Standlight for Dynamo lights

[StevelKnievel]

I've just finished a few triple and quad cree lights using some of Martin's simpler dynamo circuits. Man these things are awesome:rock:! I love the output, but I'd like to be able to get them to stay lit for a little longer after stopping. Right now I'm using the recommended 4700uF smoothing cap. For the next one I'm thinking of trying three 5.5v 1.5F supercaps in series. Will this siginificantly improve things or is there a better approach for improving the standlight runtime? Thanks!

 

[ebrady]

Putting the capacitors in series will increase their overall working voltage. Which considering that the caps are rated at 5.5V, and the generator will be at least 6, you will want to put them in series.

However, placing the caps in series will not gain you any additional storage capacity above 1.5F.

If you want to maximize storage, try to find some 1.5F caps with a working voltage greater then 6V and wire them all in parallel.

Here is a page I found searching the net on this subject.

 

[StevelKnievel]

The smoothing capacitor has to tolerate at least the Vf of the LED string (~12V for 3LEDs and ~16V for 4), hence 3 5.5V caps in series. The total capacitance of the three would be 0.5F, which is significantly more than 4700uF, so I know the standlight capacity would improve over what I'm running right now. This light is for my commuter, and I'd be willing to waste a couple of Watts while moving to have the light stay lit for 30 sec to a minute at stoplights.

If I add a resistor in series with the LEDs it would enable the cap to charge to a higher Voltage than the Vf of the LED string. In addition to storing a bit more energy, this should also make the capacitor discharge more slowly through the LEDs. This is just a brainstorm and I could be totally wrong here, so please let me know if you guys have any better ideas!

Martin, Znomit, Ktronik... You guys have any standlight tricks?

 

[rideatnight]

I have made a standlight for my 2x cree hub driven light by having a yellow LED running off a supercap.

In parallel with ONE of the CREE's (to get ~3.3V), I have the 1F 5.5V supercap, connected with a diode in series to keep the voltage from flowing back the the CREE. The yellow led has the current limited by this circuit:

http://www.nscl.msu.edu/~daniel/back/nntec/nntec.htm

At first this worked like a charm, and the yellow LED would stay lit for 3-4 minutes. However, it doesn't seem to charge as well recently, and I haven't had time to tear it apart and diagnose.

That's what I've tried. For the most part, I'm not worried about oncoming cars seeing me when I'm stopped. If they're turning left, they'll see me when I hit their windshield! HAHAHA!

 

[ktronik]

I have used 5.5v super caps... better.

I then tried 10.5v worth of 55F super caps... lots of $$, do the math & in series its not 55F anymore, but still worked sweet...

link

in the end a cheap front flasher worked out a lot cheaper...

K

 

[HEY HEY ITS HENDO]

.... "in the end a cheap front flasher worked out a lot cheaper.."
LOL and no headaches either ...
strange that nobody has suggested a tiny battery???? why?
have i missed something obvious
and, are you trying to keep a quad cree lit

 

[znomit]

Martin, Znomit, Ktronik... You guys have any standlight tricks?

Well, the glowinc glow in the dark powder is cool looking. Its quite bright but wont rival a blinky. Maybe if you really packed it in it would be just enough.
I had to make my own paint from nail polish because they wont ship internationally the solvents, I guess their stuff is better. Need a big surface area inside the light head to maximise the amount of powder you can cram in.

Otherwise liberal amounts of reflective tape up front and a good rear blinky with side pointing LEDs might be enough. I got a white cateye LD100 for upfront, still figuring out a mounting spot. I really should put my el530 on too for emergencies.

I ride mostly rural roads so not much stopping. Martins circuit give me light at walking pace so even when I arrive at an intersection the light usually stays on.

 

[StevelKnievel]

Thanks for the input everyone!

I like the idea of building a charging circuit for some small batteries, maybe three li-ion buttons in series would do the trick. I have to get another dyno hub and build it up for my lady's commuter...

For mine I think I'd like to use supercaps, just because I've never played with them before and they would never need replacing.

Thanks again for the ideas!

