LED replacement bulbs in a B&M halogen dynamo light?
My wife's Breezer came with a Shimano dynohub and a Busch & Müller Lumotec Oval Plus headlight, which uses a halogen bulb with a standard flanged flashlight base as its main bulb.
I happen to have a couple of LED replacement bulbs around, which I've been using in 3xD-cell Maglites. Just the ordinary mass-market kind from Harbor Freight -- can't remember the make or model. Touted as working with up to 6 D cells.
She's not at all picky about beam shape -- she only rides on well-lit city streets, so this is a be-seen light. Is there any reason not to try the LED replacement bulb (e.g., a chance of burning out the B&M Toplight Plus taillight, or frying the capacitor for the LED standlight in the headlight, etc.)?
I have a Lumotec N plus, which I believe has the same guts as the Oval plus. I experimented with putting an LED flange bulb in it a few years ago and I can confirm that it works. I ran mine with a bridge rectifier to convert the hub's AC to DC. The LED bulbs have their own driver circuitry in them and I don't know how much it likes to be reverse biased (if at all!), so I think a bridge rectifier is needed. Without one you'll get light half the time when it's switched on, but the other half of the AC cycle, when the polarity is reversed, you won't get anything and there's a good chance you'll fry the bulb's driver.
I can't see any harm coming to a taillight connected to the headlight, especially if it is LED-based with its own standlight. It just takes a portion of the current. If the front bulb failed, I'm pretty sure (but not 100% sure!) that the taillight has a Zener diode or something to protect it from overvoltage.
The Lumotec plus's standlight powers a small LED, so there shouldn't be any issue with a LED replacement bulb interfering with that. It presumably has its own rectifier.
I was disappointed with the light output from the LED bulb, and switched back to halogen.
I doubt the bulb failed due to overheating. I've experimented with the LED bulb that brianinc-ville was using and its thermal management seems to be adequate - hits about 75°C and reaches steady state, and that's without the benefit of airflow while riding. Well below the typical maximum junction temperature of an LED. The bulb is also designed not to take more current than it can handle.
brianinc-ville, did you run it straight from the dynamo or did you rectify the output first? Un-rectified, I wouldn't be surprised that it got fried from being reverse biased half the time.
I forgot to mention that the main barrier to the upgrade would be the optics. Incandescent bulbs and LEDs need different optics and generally aren't compatible with each other's optics. A LED might throw out more total lumens, but it is not well focused with optics designed for incandescent bulbs.