2.4W dynamo hubs

[Brucey]

apologies if this is obvious but the 3W dynamos are (nominally) intended to send 0.1A to a rear light and 0.4A to a front light. This is pretty much what happens with tungsten bulbs, too. If you are daft enough to run at speed with a front light only and you don't upgrade the bulb from 2.4W to 3W you will usually find that the 2.4W bulbs blow with monotonous regularity.

All well and good then, only almost no-one uses tungsten bulbs these days. LED dynamo lights vary; at the rear they consume between ~0.03A and ~0.1A. The latter occurs because the designers presumably wanted to make a rear light that conformed to 'the standard' and could be paired with a 2.4W tungsten bulb. (This arrangement was commonplace in the '90s and noughties, before LED front lights were so ubiquitous). The reality is that 0.03A, used properly, is more than enough to make a really bright rear light. Some of the models which use 0.1A make less light than those which use ~0.03A which presumably means they include a dummy load, just to make them compliant with the 0.1A Stvzo stipulation.

Now however the most popular dynamo front lights (as fitted to commuting machines) use LEDs. Even cheap ones are far, far brighter than anything with a tungsten bulb. You can buy powerful lights (with beams that throw 60, 80 or even 100lux out) which will use most of whatever power a 3W generator will produce. But most people on commuting bikes are happy with lights that produce 30 or 40 lux beams.

Now there are two points that are worth mentioning:

1) there is a marked lack of discussion about how much brighter your front light is, if you choose to use a rear light that draws only 0.03A vs one that draws 0.1A. The only mention of such differences you will usually find comes when someone wants to run multiple(e.g. three) rear lights, at which point you want three lights that each use 0.03A, else you might end up using over half your available generator power for the rear lights, much of that into dummy loads, presumably.

2) Most of the affordable LED front lights use LED components that have a power rating of ~1W. When they are attached to a 3W generator they reach full brightness at fairly low speeds. Unless the internal circuitry is terribly inefficient, you can usually run two such lights in parallel and get 'quite a bit more' light at typical cruising speeds.

The forward voltage across white LEDs varies with the current but a 1W LED might use (in very round numbers) 4 or 5 V and 0.25 or 0.2 Amps respectively. Because there is a rectifier of some kind in series with the LED, (and a voltage regulator if necessary) the generator will be producing around 6V as intended. But it still means that you can pretty much drive two front lamps in parallel, each with a 1W LED inside. [BTW because of the way generators work, if you use two lights with 3W LEDs inside in series, then you can get full brightness from both, i.e. ~6W power, but at a slightly higher speed than would usually be required for the lights to reach full brightness; one of the graphs on the Pilom website makes this very clear.]

So the bottom line is that unless you spend a fair amount on your front light, it won't make the slightest difference if you have a 2.4W or 3W generator; much of the power is wasted anyway because a typical front light cannot use it all. If you ride very slowly and want the lights to reach full brightness as early as possible, its worth using a rear light with a low (~0.03A) current draw regardless. If you use a 0.03A rear light and a 2.4W generator then the front light will reach full brightness at a speed that is some tiny fraction of a kph higher than if the rear light were not there.

If you have a typical commuting front light and want more light for cheap, you can simply fit a second (identical) front light. You can experiment with having the second front light in series or parallel; one way may suit you better than another. The only thing to be careful of is that the rear light is always connected in parallel with one front light only, not both in series, else it will see ~12V at speed, which it won't like at all.

[BTW the 'brake light' in some rear lights is (IMHO) of negligible benefit; most motorists view cyclists as virtually stationary objects anyway, so are not interested in whether you are slowing down or not. If the light can be (usefully) brighter (and/or flash), why not just make it brighter (and or flash) all the time?

cheers

[Brucey]

AFAIK StVzo just mentions "load", implicitly "resistive load" I guess, since related to a traditional or halogen filament lamp.

yes it is just a resistive load that is used in their test.

My point was that hub generators made to meet that standard in practice don't vary wildly in inductance. Lights can vary in the reactive nature of the load, but I think this is usually in relation to the capacitance rather than the inductance.

cheers

[andrew_s]

2) Most of the affordable LED front lights use LED components that have a power rating of ~1W.