 

[FrontRanger]

Thanks for the input everyone!

I like the idea of building a charging circuit for some small batteries, maybe three li-ion buttons in series would do the trick. I have to get another dyno hub and build it up for my lady's commuter...

For mine I think I'd like to use supercaps, just because I've never played with them before and they would never need replacing.

Thanks again for the ideas!

You've probably already considered this, but... As has already been suggested, battery power works well, but it needn't be a battery charged by the dynamo. If you ride at night, you should already have a battery-powered light that quickly disconnects from the bicycle, for use during repairs. Why not use it as a standlight also? The price, size, and weight of modern 1xAA (or even 2xAA) LED flashlights are very low, runtimes for a few tens of lumens are many hours, and spare batteries are cheap and small. A Romisen RC-G2 and a Twofish Lockblock are about $20. I like ultracapacitors as much as the next guy, but to me this seems simplest and of greatest overall utility.

 

[Calina]

Although it is always a good idea to carry a spare flashlight for emergencies and batteries have some advantages over super caps, they also have serious limitations. Most of them die in cold weather and when they are under or over charged.

Super caps don't store as much energy but they are more reliable in the long run. On dynamo driven lights, I think super caps are the way to go unless you are able or willing to deal with complex electronic circuits to control a reliable charger.

 

[ktronik]

Please take time to read my above link...

When you use big super caps, your light does not come on till the super caps are fully charged, the bigger the super cap the longer you have no light for. this is a problem, so martin & I came up with a separate charging circuit for the super caps, which robs a little power from the dyno, but still means the light comes on straight away. you can do quite alot with this circuit as you will read in the above link.

K

 

[JB01245]

How much juice does a dynamo hub put out? I found this hubless dynamo design and was wondering if it would be worth playing around with. It says that its output is 7V, 25mA pulses. Will it run a Cree or SSC LED? If not, could it be modifed to?

They also have a feature called "Standby light switch". Don't know if something like it would work for you or you are already using something similar. Here is a link

http://www.freelights.co.uk/standby.html

Ther is a video link at the bottom of the page.

 

[FrontRanger]

How much juice does a dynamo hub put out?

Most hub dynamos (Shimano, SON) are rated for 6V @ 3W average power. The average current is then 3W/6V = 500 mA. If you rectify it (and preferably filter it) you can directly drive modern high power white LEDs. There are many links on this forum to useful circuits. Martin has a nice webpage.

I found this hubless dynamo design and was wondering if it would be worth playing around with. It says that its output is 7V, 25mA pulses. Will it run a Cree or SSC LED? If not, could it be modifed to?

Not at useful light levels. Although the duty-cycle of the "pulses" is not specified, even if it were 100%, you would only get 0.175 W. That would certainly produce a visible glow in a "3W" Cree or SSC, but not the kind of light you'd want for cycling.

 

[JB01245]

Thanks for the reply and link FrontRanger.

 

[znomit]

How much juice does a dynamo hub put out?

How fast can you pedal?
Im getting about 500lm out of my triple R2 using martins circuit, its on from walking pace and ramps up till Im going around 20kph.
If you need more light just add more LEDs. You ask for more juice and the dynamo will provide

 

[FrontRanger]

Thanks for the reply and link FrontRanger.

Sure. As znomit says, you can get variable amounts of power out of your dynamo. It depends on the load. What I should have mentioned last time about the rated power of the hub dynamos is that those ratings are given assuming a 12-ohm resistive load. That was a standard value for the halogen lamps that prevailed before modern high-power LEDs were available.

As a function of speed, the output current ramps up and eventually levels off. This is because the generator has an effective series inductance and resistance. (There's also an effective shunt resistance that further degrades efficiency.) As speed increases, the inductance dominates the source impedance and limits the maximum output current to about 500 mA, assuming a reasonable load impedance. The speed at which this happens depends on the load. Stacking up more series LEDs driven at the same current requires a higher voltage, and since more power is being dissipated you have to pedal harder, as znomit says.

 

[n4zou]

in the end a cheap front flasher worked out a lot cheaper...

K

Yes; like this.