Dynamo lighting experiments.

Hi All,

Here in the Northwest as soon as daylight savings time kicks in, it will be getting dark around 5:00 p.m.. I have been experimenting with my lighting soutions once again. The following is a somewhat rambling discourse on my thoughts, considerations, and experimenting so far. I hope it might be helpful to someone wrestling with the same issues.

Last year on our tandem we used rechargeable lights. We mounted one light on my helmet and a dual beam unit on the bars. We also have a Blue LED back-up headlight in case the batteries go dead in our main units. We are using flashing LEDs mounted on each rear bar end, the rear seat post, the rear fender, and on the backs of our helmets. We also wear Illuminite reflective jackets and tights. I actually feel like we are more noticeable at night than we are during the day.

I have been frustrated by the batteries in our main system. If I forget to charge them; we are just screwed, and if we go out for a longer ride, we have to ration their use or they go dead before we finish our ride. So I started thinking back fondly on the days of my youth using a dynamo.

Some complain about dynamo lights going out when you stop, but we light ourselves up like a Christmas tree with flashing LEDs anyway, so that is not one of my major concerns. For me the biggest problems with dynamos are low power output. We are used to 20w of combined light and the typical dynamo puts out 3w.

I found 12v 6w Chinese dynamos on Ebay for $10 a piece. They are also on the internet direct from China for around $1.50, but you have to buy like 200 of them, and arrange shipping. Everyone told me that they probably wouldn't really put out 6w, so I bought four of them and four full wave bridge rectifiers ($1.50 at Radio Shack) so that I could hook them in parallel hoping to power two ten watt bulbs. This turned out not to be the problem.

You see your typical dynamo has not been engineered for maximum output, but to meet government regulations in England and Germany. The Chinese apparently haven't felt constrained by these same regulations. Unfortunately, in the case of the units I bought this has caused some other minor issues.

On our wind trainer (with the fan disconnected) monitoring the output with a multi-meter at 10mph the dynamos produce 12v with the load from a 6w bulb, which was great! Unfortunately at 15mph they are already putting out 18v. At higher speeds say goodbye to your 6w bulb! With an 11w bulb installed the voltage stabilizes at around 14v at 20mph and above. Unfortunately, the units tend to slip a bit on the tire, even when the sidewalls are dry with an 11w bulb. They also start to get pretty hot after ten or fifteen minutes.

I tried hooking up two dynamos in parallel using full wave bridge rectifiers. I discovered something interesting. The no-load voltage of one of the dynamos is around 40v AC at 30mph, but when you hook two in parallel through rectifiers the no-load voltage is over 100v DC at 30mph. By normal logic parallel connections shouldn't be multiplying the voltage. Maybe an electrical wizard can explain why my full wave bridge rectifiers are behaving this way. My original plan was to hook four in parallel; I wonder what kind of no-load voltage I could get that way? Maybe enough for the Zapomatic anti-theft system!

Hooked in parallel through rectifiers I lost some output. The circuit would only get to around 9v with a 20w halogen bulb. With an 18w bulb we get 12v at 20mph. Fortunately, the voltage output seems to be limited a bit even at higher speeds, which could be an advantage for bulb life.

Now at this point I know what many of you are wondering; what kind of drag we are getting from this system? It is fairly significant, but we haven't tested it on the road yet, because there are still other issues to be worked out. I am hopeful that the penalty won't be more than a couple miles per hour, but this remains to be seen. My thought on this is that we usually ride a little slower at night for safety reasons; with the dynamos we will still be getting a good workout.

I have ordered some rubber covers from England to put over the caps on the dynamos that will gear them down by making the caps diameter bigger. This should quiet them down some. Hopefully this will also make them less likely to slip, and keep their temperatures lower. I am now thinking of hooking up the dynamos in parallel and using one bulb at low speeds and two bulbs at higher speeds. I may design a circuit to automatically switch the second bulb on when the voltage reaches a certain threshold, and put a gel cell or zener diode or both in the circuit to add load above 12v and for battery backup. That adds the headache of having to monitor the gell cell to make sure it does not eventually get fried. The simplest and most electrically efficient solution is of course to hook each dynamo to its own matched bulb; with the wide range of outputs from my Chinese dynamos going down a hill could easily burn out the bulbs without some type of voltage limiter.

