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I am re-wiring my bike lights to add a smartphone charger and improve the connections. Wires are going in pairs, one wire in a pair being marked with a white stripe. Since I am using LED lights, polarity is important. On electronics SE and in many places on the Internet I found that the white-marked wire should go to the plus. However, my hub dynamo has the marked wire connected to the contact marked with the ground symbol (so it is connected to the frame). Usually in electronics the ground is negative. Is it a mistake of the bike manufacturer or am I missing something?

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You might be able to get a better answer on Electronics SE. That community is almost certainly going to be more knowledgeable about these kinds of oddities. – jimirings Jul 20 '13 at 17:30
There is no real "standard" as to which wire is considered "hot" and which "ground/neutral" for low-voltage wiring. And there is no assurance that "hot" is + and "ground/neutral" is -. For house wiring and power cords the white wire or the wire with the rib is "neutral". If you want to know the polarity you need to get a voltmeter to check it. – Daniel R Hicks Jul 20 '13 at 17:42
This question appears to be off-topic because it is about electronics more than bicycles or riding. – Neil Fein Jul 21 '13 at 16:56
Hint: If you're in the US go to Radio Shack and you'll likely find a small multimeter for about $10. Go to Harbor Freight and you can probably find one for $5. – Daniel R Hicks Jul 22 '13 at 21:33
As you might have noticed, I have referenced a similar question from electronics SE. The reason for this question here is that on my bike the situation was different from the answer at ESE. So I though the convention could be different on bicycles. That's why I find this question fits here. – texnic Jul 23 '13 at 9:31
up vote 2 down vote accepted

No mistake - there is no standard. 50% of the time the strip is -ve, 50% positive.

Get a multimeter - $10 for a cheapy, and remove any guess work.........

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Thanks to @ratchet I have realized this answer is actually wrong.... 33% its positive, 33% its negative and 33 its AC (both)..... – mattnz Jul 22 '13 at 1:28
You forget that at least 20% aren't working at all. – Daniel R Hicks Jul 22 '13 at 21:33
OK, thanks guys. I have a multimeter, but it does not matter: my dynamo produces AC current. Why I wanted to have it "the standard way" is that if at some point in the future I bring it to the workshop or just forget what I find out today, I want to avoid complications. In the end, that's why we have standards :) – texnic Jul 23 '13 at 9:29
But AC is "50% of the time ... is -ve, 50% positive" :-) (Not actually true if the striped wire is connected to an earth.) – armb Jul 23 '13 at 14:08
@texnic: The thing I love about standards is there are so many to choose from...... – mattnz Jul 23 '13 at 23:02

cheap dynamos create AC (which also works for LEDs) you can test that easily by reversing the polarity of the LEDs and see if they light up in both orientations

just add a diode (or diode bridge) to the connection for the charger to go from AC to DC

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The first part is great, but adding diodes to the circuit is a silly idea. You're introducing more points of failure and extra voltage drops. Much better to experiment and go with what works. – Móż Jul 21 '13 at 22:35
reversing polarity of the charging input of a phone can fry the battery, you don't want that in a smartphone, if it is AC then it needs a AC to DC converter – ratchet freak Jul 21 '13 at 22:53
if the charger you're using doesn't have reverse polarity protection on the input then swapping the leads to see which way works is going to fry it. If someone is going to open up the charging leads of their phone or build their own charging circuit that's definitely an electronics.SE question. This question is currently about using a commercial charger designed to run off a bike dyno, so the only question is the input to the charger. – Móż Jul 22 '13 at 0:19
Approximately all bicycle dynamos produce AC, not just cheap ones, with very few exceptions, and any rectification being inside lights or attached devices. But some have one terminal electrically connected to the frame, and some don't, and so do some lights, so in some cases it matters which way round the wires are connected and others it doesn't. (In some cases there is only one wire, and the system relies on the frame connections for the return. That often causes problems, especially if it relies on the greasy ball bearings in the headset to provide connectivity from frame to fork.) – armb Jul 23 '13 at 14:17

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