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I have a 16V 8W bottle dynamo and two light bulbs:

  • one is marked 6V 0.05A and is working properly
  • other does not have any inscription and ceased to work.

I would like to replace the second light bulb. What should I search for?

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    That's a very unusual dynamo rating. Can you tell us more? I'd expect incandescent bulbs to be bright and short-lived on that dynamo; LEDs may do better depending on what the regulators can handle. 6V 3W is common and I'd quite like an 8W dynamo.
    – Chris H
    Mar 18 '19 at 16:43
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    Unfortunately, there are few rules you can use for this. A small dynamo like that produces a wide range of voltages and associated currents, depending on RPM and the bulb being powered. The dynamo may claim 16v, for instance, but never realistically get over 6 when "under load" (from the bulb) and at road speeds. Mar 18 '19 at 17:42
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    @ojs, yes, they're a decent approximation of current sources which suits incandescent bulbs quite well. That's why I wonder if the rating rather than the hardware is odd
    – Chris H
    Mar 18 '19 at 17:56
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    Bottle dynamos are 6V usually, not 16V. But then most simple bottle dynamos don't have a voltage regulated output. Depending on speed it varies between 3 and 8V. 8W output means that the current will be around 1.3A which means that the added up amperage of both bulbs should be below that value with the highest rated at the front.
    – Carel
    Mar 19 '19 at 8:49
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    I wonder whether the 16V reading is just a misreading of some vertical line before a 6V... Mar 19 '19 at 20:11
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Apparently the 1970s PX-60 (see the OP's comment had a Soubitez bottle dynamo ("génératrice") on the rear wheel as sold. Similar looking vintage Soubitez dynamos come up on eBay, where sellers and photos both indicate that they're 6V 3W.

However dynamos labelled 16V 8W really did exist: They were made by Sankyo, and again appear on eBay. They were apparently fitted to Raleighs and Schwinns in the 70s. If that's really what the OP has, and the output is really that much higher rather than just being an optimistic rating, I could be tempted to try something like a 12V 5-6W incandescent bulb in the front, and 12V 1W at the back, if you can somehow make them fit. These are cheap as they're used in cars, so getting a couple for testing would be reasonable, as would getting through them quicker than you'd really like.

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Is the dynamo single terminal or two terminal? (12V bottles usually have two: a 6V terminal for the taillight and a 12V terminal for the headlight and "6V 0.05A" sounds right for a taillight.- assuming a vintage dyno using frame earthing... A modern european dyno would use a discrete earth wire and 6v headlight)

I would suggest trying a 12V 0.4A bulb for the headlight. (If the headlight requires a bayonet-base bulb, it is possible to get an adapter that accepts PR-base flashlight bulbs and some cordless tool sets include a flashlight that uses 12V, 18V or 20V PR-base bulbs which is another option...)

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I just went to the chinese shop and randomly bought a cheap lamp that fits that socket. It says 3.6V 0.75A. It seems to work fine, but I still haven't tried it at very high speeds. It is not likely to last long - just as the other light I have. I'll try to turn off the lights when going very fast.

When it stops working I will look at this in order to choose the light bulb: https://en.wikipedia.org/wiki/Series_and_parallel_circuits

Apparently I should invest in 16V light bulbs, since that for parallel circuits the potential difference (Volts) is the same for all resistors.

As for the current, it should be calculated as such:

The current in each individual resistor is found by Ohm's law. Factoring out the voltage we get:

Itotal = V(1/R1 + 1/R2 + 1/Rn)
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    No, don't turn off the lights when you're going fast. That's when you need them the most, both for you to see where you're going and for people to see you. If you do not have working lights, do not ride your bike after dark. Mar 19 '19 at 15:53
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    Your formula is for a serial circuit with resistors it won't work with lamps. The correct formula for this circuit is P/U = I >> 8W / 6V = 1.3333 A. For a bike light, the lamps are in parallel, you can't put two lamps with different current ratings in a serial circuit. if your lamps are rated at 3.6V for different current running the dynamo at nominal speed will blow the lamp with the lowest current rating.
    – Carel
    Mar 19 '19 at 15:58
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    And remember that the resistance of an incandescent lamp varies widely with current. (This makes it "self regulating" to a degree, as resistance increases as current increases.) Mar 19 '19 at 16:45
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    6V 3W are the usual figures. 16V is probably a misreading as 8W is certainly 3W!
    – Carel
    Mar 20 '19 at 21:07
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    The bicycle is from the 70s, it's a peugeot randonneur PX-60
    – Pedro Rolo
    Mar 22 '19 at 15:34
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I have a 16V 8W bottle dynamo

This is a very extraordinary rating for a bottle dynamo. Most are rated 6V / 3W or 6V / 2.4W. However, do note that dynamos are constant current devices and to get 8 watts with 16 volts, it means you need 0.5 amperes. A current of 0.5 amperes is exactly the same as for a 6V / 3W dynamo.

So I suspect some lawyer wanted to rate the dynamo for the maximum voltage it can reasonably produce at normal cycling speeds, not for the voltage it produces if you connect a normal bulb to it.

So, the correct bulb is 6 volts, 3 watts, 0.5 amperes. (I think the 0.05 ampere bulb actually means 0.5 amperes and you misread it).

If you are using a halogen / incandescent rear light using the dynamo too, then it takes 0.1 amperes so you need 6 volts, 2.4 watts, 0.4 amperes for the front. If using a LED rear light, their current consumption is so low that 0.5 ampere bulb is the closest fit for the front.

However, all of this is only of academic interest today. The arrival of cheap LED lights means you're much better off purchasing a LED front light and connecting it to your dynamo. A bulb lasts only 100 hours. At 17 km/h, this is 1700 km. Less than a chain. A LED light has higher up-front cost, but it lasts practically forever and is much brighter than any halogen light can be.

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Start with some diagnostics.

Use a voltmeter to figure out what the output voltage is, and that's half your work done.

Put your multimeter across the lamp that is working and it should be within a volt or so of the real value. If you measure the voltage with no load attached it could read very wrong (thank you to OJS on that point)

That will also show you if the ~16 volt label is close to correct, or in error. This will also exclude some available options, hopefully leaving just one.

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    This really doesn't work, because once again, bike dynamos generally aren't voltage regulated but closer to constant current sources.
    – ojs
    Dec 26 '20 at 6:41

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