For those of you wondering, many different sizes of bulbs and enclosures are available from home improvement and hardware stores in the garden and outdoor lighting areas, and also at radio shack and other electronic supply retailers. Of coarse some auto bulbs are suitable also. The garden lights are usually the best value. The best link on the headlight bulbs that I have found is: http://nordicgroup.us/s78/headlights.html

I have been experimenting with another type of generator. This may be much more promising in the long run. Many weed-wackers have permanent magnet motors. These are relatively small and work as DC generators. I took one of these and put a rubber caster wheel on the shaft and have mounted it on my test bicycle's chainstay. This can power a 50 watts bulb at 20 mph using a 2" caster wheel. It is also very quiet, because of the lower RPMs required. The motor I have right now weighs many times more than a bicycle dynamo and I haven't perfected my mount. The drag is significant. Even though a small dynamo will reach a maximum output at a relatively low speed they are still known for burning out bulbs going down hills. This generator is capable of producing much more power at higher speeds. This will take some experimentation to determine the proper sized motor, bulb, caster wheel, and voltage limiting device.

Obviously dynamos are not for everyone. I am trying to find a good compromise that will work for us. You have compromises in weight and drag, and using electronic regulation and rectifiers. The most efficient setup simply matches a bulb to the output of the generator at the speeds normally expected to be encountered.

Sincerely,
Steve M.

 

Steve
I have been using dynamoes for lighting for around ten years now. The first was a sanyo roller dynamo that ran on the top of the tire. That eventually died. Then I used a cheap numbfish hub dynamo. The light output and drag were ok but the cheap cartiridge bearings died after one winter. Now I am using a shimano hub dynamo, which is quite good. All the above were 6v 3w which matches the German requirements. All the dynamos would easily burn out bulbs at speeds of 30kmh or more although the shimano is not two bad. I use 2 6v8 zener diodes back to back to limit the voltage and protect the bulb. This is crude and probably not the most efficient but is very cheap and effective. the headlight is a cut down Arlec White Pointer Torch. This produces a very focused beam that lights up the road about 100 metres away, but also has halo that gives some spread. The downside is, it is large (about 125mm in diameter) and the aim has to be good or the light is not where you need it.
all dymamoes suffer from the conflicting requirements of producing power without too much drag. The power for the lights has to come from the rider, and as I produce around 150 watts at my usual commuting speed I do't think I'd like to have 1/3 of that going into my lights! You may feel differently.
Here a link to some dynamo light pages
http://www.myra-simon.com/bike/tips.html

John V

 

I have a 3 watt Led driven by a shimano hub dynamo. (underdriven at about 2.2 watts.) It isn't quite bright enough (av spd 16 - 19 mph) but is adequate, and is highly visible as a very bright very white point source of light.
The 12 v generator you have described sounds ideal for the way i run mine. Basically it runs at a constant current using a switchmode driver board available from here.(http://www.taskled.com/ccxw.html). It doesn't matter what voltage it is within limits (up to 30v). If you can solder basic connections and wire the system up observing the correct polarity this would work. The 12v generator would probably run up to 3 3w LED's . The LED's are made by LUXEON and are available at many places on the internet. To keep the voltage going higher than 30v you would need a zener diode as described above (only 1 for DC). Of course you would still need the bridge rectifier as well.
I'm tempted to purchase one of these dynamos myself although I have my doubts about outlaying money for the LED's and mounting them somehow, when the long term reliability of the cheap generator may be suspect.

 

Thank you for the input. Our lighting system will be used on a tandem so there is a bit more power available than from a single rider. The amount of power one is willing to allocate to powering lights is a compromise that unfortunately commercial systems generally don't give us much choice over. It will be interesting to find out what type of longevity to expect from these dynamos. Other than wondering how long it will be before the Chinese own us all; I have been pleased with the value of their goods recently.

Sincerely,
Steve

 

What you'd ideally need is a fully adjustable generator set up, something with separate generator and stator coils. By adjusting the voltage in the stator coil you'd get a similar amount of electrical energy out of it regardless of what speed you're riding at. As an added bonus the generator wouldn't be so prone to overheating and drag would also lessen the faster you're going. Running electrical motors as generatos can probably be agood place to start.

Otherwise you might consider running your current dynamo from your synch chain (or possibly the chain ring) instead of from your tyres. Advantage would be that cadence is more stable than road speed which would make it easier to set up a matched system.