Re: Bike lighting: how many lumens do you need?
Posted: 6 Sep 2024, 8:30pm
I did some experiments along these lines and I am able to share the results with you all. My experiments came in two different formats; on and off the bike.cycle tramp wrote:.......has any one tried using to lower powered leds together from the same dynamo? And if so what we're the results?
1) In the first set of trials I used a StVZO-rated generator to power two cheap LED torches which must have had some voltage multiplying trickery inside them, because they used just one 1.5v cell up to ~0.5A current. As the battery weakens both the voltage and the current droop, meaning that the lights were brightest with a new Zn-C cell installed, but had a very long 'tail' of lesser output. I configured each torch with a diode in series as protection, then wired them back to back across the generator, so each lamp would see half-wave rectification. The idea was that i'd get some light at low speeds, but the full current output of the generator (~0.9A ac) wouldn't blow the lights up either. The half-wave rectification was intended to be attention-grabbing, and the two light sources were meant to provide better definition.
I lashed each torch to either side of a set of drops and went for a ride. The results were interesting, and although nothing blew up it was not a complete success. The definition might have been improved but the strobing was terrible. It was certainly attention-grabbing, especially at low speeds, but also completely unbearable. I daresay some smoothing capacitors would have helped, but I didn't have any. Luckily I am not susceptible to epilepsy, but that would almost certainly have set me off if I was.
2) the second set of tests were off the bike and used cheap dynamo LED lights.These came from the netherlands and were sold under the FALKX brand; according to the packaging they have a 1W rated LED inside. They use an exposed emitter and a fairly simple modified parabolic reflector to create a mostly torch-like beam but with a square-ish bright spot in the centre, which I would guess is about 30 lux if pointed about 10-15 m away, Because the emitter is exposed the light is highly visible, even in the daytime/sunshine. It is certainly bright enough to be a real annoyance to others if pointed the wrong way, just like any other bright light. There is a built-in reflector. As a 'be seen' light it is excellent, and as a 'see by' light it is quite passable, very good for the money (I paid less than ten quid each I think for mine which were the Er9a model, I believe). Looks-wise you would be forgiven if you mistook it for one of B&M's offerings, having a very similar stainless steel bracket and housing design. In short, it is a good light by any standards and for a budget light it is remarkably good. It reaches full brightness at about 5v, 0.5A. It is a 3W rated light, so I think it must use a voltage regulator or a Zener diode to turn any surplus generator output into heat. Until it taps out, it seems pretty much as good as anything at turning relatively small amounts of electricity into useful quantities of light.
Simply connecting two these wee beasties in series means that you need to go a little bit faster to make any light at all (if any part of the open circuit voltage waveform exceeds ~7V there will be some light; c.f. ~3.5V for a single lamp) but most generators will manage the ~10V 0.5A required for full brightness. If both lamps are pointed in the same spot, the beam is twice as bright, which sounds a lot better than it looks It is usually much better to make the relatively small centre splodge either twice as wide or twice as long (the latter being a lot more straightforward if the lamps are mounted via a single bracket). If two lamps are wired in parallel, the low speed output is very good, but the lamps never get to full brightness, unless the generator will produce 1.0A, which most won't. Just for fun, I connected a set of four lamps to the generator, in two parallel series pairs. Just like other other parallel setup, full brightness (which in this case requires ~10.0V and ~1.0A) is never likely to be achieved. However, it does make more light.
So the sweet spot is likely to be a pair of lamps in series, if they are like the ones I tried. These can (with the use of tube spacers and studding) be mounted on a single bracket and are easily aimed so as to make a usefully longer centre splodge. In terms of not dazzling oncomers, four lights is about four times worse than a single lamp, but two is probably OK. They are probably best wired up as ' master' and 'slave', with the latter interrupting one of the wires to the former to which the rear light is also connected. If both switches are closed, all the lights are on. Turning either switch off (opening it) always extinguishes both front lights, but if the rear light is capable of running happily alone (not all are), the 'slave' interrupts to ground connection of the 'master', and both the generator and rear light have their own ground connections (which some do) then when the 'slave' switch is open and the 'master switch is closed, the rear light alone should come on.
