Why do lights have a double nature

Twice the brighter

Issue 4 November 2007

This article as a PDF

by Andreas Oehler

Anyone who wants to be out and about quickly by bike at night and is constantly dazzled by oncoming vehicles wants the brightest possible light from the headlights. Hub dynamos and modern halogen headlights deliver quite useful results, especially when 6 V 3 W halogen bulbs are used instead of the usual 6 V 2.4 W types. However, the areas illuminated by the headlights are usually too narrow or too short, so that you steer into the unknown dark when cornering or you can no longer see a pothole that has suddenly appeared in the headlights directly in front of the front wheel.

This is why hobbyists came up with the idea 6 to 8 years ago that you can also connect two 6 V 3 W headlights to hub dynamos if you connect them in series (like the classic Christmas tree light chain). You need a bit more speed until both headlights shine brightly, but you have considerably more light and can align the light cone so that a wide and / or long light field is created.

LED headlights for operation on the (hub) dynamo are still quite new on the market. You have the advantage that the light source here lives almost forever - while the halogen bulbs often fail after 30–60 hours in high-speed drivers. In contrast to halogen lamps, LEDs are particularly efficient at low currents. This means that you have a lot of light from walking pace - a big advantage on steep mountains or on winding, narrow paths. However, the light yield hardly increases with increasing speed. Headlights with 6 V 3 W halogen lamps are brighter from 15 km / h on a 28 ″ wheel, and almost twice as bright at 30 km / h. Because the light source itself is larger and more "flat" than the filament of a halogen lamp projects Most LED headlights have a more blurred, softer light field. The light of "white" LEDs does not have a continuous color spectrum like a halogen lamp. Mostly it also has a blue or green tint - sometimes even pink.

Here the differences between various interesting hub dynamo double headlight combinations are to be examined more closely. The selection is limited to typical commercial models. Self-made LED headlights built with the best available components sometimes deliver considerably more light, but you are obviously leaving the strict framework of the German road traffic regulations and in the vast majority of cases dazzling oncoming people to a considerable extent.

Figure 1 shows how it works: A conventional lighting system with 6 V 2.4 W headlights and parallel rear light is supplemented by an additional headlight with 6 V 3 W halogen bulbs and, if necessary, a capacitor.

Figure 2 demonstrates the differences between different hub dynamos in their performance when operating one or two headlights. All measurements were made at 30 km / h based on a 28 ″ wheel with an effective outer diameter of 700 mm. The following cases are considered:

aA 12 Ω resistor as a load. The effective voltage (true RMS value) at the resistor is measured. This roughly corresponds to a 6 V 3 W halogen lamp - the usual case of a "normal" lighting system on the hub dynamo.
bTwo 12 Ω resistors connected in series. The voltage is measured on a resistor - corresponding to the voltage on a 6 V 3 W halogen lamp. This case corresponds to the solution of z. B. two Lumotec with 6 V 3 W lamp connected in series on the hub dynamo.
c, d, eTwo 12 Ω resistors connected in series and an additional non-polarized electrolytic capacitor connected in series: 220 µF, 330 µF or 470 µF. The voltage is measured on a resistor - corresponding to the voltage on a 6 V 3 W halogen lamp. The case with 330 µF corresponds to the solution of E6 and E6-Z connected in series on the hub dynamo, because the E6-Z has such a capacitor connected in series.

Result: A single 6 V 3 W headlight on a 28 ″ hub dynamo reaches 6.7–6.8 V at 30 km / h. The halogen bulb is thus operated very efficiently and has a reasonable service life. 6.8 V as the optimum / maximum is therefore drawn in as a line for orientation in the diagram. If you add a second 6 V 3 W headlight in series, the voltage on each of the two headlights drops to just 6.1–6.4 V. This means that significantly more light is achieved than with just one headlight at a higher voltage , but you don't double the light output.

With a capacitor in series with the two headlights, the voltage can be increased significantly. A capacitance value that is too low can even result in problematic high voltages on the headlights. 330–470 µF have proven themselves with 28 ″ hub dynamos. However, it must be ensured that only a 6 V 3 W halogen lamp is never operated together with a series capacitor, because voltages of well over 8 V occur and the incandescent lamp can be destroyed very quickly.

The SON20 operated in a 28 ″ wheel runs smoothly, but is too weak to operate two headlights in a row. At 30 km / h, there are only 5.3 V on both headlights and only make them glow yellow. Only with a capacitor with a low capacitance value of around 220 µF can enough power be drawn from the SON20 while driving quickly - but never reaches the power values ​​of the 28 ″ hub dynamos.

