supercaps?

[cbr2702]

I'm modding a dynamo (SA X-FDD, 3W 6V) headlight (old chrome teardrop incandescent sort) with an led. I've ordered a cree XR-E (R2, WH) from cutter along with an elliptical (10/40) lens and holder. They're still not here yet, so I'm thinking about capacitors. So:

I keep seeing people here talk about "supercap"s. Are these just normal electrolytic capacitors with high capacitance? How high? How much voltage do they need to be able to take?

I was thinking of putting one (or more? that would sum their capacitances without change in voltage, right?) in parallel with my led inside the light housing. I found an old power supply with a few 4000uF 5V capacitors. Are these suitable? Or is the 5V not enough greater than the 4V I expect to have after rectification of the 6V dynamo signal?

If they're not good, are there other sorts or electronics that might be good sources?

 

[georges80]

What do you hope to achieve with a supercap or large value capacitor?

cheers,
george.

 

[cbr2702]

I don't have the led yet, so I'm totally sure what I'll need.

The primary goal is smoothing the flicker in the led. I've read that hub dynamos, especially at low speed, generate AC at 20Hz. This sounds unpleasant to look at, so a capacitor in parallel would be nice.

Secondarily, it would be nice if the led would stay on for a short time when the front wheel is not turning. This is where I would think a large capacitor would help.

 

[georges80]

I don't have the led yet, so I'm totally sure what I'll need.

The primary goal is smoothing the flicker in the led. I've read that hub dynamos, especially at low speed, generate AC at 20Hz. This sounds unpleasant to look at, so a capacitor in parallel would be nice.

Secondarily, it would be nice if the led would stay on for a short time when the front wheel is not turning. This is where I would think a large capacitor would help.

Many supercaps are rated at very low current draw (high internal impedance) and would do nothing to 'smooth' the output.

You would need a VERY large value capacitor to give you any runtime (even seconds) with the wheel not turning given a 3W LED as the load.

cheers,
george.

 

[rayhead00]

http://www.pilom.com/BicycleElectronics/DynamoCircuits.htm

shows many circuits used to acomplish the needs you have described.

 

[cbr2702]

You're saying the term "supercap" is used for a capacitor which has high internal impedance?


I'm looking at simple builds online like this one:

http://www.bikeforums.net/showthread.php?t=447235

The LED doesn't need to stay on at anywhere near full brightness to make me visible to traffic stopped at an intersection.

What about two capacitors, one with low internal impedance and another with high internal impedance wired in parallel to eachother and to the led. Would this have the low impedance one smooth the signal and the high impedance one power the led at a low current level when there was no input?

 

[georges80]

A supercap is usually a capacitor with several farads of capacitance, but many have high impedance and can only provide mA level current - which won't light up a 3W LED to make you visible to anything useful.

If you want a stand light for 10's of seconds you need batteries.

Any capacitor of a few thousand uF will do nothing to light up your LED (fractions of a second).

Assume you have 3.7V at 1F (1F high current supercap)

Joules = 1/2CV^2, so about 7J of energy given a perfect world. So, assuming the LED is drawing 700mA at 3.7V so is using about 2.5W.

That capacitor can provide 2.5W for ~3 seconds assuming the world is perfect. Since the current draw drops as the voltage drops as the capacitor discharges it will provide more than 3 seconds of light, but at quickly diminishing light levels.

It just isn't practical imho.

cheers,
george.

 

[znomit]

I'm looking at simple builds online like this one:

http://www.bikeforums.net/showthread.php?t=447235

The LED doesn't need to stay on at anywhere near full brightness to make me visible to traffic stopped at an intersection.

Thats my build and yes it doesn't get much simpler. There is no noticeable flicker at any speed you're likely to ride at.
A simple supercap across the LED will probably do for a minute or two of dim light when stopped. It seems it depends on which supercap you use.

I use one here and while acceptable its a lot dimmer than a blinky. Out last night I tried it and once eyes are dark adapted(a few seconds) I can see the white lines on the road and reflections off distant road markers.
In retrospect I probably wouldn't have bothered as I run a blinky too.

See this thread for standlight ideas.

 

[Alex Wetmore]

I'd recommend reading this other thread if you are interested in building with supercaps:
http://www.candlepowerforums.com/vb/showthread.php?t=210602

The capacitor that is working best for me is the NEC FYH series. This is the URL to it on Mouser:
http://www.mouser.com/Search/ProductDetail.aspx?qs=Z/aT8fNkoxEq6vOZMCnWuA==

My next attempt is a much more complicated circuit using a 2.5V 10F supercap and a booster circuit to drive the LED. I'm trying this capacitor for that project:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=P11345-ND

alex

 

[yellow]

on my town bike I have a front triple Cree and a rear quad 5mm red led setup.
Both circuits use 1F 5V Supercaps, at front to drive one of the Crees and at rear driving all four, when the bike does not move.

With the cap, the rear led stay relatively bright for some mins, the front Cree gets darker immediately after stopping the bike, giving light enough to navigate into the bike room, but fading away relatively quick. Say 30 secs navigating + 30 sec min glim (that can seen from the front)




I ll post both circuits and cant resisit to type what I type most every time:
I have done nearly all those circuits, ranging from easy to superduper "effective" difficult to build.
The best one I know of is JürgenH' s "MOSFET"-rectifier. Already gives useable light at speeds every other circuit does not even light up the triple-crees.


back:

front:

MOSFET:

... it simply is genius !