Dynamo problem

[grufty]

I have a Shimano Alfine dynamo which has worked admirably for 4 years in conjunction with a Buechel front lamp. However, out on a run this morning, the front lamp stopped working, the capacitor for the standlight is charging up as the standlight lights. I thought it might be a switch problem, so having a spare front lamp I replaced that . The main would light for a while but then conks out leaving the standlight on. Anyone any ideas, is my dynamo bust?

[Brucey]

if you don't have anything much else to test it with, you can test the generator by putting a dead short across the output, and spinning the wheel. The wheel should slow quickly when driving either a good lamp or a dead short. This should help you discriminate between a bad generator and a bad lamp.

cheers

[andrew_s]

It's unlikely.
There's not much to go wrong with a dynohub, electrically speaking, so they either work, or don't at all.

I'd suspect the wiring (including the plug at the hub end).

[grufty]

Ok, I tried Brucey's shorting plan, and yes , the wheel does stop spinning fairly quickly. I've tried bother the original light and the new, out of the packet light again and they both give good light at v slow wheel rotation, but go dead on increasing the speed, the standlight coming on when rotation ceases. Could it be that once the capacitor is charged, things go pear shaped, perhaps the dynamo has fried the innards of the lights?
I have tried both lights with a different dynohub (DH3D37) with the same disappointing results, but that bikes front light ( same type as the others) works fine.

[Brucey]

one remaining thing that might make a difference is the wiring polarity; both your hubs may have a ground return connection in the hub, which will then make the headlight polarity-sensitive if it too has a ground return connection.

One test you can do is to lay the bike on its right side and to plug a different (loose) headlamp into it, i.e. with no ground return connection possible to the headlight. If the lamp does the same thing it is almost certain that the lamp is at fault somehow.

Another test is to simply reverse the connections in the plug to the hub, and see if that makes any difference or not.

cheers

[grufty]

Thanks Brucey, I tried reversing the connections with the same result, the lights worked at slow rotation for a while then stopped, the standlight came on when the wheel stopped. I've been testing the lights off the bike, so no frame connection.
This has all appeared out of the blue, so I suspect that a trauma of some sort in either the light or the dynamo has caused a problem in the other component(s). My knowledge of electrical stuff is very limited so I'm tempted to start from scratch with a new hub, any recommendations?
Many thanks for the help and responses

[Brucey]

All the indications are that the hub is not at fault. All you need to confirm this is a 'known good' headlight, even an old one with a tungsten bulb would do. Since cheap LED headlights are available for about ten quid (and might prove useful on a hack bike...?) one route ahead is to buy one of those and to test the hub more thoroughly. If the hub ever spins freely (like it does with no load connected) when there is a known load (or dead short) then the hub might be faulty, but you have not reported that fault. Do I take it that the lamps don't work with the other hub as well?

It may seem unlikely that two headlights (one unused) may suffer the same fault but this is more common than you might expect; electrolytic capacitors (for example) can fail and 'bad batches' of lights are not exactly unknown either. More regular use might explain why the third lamp (on the other bike) still works OK; one of the failure modes is that unused electrolytic capacitors tend to depolarise, and when they are brought back into use, they respond best to gentle treatment (look up 'reforming electrolytic capacitors').

A very simple test is to fit the alfine-hubbed wheel into the bike with the lamp that works (or just plug the lamp in with the bikes side by side) and to try that; even if you can't ride the bike that way (eg because of tyre or brake compatibility problems) simply spinning the wheel ought to tell you something.

As regards new parts, a lamp with a good warranty is a good idea. They come with up to five years warranty.

cheers

[2_i]

Another source of problems is that humidity works its way through any coatings that the lamp's PCB has, and starts shortening there. Isn't there a taillight there? You can connect any lamp to the dynohub to test it, while spinning the wheel by hand. You can test any lamp with a DC, battery or USB output - please no repeated discussion of that here. Dynohub's failure modes are not complicated, either noise and increased friction or broken wire - that is about it - so what you have is unlikely dynohub, repeating what was said before.

An experienced person might open the lamp, clean the PCB and bring the lamp back to operation. However that is not a route for you and, even if it were, it is not an economic route in terms of a pay per hour - a new lamp is cheaper.

[MikeF]

A review from Amazon.co.uk
"Poor quality - I have had this light fitted for around 5-weeks. At first I was impressed with the ease of fitting, simple operation and the brightness. Now however I have the problem that the light fails to operate when the wheel is turning, but the stand light comes on when the bike is stationary - useless and dangerous. Translate some of the reviews from other European users below and you’ll see this is not an uncommon problem and is thought to relate to a poor switch design. Unfortunately I am now outside of the returns window and will just have to buy a better light. Buy cheap, buy twice as they say...."

Looks like an issue others have had.

[Jdsk]

My knowledge of electrical stuff is very limited so I'm tempted to start from scratch with a new hub, any recommendations?

Do you want to buy a whole package: dynamo built into wheel, lamps, wiring? If so I recommend looking at the bundles from Spa Cycles:
https://www.spacycles.co.uk/m10b0s209p0/Wheels/Dynamo-Wheel-and-Light-Bundles

If that is of interest there is a useful discussion of their options in the archives.

Jonathan

PS: And my latest bundle is expected... today. : - )

[grufty]

I've just done as Brucey suggested and 'borrowed' Mrs Grufty's light as she has the same setup, all the signs look promising, just need a test run when the rain stops!
I assumed that because I was replacing the original dud light with a brand new one ......
You're correct with the buy cheap, buy twice advice, will be looking at B & M!
Thanks to all for the help and advice!

[fausto99]

... one of the failure modes is that unused electrolytic capacitors tend to depolarise, and when they are brought back into use, they respond best to gentle treatment (look up 'reforming electrolytic capacitors')...

As well as depolarising, electrolytic capacitors also dry out with age and temperature. This is generally irreversible and they have to be replaced.
Having said that, I did once resurrect an old electrolytic by very slowly injecting distilled water into the bulge on the top while applying a current limited reforming voltage, but that was an old vintage radio where the look of the old component was important and there was nothing to lose.

[MikeF]

My knowledge of electrical stuff is very limited so I'm tempted to start from scratch with a new hub, any recommendations?

Do you want to buy a whole package: dynamo built into wheel, lamps, wiring? If so I recommend looking at the bundles from Spa Cycles:
https://www.spacycles.co.uk/m10b0s209p0/Wheels/Dynamo-Wheel-and-Light-Bundles

If that is of interest there is a useful discussion of their options in the archives.

Jonathan

PS: And my latest bundle is expected... today. : - )

He needs a only replacement light! Why buy a new wheel and generator?

[Jdsk]

I couldn't make out was known to work and what wasn't. So I suggested that source if the whole set was what was wanted.

Jonathan

[ConRAD]

... is my dynamo bust? ...

... well, just in doubt do the following:

1. Wire one of your dynamo poles to the handlebar
2. Connect and keep in your hand the second wire
3. Start and keep your speed at approx 20 kmh
4. Touch repeatedly the handlebar with the second wire
5. If you hopefully note some sparks then it means your dynamo's working

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