The most powerful hub dynamo for operating two headlights is the Shimano DH-3N71. When operated with a series capacitor, however, it is easy to get into dangerously high voltage ranges. However, the differences in the electrical properties of hub dynamos for "large" wheels are not world-shattering overall. Therefore, the behavior of the SON28 is considered in the following as an example.

A SON28 with a 28 ″ impeller is operated at speeds of 5–50 km / h. The resistors or capacitors mentioned under 1.) are connected in series. The voltage is measured at a 12 Ω resistor in each case.

Result: The use of a series capacitor is interesting to significantly increase the performance of a hub dynamo (up to 2 watts more) at speeds of more than 20 km / h. However, the capacitor circuit has the disadvantage of significantly reducing the power when driving slowly at less than 10 km / h compared to the case without a series capacitor. The lower the capacitance value, the more extreme its effect. 330 µF and 470 µF have proven to be favorable values.

Since non-polarized electrolytic capacitors are difficult to obtain, you can also make do with connecting two identical polarized electrolytic capacitors in series with opposite polarity. The resulting capacity of this combination is half its nominal value. Two capacitors with 1,000 µF / 16 V are a good starting point for experiments and are available from any electronics dealer for a few cents and will come very close to the values ​​shown in Figure 2 for 470 µF.

Various lights and their combination as »double headlights« are operated on a SON28 in a 28 ″ impeller at speeds of 10–30 km / h. The maximum illuminance (in lux) is measured on a wall 10 m away.

aan E6 with a 6 V 3 W halogen lamp
ban E6 and an E6-Z (with integrated 330 µF series capacitor) connected in series, each with 6 V 3 W halogen lamps, the light cones are arranged next to each other so that they clearly overlap
ctwo Lumotec connected in series, each with 6 V 3 W halogen lamps without a series capacitor, the cones of light are arranged next to each other so that they clearly overlap.
dan Inoled Inolight 10+ LED headlight
etwo Inoled Inolight 10+ LED headlights connected in parallel (!), light cone aimed at the same area
fa DLumotec Oval S + LED headlight
Ga DLumotec Oval S + LED headlight and an E6-Z connected in series, each with 6 V 3 W halogen lamps, the light cones are aimed at the same area

Result: The combination E6 plus E6-Z connected in series is by far the brightest at 20–30 km / h. At> 30 km / h the double Lumotec (without series capacitor) catches up. At under 20 km / h for the double Lumotec or under 15 km / h for the E6 plus E6-Z, it makes sense to switch back to just one halogen headlight to get more light.

Currently commercially available dynamo LED headlights are only brighter at less than 10 km / h than a good 6 V 3 W halogen headlight. At higher speeds, the light output of LED headlights does not increase at all (Inolight 10+) or only slightly (Oval S +).

The combination of Oval S + and E6-Z is interesting because it allows a lot of light when driving fast (primarily through the halogen headlights) but does not require manual switching back to just one headlight when driving slowly.

Due to their built-in switching regulator, the Inolight 10+ are not suitable for series connection. When driving faster, they consume less and less power - but the input voltage increases. However, it is possible to operate two Inolight 10+ in parallel and thus almost double the light output. In some speed ranges, however, there are fluctuations in brightness because the switching regulators are not designed for this use. The high light output is impressive even at <10 km / h and the bright parking light.

The headlights are operated on a SON28 at 30 km / h and shine on a wall 5 m away. In the case of double headlight combinations, the light cones are sensibly arranged, i.e. H. mostly next to each other with a slight offset to the side in order to get a little wider illumination.

All pictures are taken with the same manual camera settings. In order to draw the eye to the essentials, the recordings were converted into coarse black-and-white images using “Floyd-Steinberg dithering”. These photos only give a qualitative impression of the light distribution and allow a rough comparison of the width / length / shape of the light field.
The brightness cannot be compared with it. Image 5f in particular with two Inolight10 + looks very bright, although it is clearly inferior to E6 plus E6-Z when driving quickly.

With the Inolight 10+, which is already beautifully broadly illuminated, the light cones are exactly in the same position. With the combination of Oval S + and E6-Z, the Oval S + is aligned a little lower in order to illuminate the close range particularly well when driving slowly, while the E6-Z as high beam enables a wide view when driving at high speed.

Cyclists are unlikely to reject more light, at least when driving faster, unless they have financial or aesthetic concerns about buying or installing a second headlight. Hardly anyone wants to put more drive power into the hub dynamo - especially when they want to go fast. During the measurements for Figure 2, the drive power of the hub dynamo (at 30 km / h) was therefore also determined. All hub dynamos behave in a relatively similar manner to one another, so that the representation on the basis of one example is sufficient.