Are you sure about that?
1 W is only around 300 mA.

When I was making DIY front LED lights, back before decent commercial front LED lights were available, the Cree LEDs I used were rated for 1 A continuous, given a reasonable heat sink. I didn't bother with anything fancy - just put 2 LEDs in series across the output of a bridge rectifier IC, and the hub across the input, figuring that the hub wouldn't give enough current to trouble the LEDs, which held good up to 35 mph.
I never got the housing and optics anything above adequate, so I gave up as soon as I could buy something better.

It's probably heat being the limiting factor in a cheap plastic front light that keeps the output down to 30-40 lux

[BTW the 'brake light' in some rear lights is (IMHO) of negligible benefit; most motorists view cyclists as virtually stationary objects anyway, so are not interested in whether you are slowing down or not. If the light can be (usefully) brighter (and/or flash), why not just make it brighter (and or flash) all the time?

I have on rare occasion found the brake light useful (B+M Toplight Line), as a following rider on a Belgian cycle track.
I always assumed that the extra brightness came out of the standlight capacitor, which would mean it's not available all of the time.

[Brucey]

I've built lights using 3W rated LEDs too; they tend to overheat (and/or expire) if they are given the full whack and are not mounted to a pretty decent heat sink. Powerful LED front lights have heatsinks etc which are in the same vein, but cheaper lights (typically with narrower beams up to about 30lux, claimed) do not. They have much feebler heat sinks, no way are they good enough for a 3W rated LED. Some lamps (which are plenty bright enough BTW) even say they have a 1W LED inside them.

If a 1W LED is of the usual efficiency (for bike lights) it can be around 150-200 lumens per watt, which translates to a reasonable (not massive) sized ~30 lumen 'splodge' on the road, which is what you get, so it all makes sense.

Anyway if you have a front light with a 1W rated LED inside it then unless it is spectacularly strangely/stupidly designed then, in essence, it won't matter if it is connected to a 2.4W or 3W generator or not.

I've converted several Pashleys (which are routinely fitted with a tungsten front light only connected to a 2.4W generator) with (1W) LED front lights and an LED rear light and the result is both a massive improvement over the original and more than good enough for commuting on.

cheers

[peterh11]

Anyway if you have a front light with a 1W rated LED inside it then unless it is spectacularly strangely/stupidly designed then, in essence, it won't matter if it is connected to a 2.4W or 3W generator or not.

I've converted several Pashleys (which are routinely fitted with a tungsten front light only connected to a 2.4W generator) with (1W) LED front lights and an LED rear light and the result is both a massive improvement over the original and more than good enough for commuting on.

cheers

Hi Brucey,

Sounds useful, thanks for mentioning that. What kinds of lights did you use for the conversions? B&M or a different brand? By default I’d use a B&M Cyo and a B&M Topline rear which I have already in my cupboard as spares.

I’m wondering specifically which type of LED B&M use on their mid-priced lights e.g Cyo, Cyo Premium, IQ-X. Are they all 1W LED? I would suspect so since the ones on my bikes reach their full brightness at fairly low speeds.

Is there a general source of information on which lights use how much power (e.g. which rear lights are the 0.03 and which the 0.1A)? Or did you find out by dismantling them or measuring the voltage and current in operation? (I did a quick search but didn’t find anything.)

I’m inclined to buy the bike and then change the dynamo if it turns out to be a problem, but if careful selection of a rear light can avoid issues, that sounds like a good option.

Peter H

[Brucey]

most of the lights you mention have LEDs that are probably more than 1W rated.

IME an IQ-X connected to a 3W generator will continue to brighten until more than 15mph and probably won't quite reach full brightness using a 2.4W generator. But that is a light that produces a fairly wide beam of about x3 brightness vs a typical light that would be used for commuting; it is one of the best lights you can get, overkill for typical commuting duties.

cheers

[peterh11]

most of the lights you mention have LEDs that are probably more than 1W rated.

IME an IQ-X connected to a 3W generator will continue to brighten until more than 15mph and probably won't quite reach full brightness using a 2.4W generator. But that is a light that produces a fairly wide beam of about x3 brightness vs a typical light that would be used for commuting; it is one of the best lights you can get, overkill for typical commuting duties.

cheers

Ah. Thanks for that. I think I am going to end up putting a 3W hub in then. I’ll test it out as is but budget for the upgrade and research to check that I can get one with the same flange as the 2.4W one which comes as standard on the bike, so as to avoid replacing the spokes as well.

