Homebrew light setup - advice needed
 

I've been picking brains over on the C&V forum about my setup and by their suggestion I've come here to see if I could get a little more input.

I have an old headlight that I have put 25 leds in, replacing the original incandescent bulb. It's 25 leds soldered together in parallel on a radioshack pcb. I have an sram dyno-hub on the bike now that I want to power the light with. When I was initially putting this together my plan was Dyno-hub > bridge rectifier > headlight > 4 AA rechargeable battery pack.

Now, I've been told that I need a capacitor and that using a battery pack in place of a capacitor is not a good idea (something about exploding batteries).

From what I understand, I could just wire it up with the bridge rectifier and be good to go. However, I would like to have the battery pack in there to keep the light on when I'm not moving.

What is the right/safe way to implement the battery pack to keep my light on while not moving?

There is a theoretical risk of overcharging your batteries, but in the real world your idea will work. I personally think it is way easier to blow up a capacitor than a battery with a bicycle dynamo. At least I have managed to make a 16V capacitor that was part of my bicycle USB charger (prototype only) bulge nicely.

To get back to your setup : I plan to make a similar thing for one of my bikes. My plan is to always run the dynamo (bottle model on this bike) when the lights are on, unless I feel/guess/fear that the batteries are overcharging.

The only question I have regarding your setup is why all those leds in parallel? You plan to use a dynamo that produces 6V RMS and your battery pack will have a similar voltage. Better then to put the series-paired leds in parallel (each time put 2 led in series and put x-number of those pairs in parallel). Else you risk to over-volt your leds. Have a look at www.pilom.com if you have not done so yet.

FYI : I'm not a electronics expert, but since I started tinkering with dynamo powered USB and led lights I have been fascinated by it.

Just saw your forum ID. Considering you call yourself "flammenwurfer", maybe you should be very careful after all ;-)

Just checked out your post in C&V forum. So a few more idesa.

Lights (and leds more than bulbs) will flicker when powered by a dynamo at low speeds (hub dynamo will cause more flickering than bottle dynamo). One of the solutions is to ride faster. But there may be a particular issue with your setup. You have 25 leds in parallel. I don't know how much milliamps they need each, but there is a good change that 25 of them are not getting enough power from the dynamo hub. This would be solved if you put 12 pairs of series leds in parallel as I mentioned in my previous reply.

Adding a capacitor or batteries will equally help. But when you use batteries make sure that you use some resistors if necessary to limit the current through your leds.

But your AA batteries will not explode under any normal circumstances.

So are you saying that having them in parallel will be dimmer than if I did 12 pairs of series leds in parallel? Sorry, I don't know a whole lot about electric circuits. I have another pcb and tons more leds, so I can make another light if it would work significantly better.

hope you don't feel like I'm stalking you :)

I hadn't noticed the unique setup you have chosen. You probably are dragging down the voltage of the dyno too much with so many leds in parallel. Even the best dynos only put out 500 milliamps. I have no idea what the current rating of your leds are, but you may not be driving them to their full brightness. For example, if they were rated at 40 milliamps, you would only be driving them to half capacity 40*25=1000, but you're delivering less than 500.

dupe

Just a note on capacitors - they can be used to smooth out a flickering power source to some extent which may be why it was suggested. My electronics experience is mostly in audio amplifiers and I use a capacitor in this nature when running a signal through an old school needle-type level meter to smooth out the signal hitting the needle, this prevents it from jumping around faster that you can track it.

I think it would probably help in lighting as well to prevent it from rapidly flickering as the dynamo's output varies.

Right indeed. I don't know what type of leds exactly you are using, but I guess they must be rated around 3V and using close to 40mA a piece. You have put 25 in parallel which means that each of them gets around 6V and that they ask for 25x40mA. Not a good fit for your 500mA dynamo.

If you do the 12 "pairs-of-leds-in-series" in parallel, then each pair of leds will get 2x3V (Voltage drop of 3V per led) and they will ask for 12x40mA which is 480mA. Considering that the bridge rectifier diodes will burn some of the power from the dynamo, the leds should be fine handling the rest. Leds are more sensitive to too much current, but can handle over-voltage better. Standard bulbs do the opposite.

Of course you must check the exact voltages and mAmps for the leds you are using and adjust the maths to determine how many pairs of series leds you should put in parallel.

Looking forward to your test results...

this complexity is why it probably would have been better just to get a Cree XPG on a 20mm heatsink. You can run it directly if you're lazy, or add a bridge, or bridge + cap.

I don't know a lot about this but your explanation makes sense carrefour. I don't remember exactly, but I think the leds I'm using were rated 20-30ma.

So if I want the light to stay on for short period of time when stop, what would be the easiest/simplest way of accomplishing that? In another thread I saw somebody mention using a supercapacitor in their setup. Is there a way to use one of those to keep the light lit up briefly when I stop at stop signs and such?

