Cycling UK Forum

rjb wrote: ↑4 Jul 2022, 8:38pm Dynamos, hub and bottle are current generators so don't have the issues of overvolting LEDs unlike incandescent bulbs which fail.
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The reason that describing dynamos as current generators is not helpful is shown by the continuation of the sentence, which draws an entirely erroneous conclusion about overvolting.

The well known facts that you can run two filament headlamps in series, in which case they will achieve normal brightness at a higher speed, or that a filament tail lamp will blow very soon after a headlamp fails are both demonstrations that a higher load impedance causes more of the emf to be developed over the load, overvolting in both cases. As has been pointed out by others, LED lamps contain electronics to condition the voltage applied to the diode to a suitable range, about 3 V. Place suitable electronics around a filament bulb and it too will be protected against overvolting even if it is not the correct resistance. LEDs are unlikely to have the impedance required by the traditional dynamo circuit, and unlike filament bulbs, which are fairly cheap replaceable components, they are also expensive sealed units, hence the electronics.

The late filament age way of protecting filament bulbs was fitting the dynamos with zener clippers, but the dynamo instructions usually include a warning that the zeners will be damaged by prolonged running without a load.

Thévenin not withstanding, the simplest, most easily understood model of a dynamo is an electromotive force (emf) proportional to frequency in series with an inductor and resistor. The potential divider formed by placing the load resistor, or bulb, across the generator terminals divides the emf between the load and generator internal impedance. Because the inductance of the inductor is also proportional to frequency the generator internal impedance increases with frequency and so the proportion of the voltage across the load reduces as frequency increases, hence the phenomenon of tending to an asymptotic current.

So you're saying that, in a sense, a STVZO-compliant dynamo behaves almost like a constant current generator when the cyclist is up to speed (i.e, when the reactance of the dynamo coils dominates the overall impedance of the circuit formed by the dynamo and light) ?

wjhall wrote: ↑18 Jul 2022, 9:19amthat a filament tail lamp will blow very soon after a headlamp fails

It's a moot point whether the bulb fails because it's overvolted, or, being rated at 0.1 A, because it can't cope with the 0.5 A the dynamo is trying to push through it.

wrote: ↑18 Jul 2022, 11:49am So you're saying that, in a sense, a STVZO-compliant dynamo behaves almost like a constant current generator when the cyclist is up to speed (i.e, when the reactance of the dynamo coils dominates the overall impedance of the circuit formed by the dynamo and light) ?

The second half of your sentence is a neat summary of why the current into a resistive load has an asymptote as speed, hence frequency, increases, but since even with a Thévenin equivalent circuit using a frequency dependent current sources and admittances you will not find a constant current source in the model it is a moot point why the belief in a constant current source has become almost theological.

If you do use a current source & admittance model the you will find the frequency dependent current source pushing rather more than half an amp down the shunt susceptance in the generator model.

....my brain hurts.....

wjhall wrote: ↑27 Jul 2022, 12:54pm The second half of your sentence is a neat summary of why the current into a resistive load has an asymptote as speed, hence frequency, increases, but since even with a Thévenin equivalent circuit using a frequency dependent current sources and admittances you will not find a constant current source in the model it is a moot point why the belief in a constant current source has become almost theological.

You won't find a current source in a Thévenin equivalent circuit simply because by definition it doesn't have one; a Thévenin equivalent circuit consists only of a voltage source and an impedance in series with it. However we also have the Norton equivalent circuit, which itself consist of a current source and an impedance in parallel with it. The Thévenin-Norton equivalencies can both be used to represent the same network of linear voltage and current sources and impedances at a given frequency, and can also be equivalent to each other via well-known relationships. For example, a constant current source can be approximated by a sufficiently large voltage source with a large resistor in series, not unlike the STVZO dynamo under the conditions already discussed above.

If you do use a current source & admittance model the you will find the frequency dependent current source pushing rather more than half an amp down the shunt susceptance in the generator model.

As an example of this, page 6 of Magdowski's 'Modeling a hub dynamo' has measured the short-circuit current to be a maximum of 0.71A (maximum in the sense I can't pedal that fast for long). On page 13 Magdowski shows the maximum voltage across a 12 ohm resistor to be about 7.5V, equivalent to a maximum current of 0.625A. So not exactly 'constant current' with load resistance, but it's not a bad approximation, particulary as the short-circuit current would be very much different if the dynamo was behaving as a 'pure' voltage source. The same plots also shows the current levelling off quite nicely with speed.

I appreciate your point here has been that "describing dynamos as current generators is not helpful" but I'm still not sure I understand why you think that.

iandusud wrote: ↑30 Jun 2022, 10:09am We have just acquired a second tandem with a view to using it as our everyday bike and saving the "good one" for weekend rides and touring. Part of the thinking behind this is that our "good one" owes us getting on for £5k and as we don't have a car and cycle everywhere we often need to leave our tandem locked up outside and would rather have £600 worth of bike stolen should that happen. I love the dynamo lights our best tandem (we regularly ride on unlit paths where good lights are essential), and the new one has a bracket for a bottle dynamo. I don't want to go to the expense of replacing a very good wheel (Phil Wood hub) with a dynohub. So how about a bottle dynamo. What are peoples experiences of current offerings? Your thoughts greatly appreciated.

You could buy a dynamo hub and replace the hub on your wheel with it. I did it (on 2 bikes) a few years ago and rebuilding the wheel wasn't that difficult - getting started being the hardest part (I made a jig to hold the hub and rim in the right place relative to each other).

Arent the Shimano Nexus C3000* dynamos good enough for most now, and available in good value built-up wheels? The C6000 series seems newer and further improved?

*I have one on my tourer/all purpose bike.
Given that current manufacturers insist on routing cables in the axle in a trough under the bearings, thus making bearing adjustment/ lubrication unnecessarily tricky (if one wants to not break or weaken said cables).

I don't see the point in paying more until someone makes a better cable routing design... (like the sturmey gh6 dynohub arrangement, long out of patent...)

Dynamos are a dying breed, what with the rise of ebikes with lighting outputs and rechargeable lights with solar cells. In a few years they will be no more.

rjb wrote: ↑28 Jul 2022, 11:39am Dynamos are a dying breed, what with the rise of ebikes with lighting outputs and rechargeable lights with solar cells. In a few years they will be no more.

Did you take the pin out of that grenade?

rjb wrote: ↑28 Jul 2022, 11:39am Dynamos are a dying breed, what with the rise of ebikes with lighting outputs and rechargeable lights with solar cells. In a few years they will be no more.

E-bikes are for when I'm too old & knackered to pedal one, which isn't yet.

On a dull day in winter, my dynamo light is often on all day (as determined by the sensomatic switching).
Good luck doing that with a solar rechargeable light.

I was intrigued to see a bottle dynamo attached to a horse-drawn cart in a museum in Spain. It needed some gearing via two rubber-coated pulleys, to get traction from an iron rim with no tyre, and increase the rotation speed I suppose. (Rubber since perished.)

Strictly speaking, the iron rim is the tyre! But intriguing to see a dynamo there.

Our Raleigh Leeds has a hub dynamo so you can get home even if the battery has run out

jb wrote: ↑30 Jul 2022, 7:27pm Our Raleigh Leeds has a hub dynamo so you can get home even if the battery has run out

https://e-motionevc.co.uk/store/product ... id142.html

23kg of bike and a flat battery sounds like bad news to me, but maybe I misunderstand.