Timeline for Is there a benefit to turning off dynamo-powered lights?
Current License: CC BY-SA 4.0
40 events
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| Oct 28, 2022 at 4:19 | comment | added | cmaster - reinstate monica | @leftaroundabout Couldn't have said it better :-) | |
| Oct 27, 2022 at 19:09 | comment | added | ojs | @leftaroundabout thanks for the explanation | |
| Oct 27, 2022 at 18:51 | comment | added | leftaroundabout | ...used this way like any other generator/motor, which is what's done in most applications because this is the most efficient working domain. What @cmaster-reinstatemonica is referring to is that the single-coil design of the dynamo has a higher self-inductance than multi-coil ones, i.e. at high speed & high current the wire's own field counteracts the ∂𝛷/∂𝑡 from the magnet-rotation. For most motors/generators this is a negative effect; for the old light bulb use it happened to be useful to prevent overvoltage, but that doesn't mean a dynamo must or should always be operated in that domain. | |
| Oct 27, 2022 at 18:48 | comment | added | leftaroundabout | @ojs I'm not saying they're supposed to be operated at specifically 190 mA either. You're both way overinterpreting what a dynamo does. It's just a magnet and a coil with multiple poles. It behaves, in essence, no different from any other generator/brushless-motor: voltage is proportional to ∂/∂𝑡 of the magnetic flux minus 𝐼·𝑅 losses through wire resistance 𝑅. At sufficiently low current, the magnetic field of the wire is negligible so all the ∂𝛷/∂𝑡 comes from movement of the magnet, and the 𝐼·𝑅 is also negligble. So that's a const-voltage source, and thus can be... | |
| Oct 27, 2022 at 17:52 | comment | added | ojs | @cmaster-reinstatemonica I know that. I'm looking for a source for the idea that dynamos are supposed to operated or most efficient at 190 mA. | |
| Oct 27, 2022 at 17:30 | comment | added | cmaster - reinstate monica | @ojs The design goal of bike dynamos was to power a 3W light bulb without voltage regulation across a large range of bike speeds. You didn't want your bulb to blow out just because you were descending at 50km/h+, but you also wanted full "brightness" from 20km/h+. This lead to the current design of dynamos, where you have a rather complex anchor that alternates the magnetic flow through a fixed coil. A design that is specifically not designed to guarantee that the magnetic field lines actually cross through the copper windings, producing power. This design persists even in hub dynamos. | |
| Oct 27, 2022 at 15:35 | comment | added | ojs | @leftaroundabout do you have any sources for the "supposed to be operated at"? To me the whole thing still looks like it's a designed around the now obsolete idea that the dynamo should be able to power a 3 watt bulb without voltage regulation and it can do anything when the load is radically different. | |
| Oct 27, 2022 at 15:22 | comment | added | leftaroundabout | @ojs the “specified current” really just tells what current the dynamo can reliably provide without risk of overheating. It's not the current that the dynamo is supposed to be operated at, nor the current at which it provides the most power (that happens in the middle at the transition between constant-voltage and constant-current domains), and definitely not the current where it's most efficient (on the contrary, the constant-current domain is when most of the energy is wasted to internal resistance). | |
| Oct 27, 2022 at 14:51 | comment | added | ojs | @leftaroundabout interesting, but to me it looks like they measured one hub dynamo that has produces voltage that is roughly linear to speed when connected to current sink that is set up to take a specific current that is less than half of the specified. I agree that it is not constant-current with load that is that far out of spec, but I wouldn't call it exactly constant voltage. I also don't see why the results are generalisable, when the bottle dynamo they measured is clearly different, or why the generator would be efficient when it's run far outside spec. | |
| Oct 27, 2022 at 14:06 | comment | added | leftaroundabout | @ojs how they actually work. The source I used: pilom.com/BicycleElectronics/HubDynamo.htm, admittedly rather amateurish but don't see a reason to doubt the correctness. The constant-voltage region is the linearly rising part on the left of the 𝑃 vs 𝐼 diagram, i.e. up to 150 mA for the Shimano DH; the voltage looks like 25 V at 15 km/h, or 60 V at 40 km/h. The constant-current region is the dropping part on the right of the diagram, starting at about 220 mA. | |
| Oct 27, 2022 at 13:43 | comment | added | ojs | @leftaroundabout I'm sorry that I have to ask about this, but are you writing about how you think dynamos should work without any connection to reality, or how they actually work? And just for reality check, what is this constant voltage that you keep mentioning in volts? | |
| Oct 27, 2022 at 13:35 | comment | added | leftaroundabout | @ojs they're certainly not constant-voltage with a load as low as 2 Ω, but that's irrelevant. They are constant-voltage when the current is kept low enough, that's what matters. A reasonable equivalent resistance for a dynamo light is something like 200 Ω (but of course the regulating circuit will mean this constantly varies). | |
| Oct 27, 2022 at 13:28 | comment | added | ojs | @leftaroundabout please come back when you understand that bike dynamos are not constant-voltage sources even though they look like that when connected to 2 ohm resistive load. | |
| Oct 27, 2022 at 13:20 | comment | added | leftaroundabout | @ojs the dynamo is not “pushing extra current”. In the efficient operation domain, it acts as a constant-voltage source; it's not a problem at all if the currents are arbitrarily small. On the contrary, what's a problem is when the current is too big such that the voltage drops and you get the constant-current behaviour you describe. Nightrider is exactly right that these lights are just braindead designs; I too am surprised that they seem to be still common. (Moreso since the last dynamo light I had, about 10 years ago, did have proper regulating circuitry.) | |