If they are 20-30mA, then you will need at least 15 pairs of leds.

In theory supercaps are the way to go if you want a stand light. However (1) I have not tried them myself... (2) from what I believe to understand about them it would not be as easy to use them to keep the light going for a while after stopping cycling.

Let me clarify a bit. You should not expect that a supercap will keep all 25 leds on for let's say a stop at a red light. The way to go is to only run 1 led for the stand light. All very easy to do with electronics for experts, but that is not me.

I would just go with batteries : dynamo, bridge, 4 AAs, leds. But with batteries you may have to add resistors (very cheap) in the circuit to avoid over-currenting them. This is where you use Ohm's law.

Also take into account that if you plan to use the light only once every x months your batteries will be empty. No problem to charge them with the dynamo while cycling, but the question is if your lights will work properly at the beginning. The batteries may take all the current till they reach a minimum level of charge. Best way to find out is to try.

I second Unterhausen in getting a or some power leds. I think that www.pilom.com has some very simple, but great ideas for that.

Some good stuff at that site, but some of the fabrication of heatsinks and such, I don't have the tools to do myself.

I am intrigued by a couple of the ideas on there though. Mainly this and this.

I could get a Luxeon EndorStar 3 led, with a capacitor (not sure what one I need) to smooth out the low speed flicker. But I'm not sure how I would heatsink it and mount it in my light housing. I would like to keep the bike looking somewhat vintage. I just don't think a new looking fabricated light would look right on the bike. And then I would still have to work in the batteries or supercapacitor.

How big/how many supercapacitors would it take to run that Luxeon 3 led thing for 30 seconds?

You are right. The mechanical part of LED lights is often harder than the electronic part. I forgot that you started the idea on the vintage forum so I see your point on the looks. But then my question is : Do you really need the stand light function? Don't get me wrong, I'm not trying to tell you what to do, but the point is that without that your circuit can become really simple and small. For the heat sink there are different solutions. You can use thermal glue and depending on which led you use, you may not need a big heat sink anyway. (I don't know by heart which leds that would be. I have read through many websites over the last months and some DIY projects out there mention that the need for cooling is very limited.)

The typical supercap is 1F at 5.5V and cost a few $. But I have not seen many succesful DIY, if any. The general idea is that if you need a stand light it is easier to add a small battery light. Don't know if that is an option for you. I'm just trying to help with ideas.

A single 5.5V 1F supercap on one led would keep it bright enough to "be seen" for 30s.
Again 3 LEDs is over the top and you triple your heat problem. I'd stick with one. Maybe even one on a doubler circuit(see pilom.com) to limit it to 250mA.

The endor star is a very very floody beam and not really what you want on your bike.

Are those Luxeon and Cree leds that much better than the cheap ones you find on ebay? Is 1 led really bright enough to light my way going 15-20mph?

I guess I just have never seen these in person. I got what I have hooked up finally. Dynamo > Bridge Rectifier > 25 leds in parallel. It's definitely acceptable and I only notice a slight flicker if I ride so slow that I could almost be walking. I feel like 25 leds should be a little brighter, but it still does a decent job of lighting my way. I think it could have something to do with the lens though. I have the 25 leds shining out through the lens from the old Union housing that was meant for an incandescent bulb.

I think I don't want to have to mount or stash a battery pack anywhere on the bike. It's starting to feel too cluttered. Unless I can get a supercapacitor to work and fit it inside the housing, I will probably just nix the stand light. I really don't think I will be riding all the frequently in the dark and as it is now is acceptable for me.

Thought of something else. With my current setup, would a 1f 5.5V do anything?

If I were to try a super capacitor, where would I wire it in? Between the rectifier and the leds? or between the dynamo and the rectifier?

How bout one of these?

The supercap should be OK. Don't know for how long though. The supercap goes between rectifier and leds. NOT between dynamo and rectifier!

You could try to put the supercap over 1 of the in series placed pair of leds. That way only 1 pair would pull the power of the supercap. You should know that the supercap will probably delay your lights for a couple of seconds too when you start cycling as it will need to get charged first. Simply the opposite logic from when you stop, when the supercap should get the light going for a short moment. I wouldn't expect anything fabulous though.

I would suggest you have a look at Znomit's http://www.candlepowerforums.com/vb/...5&postcount=68 . He has a lot of experience.

Thanks guys, for all the info. I really appreciate it!

I think I read somewhere that you can use up to 3 supercaps in series to get more time out of them. Is that correct? Would it hurt anything to hook two of the 1f 5.5V supercaps up and see what happens with my current light?

One is plenty. Yes it needs to charge up before your light will come on, takes a few seconds the first time you ride. As noted there are better ways to do it but one supercap works ok.