My commute includes stretches of poorly lit and unlit shared use paths across parks in Cambridge. I have been using my Brompton as my daily ride while researching the replacement for the stolen utility/commuter. It has an AXA supposedly 30lux headlamp which is just not good enough to see my way among the potholes, unlit bikes and pedestrians in dark clothing. It is nowhere hear as good as the Cyo 40lux one on the old bike, and I am seriously considering putting my spare Cyo on, even if temporarily (it has a 3W Shimano dynamo hub but the wiring will be a bit of a faff to redo).

Yesterday evening I rode the same route on one of my touring bikes which has a Cyo Premium 80lux headlight - such a difference! I agree that the IQ-X (have that on the other tourer) and the Cyo Premium are fantastic lights for the money they cost (especially if you buy direct from German web traders).

Cheers
Peter H

[Brucey]

if you want to do the swap as quickly and easily as possible, you might just be able to swap the stator assy into the old hubshell and thus avoid having to rebuild the wheel. You ought to be able to do this provided you buy the same model hub dynamo in 3W form. So for example if you have DH-2N31 at present, DH-3N31 ought to provide the correct stator assy. There are however a few possible snags;

1) that you will need the octagonal spanner to release the stator assy from the hubshell
2) that on a loose hub the stator assy might be difficult to unscrew (less so than with a used hub IME)
3) that the magnets are different in a 2.4 W hub from a 3W hub. I don't believe this is the case but nowhere is it written one way or the other.

On the plus side the worst case is that you will end up having to build the wheel (like you would anyway).

cheers

[bgnukem]

Question for conRAD: The output table for the dynohub is very interesting, with peak power of around 10.4W into a resistance of 37 ohms at 40kph, far in excess of the usual 3W output rating of these hubs! Were the resistive loads in your table chosen to replicate LED front light set-ups?

I currently run a 3W hub (Shutter Precision) powering a front light only. I've had a couple of LED front lighting systems made for me by a friend, one based on two '3 watt' focusable head torches and the other based on two focusable aluminium-bodied LED torches of the type available from China via eBay, each with a '10 watt' LED in it. I like the twin light arrangements as I can focus one on the road surface and one further ahead for visibility to traffic, etc, though there is no beam cut-off.

I don't understand electronics but I think my friend deliberately used over-rated LEDs to avoid the possibility of burning out the lights on fast descents, etc, as the dyno-hubs produce more than the rated power at such speeds (as shown by conRAD's table). Despite potentially having '20 watts' of LED on the front of my commuter bike, I find the system gives continuous light from around 6mph and produces plenty of useable light at normal riding speeds (say 12 - 18mph) so it seems it is not necessary to restrict the rating of the LED to the hub rating (at least in wattage terms).

[ConRAD]

...were the resistive loads in your table chosen to replicate LED front light set-ups?

NO, the scope of my test was actually to demonstrate that when a dynamo is loaded with a high resistive load its output current varies significantly with the speed, i.e. it is not a “current generator” as generally regarded.
The values of "my" load resistances are coming from a calibrated resistor-patch box where I combine resistances to get the final value that I want.
So nothing to do with the LEDs.

[ConRAD]

QUESTION: what does it mean 2.4W for a headlight, how is it measured?
Just wondering how voltage, current and power are generally measured when the load is not a pure resistive load.
Talking of AC alternate currents common multimeters generally give RMS readings, more sophisticated multimeters give TRMS readings to properly keep into account harmonics. But when scrappy voltage and current are everything but a beautiful sinewave … how is measured power?

Picture 1 shows Voltage and Frequency at no-load conditions at 20 km/h on a 28" wheel
Picture 2 shows Voltage and Frequency on a 12 Ohm resistor at 20 km/h on a 28" wheel
Picture 3 shows Voltage and Frequency on a 40 Ohm resistor at 20 km/h on a 28" wheel
Picture 4 shows Voltage and Frequency on a non-linear load (in the specific picture below the load is an e-werk)

[Brucey]

there has to be something limiting the voltage in your open circuit tests; probably some kind of clipping diode, but possibly something else.

