spikes, more like an automotive "load dump" where you have sustained low impedance high voltage to contend with. However having said that, I would just put a 10-12V  clamping diode instead of R1, capable of handling a sustained 3W of power (plus a safety margin).

(but as I said there is probably one somewhere on the bike either in the dynamo or front headlight. They do often burn out though if the bike is ridden for long periods with the front or both bulbs blown)

Then the issue becomes how to get rid of the heat. Fortunately by the nature of the application you have plenty of air cooling !

[Circlotron]

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.

I'll second that.
The dynamo on the bike I had many years ago measured almost exactly 1V /  kph road speed with no load connected. Was quite surprising.

[Codebird]

You'll need a few bits of control electronics in there too. Some phones are rather unhappy about getting under-voltage power, it can be unpredictable, so you'll need a simple under-voltage cutoff too - just to cut power to the output when the voltage is under perhaps 4.8V, and turn it back on once the supercap has charged up again. You might also want to swap the supercap for a li-ion cell - not so much for the lower cost and higher capacity, but because it might be easier to manage charging. The output will always be a step-up so you can use a simple boost converter, and there are a ton of tiny li-ion charge controller and discharge-protection ICs. You'll also want a little buck converter for the input side, because it's more efficient than trying to just load the dynamo down to the required voltage under high speed.