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. – Daniel R Hicks 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... – cmaster - reinstate monica Mar 19 '19 at 20:11

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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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. – David Richerby 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.) – Daniel R Hicks 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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