Keeping track of power is fairly simple as long as the load is resistive, and the current remains in phase with the voltage. However when they are not in phase with one another you can have a true power that is completely different from the (RMS) Volts x Amps value.

ps I have just looked inside a bottle dynamo and there is a diode of some kind across the output. I don't recognise the markings perfectly but they are as follows

- DB (run together as a single character, I think this may be a maker's mark)
- P 709
- P6KE8.2CA

I suspect this is a suppression diode of some kind, eg. containing a pair of Zener diodes back to back perhaps. Possibly presently available hub dynamos can contain the same kind of thing? (they didn't always; some generators were measured to produce up to 100V at speed into an open circuit). Note that when a diode like this conducts, it dissipates power and that effect will be seen as a reduced efficiency (power in vs power out) if all you can do is measure the terminal output.

[edit Farnell list P6KE8.2CA as a bidirectional TVS diode. datasheet http://www.farnell.com/datasheets/2861222.pdf]

cheers

[ConRAD]

there has to be something limiting the voltage in your open circuit tests

NO, there's nothing, only when I connect some resistive load the the waveform becomes a little bit smoother.

Generally hub dynamos are not voltage protected so, when not connected to anything or connected to a resistive load, the output waveform is a sort of an AC-alternating current, yet not sinusoidal.
Whenever connected to electronics such as LED lights (e.g. SON Edelux) or AC/DC converters (e.g. B&M e-werk) the wave becomes somehow chopped either because the presence of a back-to-back voltage protection zener (e.g. SON Edelux) or a rectifying diodes bridge (e.g. B&M e-werk).
In such a case the waveform is still an alternating periodic wave but its shape is so much spoiled that normal multimeters designed to calculate RMS starting from the formula RMS = 1.41 X Pick_value do not give at the end a sufficiently reliable value.
So my initial question was: how 2.4 W is measured on a 2.4 W headlight? (e.g. SON Edelux)?
Note that SON Edelux, like most of LED headlights, is zener voltage protected at its input.

Here below: back-to-back Zener voltage protection symbol

[peterh11]

if you want to do the swap as quickly and easily as possible, you might just be able to swap the stator assy into the old hubshell and thus avoid having to rebuild the wheel. You ought to be able to do this provided you buy the same model hub dynamo in 3W form. So for example if you have DH-2N31 at present, DH-3N31 ought to provide the correct stator assy. There are however a few possible snags;

1) that you will need the octagonal spanner to release the stator assy from the hubshell
2) that on a loose hub the stator assy might be difficult to unscrew (less so than with a used hub IME)
3) that the magnets are different in a 2.4 W hub from a 3W hub. I don't believe this is the case but nowhere is it written one way or the other.

On the plus side the worst case is that you will end up having to build the wheel (like you would anyway).

cheers

Thanks Brucey.

For the record and for the benefit of anyone else reading this, the bike is a Gazelle Chamonix, and the supplied hub is a DH-C6000-2R-N. This is one in a range of hubs with different power outputs, spoke counts and axle types. According to the Shimano web site, the DH-C6000-3R-NT has exactly the same dimensions and appears to use the same shell. I’ll delegate this to the bike shop, but we may be able to take the inner out of the 3W one and put into the wheel as is. As I am a cautious person, I will tell them if in doubt to rebuild the wheel (should be straightforward) and maybe someone would take the 2.4W one off my hands.

But first to get the bike, upgrade the lights and test it. Sadly I have to wait till January to get it on the Cycle 2 work scheme through my employer’s systems. So the job for this weekend is to put a better front light on my Brompton!

Cheers
Peter

[Brucey]

there has to be something limiting the voltage in your open circuit tests

NO, there's nothing, only when I connect some resistive load the the waveform becomes a little bit smoother.

well something is truncating the waveform in the open circuit test, the voltage form doesn't get to be that shape without there being some reason. If there were a protection diode, it would probably be buried inside the hub generator. I've never seen one but that isn't to say there is no such thing; for a long time shimano shipped their hub generators with an external voltage protection device, but they don't seem to bother any more, so maybe they have a component inside the hub.

cheers