Electrical load1 × 12 Ω2 × 12 Ω2 × 12 Ω + 330 µF
Electrical power3.8 W.6.3 W7.8 W
Drive power7.6 W10.1 W12.0 W
Efficiency50 %62 %65 %

So if you want more light, you don't get anything for free, but for twice as much electrical power and thus twice as much light you need far less than twice as much mechanical drive power. The reason for this is that a considerable part of the hub dynamo losses is independent of how great the load resistance is. Losses at bearings and seals remain the same, but so do the losses due to the 2–3 Ω internal resistance of the dynamo coil.

Final words: what is the ultimate hub dynamo lighting system? Unfortunately, there is no clear answer to this, as driving speeds, route choices, traffic density and night vision differ greatly. The racing cyclist who trains on straight country roads on winter evenings will presumably prefer the E6 and E6-Z to be as bright as possible on a still quite narrow but long belt. The mountain biker on a winding forest path will enjoy the wide, soft light from two Inolight10 +, which deliver this even at a good walking pace and also illuminate the path usefully even when stopping. The everyday cyclist with occasionally fast stretches over the country might like the combination Oval S + and E6-Z, as it combines the convenience of automatic twilight and parking light with the long-range high beam. Individual fine-tuning of the light distribution is then still possible through the installation height of the headlights and their alignment with one another.

Double double

Up to September 2007, "Double lightning better" was up to date. At the autumn trade fairs, however, two new StVZO-pious LED headlights were presented that shine twice as brightly as usual: The Busch & Müller Lumotec IQ Fly and the Inoled Inolight 20+ model 2008. Both lights use the Cree XR-E 7090 LED that works significantly more efficiently than the Luxeon LEDs of the older generation of headlights. In addition, the light-emitting surface is small and the beam angle is narrow, which enables clean light bundling with a comparatively small reflector.

The new Inolight20 + (model 2008) hardly differs from the previously tested Inoled 10+. The main difference is the Cree LED instead of a Luxeon PW09. The light field has a similar shape with slightly sharper contours. Since only a single pre-production copy was available, double headlight tests were not possible. In order to safely rule out damage to the electronics when driving at high speed, the Inolight20 + was always operated with a Dtoplight + rear light, which makes it a little worse off when driving slowly than the IQ Fly without a rear light. A parallel connection will be possible with the Inolight 20+ as well as with the Inolight 10+. It has the advantage that the switching regulators as well as the LEDs work very efficiently up to high speeds and the somewhat short light field can be compensated for by a vertical angular offset.

The Lumotec IQ Fly shines a little narrower than the Inoled, but the main light field is longer and the near field is also well illuminated by diffuse light. The switching regulator in the IQ works as a current doubler, which operates the LED with up to 1200 mA when driving quickly without a rear light and thus outside the LED specification. A parallel connection is therefore also a good idea here in order to protect the only moderately cooled LED and to operate it more efficiently. However, a series connection is also possible. By using a series capacitor, both combinations can be used to achieve significantly more power at medium to high speeds. 330 µF is a favorable value in both cases. In parallel operation, there is a slight increase in the resonance of the increase in power at medium speed. When connected in series, you could draw even more power with a lower capacity when driving faster, but with the risk of operating the LED with more than 1200 mA. This should be avoided in any case. It is advisable to align the luminaires with a 4–6 ° horizontal offset. This creates a light field that is a little wider than that of the Inolights and yet has a clear overlap of the areas of maximum illuminance. Measurements were made here with a 4 ° horizontal spread.

The measurements for the above diagram were made with the same measurement setup as for Figure 4. For easier comparison, the values ​​for the halogen headlamp E6 and its combination with the E6-Z have also been recorded here.

The IQ parallel operation with 330 µF represents the optimum, which can be achieved with currently commercially available dynamo headlights at slow speeds of up to 20 km / h. Those who drive fast on rather straight country roads might like the combination IQ (possibly with parallel taillight) and halogen headlights E6-Z (with integrated series capacitor, halogen bulbs 6-V-3-W), whereby the IQ is more for the broadly illuminated Close range and the E6-Z are responsible for the distance. As in combination with the Oval S +, the E6-Z does not have to be switched off manually when driving slowly: the IQ does not get noticeably darker.

Conclusion: The new LED headlights represent a clear step forward and are already brighter than even the best halogen headlights in all driving speed ranges. However, the LEDs are very hot when driving fast and therefore operate inefficiently. Anyone who can or wants to afford it can implement impressively bright and efficient hub dynamo solutions with double headlight solutions.

To the author

Andreas Oehler (40) works as a mechanical engineer for bicycle lighting manufacturer Schmidt Maschinenbau. He heads the ADFC Technical Committee on a voluntary basis.

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