Rob.

Yes, agree, the unspended mass of the dyno hub, in this case, isn't imo, a reason not to go forward with what you are doing. I did poiint this out in an earlier post of mine.
🙂

"I didn't mean to come across aggressively/over defensively, I just got a bit frustrated when posting looking for advice, having got what I considered a nice bright light working for a zero weight penalty, that people who hadn't even tried a dynamo light with power LEDs, let alone tried making one, or perhaps hadn't even set eyes on such a thing, were guessing at supposed negatives, without anyone guessing at supposed positives!"

Yeah, understood, I had considered it myself, during early 09, but elected to do something else. So my Qs and comments were intended to learn more about what you were proposing, is all. So some, what I thought to be, balanced, testing Qs was all I was asking.

Yes, it was your legendary thread on STW's previous forum that got me building battery powered LED lights.
😉

What hubs are you and Joe using at the moment ?.

As I posted earlier, I am midly intrested, but I see issues with the current format is all.

Ta.

😀

L.

Just to harp back, your voltage across your cap drops as so with any power drawn from it, my comment about it dimming is true if you're working at or below the nominal voltage of the LED, you just haven't noticed the dimming effect or you're initially operating above normal LED voltage and so are in that plateau where current and output break ties and start to cause breakdown of the LED?

Modern dynos do have capacitor backup for short stops, but require a bit of control. You'd need to pass the output of the dynamo through a diode to stop the cap back-feeding the dynamo coils, and you'd need to consider that while the dynamo is a current-limited delivery device, the capacitor is very much not – it'll melt the leads off your LED given enough potential difference, so if you're charging it to much more than the standard forward voltage of the diode expect to have to take more care. So I think you'll be looking for :
dynamo>diode>current reg>LED. The size of a decent cap may be prohibitive, especially compared to the size of a pack of rechargable AA's that will do a similar job (though not like being totally discharged so that needs to be accounted for).

http://www.candlepowerforums.com/vb/showpost.php?p=2700461&postcount=52 has what looks like a pretty simple circuit for a standlight using extra diodes, resistors and a super-capacitor, which reportedly works okay.

There is lots of talk on that thread too, some of it from people who've actually built these things. It appears that all the talk of needing constant current devices and blah is not really true in these designs where you have the capacitor in parallel with an LED, as the maximum capacitor voltage is regulated by the LED max voltage.

What hubs are you and Joe using at the moment ?.

I'm using Shimano DH-3N80 (on a road bike). I think the 3d72 is the disc version but I'm not sure. Be careful of older versions of the hubs, as they are less efficient and much heavier.

Joe

That thread is what got me started on super capacitors

The "super" capacitor is 20mm diameter and 7mm deep. It weighs less than 1g. It's rated at 5.5V 1.5F, I charge mine across two LEDs using a diode to both control the discharge through one LED and to drop the 6.4V from the Vf of two LEDs down towards the 5.5V rating.

I wish people would stop making assumptions about capacitor charging and discharging and saying thins will go pop and keep telling me it won't work, without trying it, when it quite clearly works very well indeed. I've been running this for a week or so (>20hrs of riding, lots of charge discharge cycles) and nothing has gone pop. The capacitor charge is controlled as described above. Discharge is controlled with the very simple technique of a current control resistor as it's only a backup and I'm not so concerned about efficiency. I'd rather use a 2p resistor and an extra capacitor than one less £2 capacitor and a TaskLED controller board!!! The post was started to ask people who had tried it, or considered it, advice about how to make my solution THAT WORKS about how to make it better. Stop telling me it won;'t work people!

Coffeeking, my original post described the capacitor voltage control technique.

I'm using an alfine dynamo disc hub. It quite happily drives 4 LEDs just fine without any modifcation. It has neither capacitor backup (no hubs have this that I[m aware of, normally this is in the lamps, what hub are you thinking of?). I've also not come across a hub with a zenetr diode voltage limiter in, although this is talked about on other forums from time to time.

People seem obsessed with the size of the super caps. They're tiny, much smaller than a single AA, let alone the charge and discharge controlling required to make a battery solution work.

Erm. That'll do for now. Any actual advice? Although Luminous has raised a few genuine questions, that I hope I've helped to answer.