Originally Posted by
A10K
This is false. The 3 watt rating of a dynamo comes from the German rating system which assumes a fairly low cruising speed (8mph?). A dynamo measured at the speed with a standard load (200 ohm, iirc) must put out 3W, no more, no less, and power a 6V halogen bulb. Some older sidewall dynamos had over-voltage protection, to prevent frying the bulb. Hub dynamos don't typically have that protection and can easily exceed that voltage during most of a ride. Same site OP linked to, which is a great resource for all things dynamo related, has tests of hub dynamos at different speeds and their corresponding power loads:
http://pilom.com/BicycleElectronics/HubDynamo.htm
Not false. I don't think you looked at the source he cited. It has a section that claims to produce up to 10W by 'manipulating' the 3W of nominal power (constant current and assumes unlimited voltage as speed increases) from the dyno hub at least that is what my quick perusal showed. In short the power the light has available is something a little less than the power being generated from the hub, no amount of 'manipulating' is going to change that.
In short it expects that the dyno will have no limits (zener or saturation) applied to the output voltage. The cite also only tested two different dynos.
Originally Posted by
A10K
The power of the OP's light can be measured by multiplying the saturation current (.5A) by the Vf of the LEDs at that current, which for 4 white LEDs will be in the area of 3.3*4 = 13.2V, leading to 6.6W. He could get even more by adding additional LEDs in series, which will linearly increase the total power draw, but increase the minimum speed at which saturation happens. In my experience I've found 2 white/2 red LEDs is a good compromise between peak and low speed output.
No the power of the light can be estimated, not measured, with the method you propose and then only if one also measures the temperature of the LED. Optical power measurements require an integrating sphere light meter.
Originally Posted by
A10K
4 LEDs will easily have far more output than a cheapie halogen light head, optics or no optics. Versus better optimized commercial dynamo heads, most still only have one LED, so unless he is prepared to buy and chain several in series, his setup may be still more suited to his needs (the mixing of the XR-E for far lighting and XP-Gs for up close shows that he did put some thought in this.
Finally, a big advantage of dynamo lights over battery lights is that they are always on--there is no penalty for running a dynamo light during the day, making you more visible to others.
Few dyno lights are halogen anymore. The cheapest are using the same high cap leds the op is using. Their big advantage is they have a correctly designed optical system that places a gradated degree of the light output from near to far. A very good quality dyno light (that easily exceeds the performance of the OP's design can be had for $50...
Yes, I am aware of the advantages of dyno lighting, because that is what I use.
Originally Posted by
A10K
Light meters measure lux/brightness, not lumens/output. A better measure would be to pick up a multimeter/current meter and take some measurements while riding. Take the highest sustained current, multiply it by the number of LEDs (in series), will put this discussion to rest.
An integrating sphere light meter is what is used to measure the output of leds (and other light sources)... That is how they derived those nice graphs in the data sheets. While an expensive instrument, there have been a number of amateur science type articles over the years detailing how to get a pretty good approximation at home.
This is particularly important since the temperature of the LED significantly affects its ability to produce light. That is why those graphs have temperature ratings associated with them.
Which is why dyno lights are typically have the brightness at some fixed distance in front of the light, where the light is needed while riding, it simply makes more sense than led output power which is largely a marketing gimick. That is why dyno lights with 40 lux ratings are usually superior to battery lights with 400+ lumen ratings. Heck the effectiveness of the old Halogen lights demonstrates that light output is the least important aspect of a good and useful lighting setup for a bike.
Originally Posted by
A10K
Also, good job, MrBubbles. What color temperature are the XP-G R2s? Since you can get 4000K warm white XP-G2s (2nd gen) in the R4 bin nowadays, you could get a significant boost in output no matter what your color preference.
It is a cool hack, but simply not cost effective or practical. Further, you are likely going to have near bright spots that reduce your night vision and reduce the ability to see at night--a common problem with the lighting systems with simple conical optics.