Can I use a B+M S12 with two 6V LED lights in series?

Plus, is there a problem with using zener diode voltage limited dynamos, like the S6 and the S12 with LED lights?

You cannot just put two LEDs to a generator. You have to put some resistors in parallel to limit the current. You could put two resistors in series and then put a LED in parallel to each of the resistors.

I guess what I meant was that I should be able to put two Lumotec Cyos in series with a 12v dynamo because the current levels should be the same as that coming out of a 6V regulated dynamo, would it not?

While you can put two bulbs in series, and they should share voltage well, I'm not sure on these LED lights. I want to say "yes" but without knowing what is inside them, I don't know. Odds are, they just have a ballast resistor, in which case you should be fine. Actually, I think you'll be fine regardless, but I'd hate to myself wrong. I take it you have a mis-mash of parts, and are trying to get something to work?

No mishmash of parts. Just want to upgrade the lighting system on my bike. Riding in the city with lots of fast cars (many expressways at 50 mpg and the like). I was debating about putting a B&M 12s dynamo or a 6s dynamo on my bike. The 12s is more efficient and generates more power, but it is not generally sold in conjunction with the LED 6V lights. The 6s is also not guaranteed to work with the new LED lights as well.

Although a LED is more efficient at creating light than a filament bulb, a LED has a much narrower view angle so it may be necessary to change the optics to match the LED light output.

If you're buying all new stuff, I'd just buy it to match. Also, I'm not sure if you'll be able to outshine cars; what you might want instead is a decent hub setup and then some sort of high-brightness battery operated light. That way you'd have a backup on you bike, for night time riding (in case of breakage). 3W isn't much to light with.

My understanding is that the regulated dynamos were designed to keep from blowing incandescent bulbs at high speeds.

Some LED headlights such as the Supernova ones are designed to take advantage of the higher voltage of an unregulated dynamo to use the extra power output for more light. A regulated dynamo such as the S6 will not allow them to reach full brightness per my understanding. Ask B&M tech support about your proposed hookup as both the S12 dynamo and IQ Cyo are their products and they should know of any technical problems with what you propose.

The S12 costs as much or more than a dynamo hub wheel and I would personally go with the hub dynamo.

Don't forget to rectify the current and put a capacitor across the LEDs to smooth out the fluctuations in voltage. From what I've read, a dynamo is constant-current source, so you shouldn't need to worry about exceeding their max rated forward voltage. We're talking about power LEDs, right? A standard 6V/3W dynamo should deliver 500-600mA which is fine for most power LEDs that I'm aware of, but check your specs.

This website has loads of information on circuits for bike lights. If it starts to get a bit complicated, I recon circuit 6 is fine. Beyond circuit 6 is a slippery, headache-ridden slope to 12 and beyond! I'm working on something right now that will include standlights for when I have to stop.

i don't think so as the led electronics are designed for 6 volts. i'm surprised they don't make led systems for 12 volt dynamos, but then again 12 volt dynamos ain't really that common nor needed with the new 6 volt led systems. the new higher power led lights are making thje old halogen bulb systems obsolete. the s12 halogen system would com close to the brightness as my 6 volt b&m lumotec iq fly led front light. it lights up a large patch of road & i only need 1 light 2 would be overkill. for what that 1 s12 dynamo costs you can sell it & buy yourself a 6 volt led system...