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I have a question about LEDs on 6V Shimano dynamo hubs. I built up an old Schwinn with a Nexus dynamo hub, and the headlight I'm using is based on an LED 6V headlamp. The LED is a Chip-on-board design, and I've wired it directly to the two terminals on the hub without using the frame as a ground. I have the pulses set up to alternate between the headlight and the tail lights. The light works great, but I wasn't aware that the Shimano dynamo is putting out reverse-polarity pulses as well as the ones properly oriented to light the LED. Is this bad for the LED bulb? Will it burn out prematurely because of the reverse pulses?

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    It's a little iffy. The dynamo is likely designed to drive lot more current than the LED uses, and the voltage regulation would be relatively poor. – Daniel R Hicks Aug 30 at 21:56
  • I'm reminded that there are AC dynamos and DC ones (perhaps more properly called "generators"). Probably AC is predominant, and generally unsuited for powering a "bare" LED lamp. A reasonably astute electronics hobbyist could easily rig an adapter, however (and there may be some for sale on the Interweb). – Daniel R Hicks Aug 31 at 0:21
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    @DanielRHicks essentially all bike dynamos are AC, though the word "dynamo" refers to a DC source in many contexts. I haven't found a complete adaptor except expensive 5V regulators with a USB socket, but the design is fairly simple: bridge rectifier - capacitor - voltage regulator (if the LED comes with a current regulating driver) or current regulator (for a bare LED). I'm working on something similar to allow a light that also works off a 5V battery pack. Downside is the lack of a standlight without more complex electronics – Chris H Aug 31 at 9:52
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Reverse pulses at low voltage probably won't damage the LED, but the COB (chip on board) that gives the flashing modes might not be able to cope with the full reverse voltage.

The problem is that the hub dynamo is more accurately a constant current source, and will supply about 500mA. One common trick is to run two 6V lights in series off a hub dynamo, and most dynamos will give full brightness from both... they put out 12V. The actual voltage you get out of it might be clamped by a zener, but it's unlikely to be a 5W zener that can take the full power of the hub forever. Not that you want that anyway, it's extra drag.

I have run bare 5W star LEDs straight off a SON hub dynamo (well, with a diode and capacitor to reduce flickering) because they're designed to take 500mA and that's what the dynamo supplies. It worked really well, just using a chunk of aluminium channel as a heat sink/weather sheild/reflector. I don't really know about longevity, after ~10,000km/a year or so I got a proper light cheap so switched to that.

So if your 6V LED is actually a 1W light it might be getting 12V or more. Whether it survives that is anyone's guess. Worse, even a 5W light is likely to be getting +6V on that half of the wave, then -12V on the other half when it's not drawing any power. I suggest a basic diode or bridge rectifier in series, capacitor in parallel to give you DC.

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  • I suspect the LED itself will be fine, protected by the driver, but the drive circuit won't last long – Chris H Aug 31 at 9:45
  • +1 for pointing out the dynamo is a source of current rather than voltage - so the voltage can rise – cbeleites unhappy with SX Aug 31 at 13:53
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LEDs don't like negative voltages. They generally fry at some 3x-5x their normal forward voltage.

Then again, you don't have a bare LED, but a LED and a LED controller and it is the controller that is likely to suffer first. The bad thing is, we don't know much about it.

Another bad thing is that the 6v hub is actually 6v when properly loaded w/ 6V, 3W bulb. Otherwise it can go very high, like 50V.

You'll get prettu much extra complexity if you try to do the proper thing.

You can as well opt for a simple & stupid solution like hub => bridge rectifier => 1.5W white led (bare, without controller). It will behave almost like a bulb, except for the brighter light and about half the drag.

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6v is probably low enough to not damage the LED, but you might be vulnerable to a voltage spike. You could use a regular diode in series to help protect the headlight.

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    I don't see how a diode in series is going to help. – Daniel R Hicks Aug 31 at 0:01
  • @DanielRHicks the forward voltage drop of the diode lowers the voltage seen by the light, and the diode also prevents the reverse polarity half of the wave from being seen by the light. The latter is probably more important. But more information is needed in the answer to really know. – Móż Aug 31 at 0:15
  • Try measuring actual output from a hub dynamo. It's nowhere even close to nice 6v. – ojs Aug 31 at 0:35
  • Using a diode in series will split the amount of reverse voltage that the headlight receives, which seems to be what the poster is concerned about. Depending on the parameters of the diode there should be minimal impact on the brightness. – BetterSense Aug 31 at 0:43
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    The poster is concerned about running an LED light on AC output. As other posters have also pointed out, a diode in series forms something like a half-wave rectifier, except an LED is not a resistive load. With a resistive load, all the voltage would drop across the rectifier diode. With a diode load, the voltage drop will be split between the rectifier diode and the load diode according to their respective V_f specs, but reverse voltage load on the headlight will be still reduced. Forward current will be almost unaffected by the rectifier diode. – BetterSense Aug 31 at 2:50
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I think the best solution is to get a light built for that particular hub. That way I can be sure that it's engineered to work properly with that system. It's a shame--that light looks so good with this bike. the bike.

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  • You could probably find a way to mount the electronics of a simple dynamo light inside the existing light housing to get the best of both. – Emil Aug 31 at 8:30

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