| Oct 27, 2022 at 12:47 | comment | added | ojs | I have measured 50V from just spinning an hub by hand, so I wouldn't be surprised if fast riding would go over 72V. The other question is, what are you going to do with the extra current that the dynamo is pushing when the converter takes 0.2A or less. | |
| Oct 27, 2022 at 11:43 | comment | added | nightrider | The converter like distrelec.ch/en/dc-dc-converter-72v-5v-fixed-recom-78he5/p/… can take from 6.5 till 72 V and output 5 V, up to 1.5W. Its efficiency is 83 %. Larger voltages are getting dangerous already. | |
| Oct 27, 2022 at 11:29 | comment | added | ojs | Can you link to this power converter, and maybe explain how it works with bike dynamos? Dynamos are weird. | |
| Oct 27, 2022 at 10:48 | history | edited | nightrider | CC BY-SA 4.0 |
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| Oct 27, 2022 at 10:44 | comment | added | nightrider | Yes. I was designing and building my own electronic device that required the power converter, and since then I know that such converters exist, what they can do and how much do they cost. I am surprised that this technology is not used with these lights. | |
| Oct 27, 2022 at 10:31 | comment | added | ojs | Do you know anything about electronics, really? | |
| Oct 27, 2022 at 10:02 | history | edited | nightrider | CC BY-SA 4.0 |
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| Oct 27, 2022 at 9:53 | comment | added | nightrider | Looks like even high end dynamo lights are not using the modern power converters and draw about the same power as the older halogen lights, relying on the stabilizers of 1970s design for mitigating the wide voltage range of dynamo. Shame, they probably do not see 10 W as worth considering. I have adjusted the answer. May be a good idea for a startup. | |
| Oct 27, 2022 at 9:48 | history | edited | nightrider | CC BY-SA 4.0 |
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| Oct 27, 2022 at 8:58 | comment | added | ojs | "Reality is wrong, my opinion is how things should be" is an easy but not very productive way to approach problems. | |
| Oct 27, 2022 at 8:08 | comment | added | gerrit | This seems to be an answer based on what could be achieved in theory, rather than what is achieved in practice. | |
| Oct 27, 2022 at 7:58 | history | edited | nightrider | CC BY-SA 4.0 |
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| Oct 27, 2022 at 7:45 | history | edited | nightrider | CC BY-SA 4.0 |
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| Oct 27, 2022 at 7:44 | comment | added | leftaroundabout | @cmaster-reinstatemonica yeah, but again – there's no good reason nowadays to have lights, whether front or rear, incapable of surviving much higher voltage than the rated one. Of course the manufacturer can save a couple of cents by omitting this circuitry, and many will do it (perhaps reasoning that if one light goes then the users may anyways just replace both), but that doesn't change the fact that it's wrong. | |
| Oct 27, 2022 at 7:39 | history | edited | nightrider | CC BY-SA 4.0 |
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| Oct 27, 2022 at 7:13 | comment | added | cmaster - reinstate monica | @leftaroundabout You are forgetting that there may also be a rear light connected to the same dynamo. The rear light never drew as much power as the front light, and simply relies on the presence of a front light in order to not get toasted. I have some first hand experience with that where a failed front light caused the rear light to get toasted. | |
| Oct 26, 2022 at 20:54 | comment | added | leftaroundabout | @cmaster-reinstatemonica no, the light would need to be designed to be capable of operating with variable voltage, which it should be. (Yes, many aren't, but that's just bad design.) | |
| Oct 26, 2022 at 20:11 | comment | added | cmaster - reinstate monica | Also, even if an LED light only needs 0.2W of power, the light would still need to draw at least 3W from the dynamo. This is necessary to avoid the voltage skyrocketing and frying the LED: The dynamo is designed to deliver the correct voltage across a large range of bicycle speeds under the assumption that it can feed 0.5A into the system. Take away the current drain, and you get voltages that hurt. | |
| Oct 26, 2022 at 20:03 | comment | added | cmaster - reinstate monica | Please take a look at calofr's answer: bicycles.stackexchange.com/a/86359/36555 | |
| Oct 26, 2022 at 16:41 | comment | added | ojs | You conveniently ignored the part that says 1.2W is tolerable "for riding at a moderate pace once the eyes were dark-adapted". If you got the 6W from where I think, you should probably be aware that the light is unusually powerful and the standard is 2.4 or 3 W. | |
| Oct 26, 2022 at 15:46 | history | edited | nightrider | CC BY-SA 4.0 |
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| Oct 26, 2022 at 15:45 | comment | added | nightrider | Added the source that specifically references bicycle lights. 1.2 to 2.4 W so mostly that I said. | |
| Oct 26, 2022 at 15:32 | comment | added | ojs | Those 4.5 volt torches were also incredibly dim compared to even non-halogen dynamo lights. 1.125W LED would be unnoticeable during day, and maybe just barely enough for riding if there aren't other light sources around and you have enough time to adapt to dark. | |
| Oct 26, 2022 at 14:52 | history | edited | nightrider | CC BY-SA 4.0 |
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| Oct 26, 2022 at 14:47 | history | edited | nightrider | CC BY-SA 4.0 |
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| Oct 26, 2022 at 12:45 | history | answered | nightrider | CC BY-SA 4.0 |