IIRC, that was a somewhat recommended practice.
To some degree, it was to protect the taillight bulb when the headlight bulb blew out. The headlight bulbs were only rated for 100 hours life, so it wasn't as if you expected it to last years and years.

I've got a little Jos dynamoblock light that is the only sidewall dynamo I've used, and I've never used a tail light with it...



.... but now I'm wondering how well the bottle dynamos regulated their current, at least in comparison to hub dynamos. Their inductance must be a lot less, but their AC frequency is quite a bit higher. Maybe their current regulation is similar to hub dynamos??

Steve in Peoria

I would have sworn that you said this:

What I do know, however, is that LED bulbs (Light Emitting Diodes) must have DC in order to work. .......
So if the generator hub is producing AC then there is a bridge rectifier somewhere, either in the hub itself, or in the housing of the light, but you cannot present an LED light bulb with AC, it simply won't work. ......

If I'm mistaken,, then I apologize.

Steve in Peoria

Thanks a lot for that, [MENTION=113466]steelbikeguy[/MENTION]. I don't follow all of it, but it helps. So the lights have various different designs, so we can't generalize how they would act when treating them not as intended.

I have a sidewall dynamo and ran a voltmeter across it with no load. Just on the repair stand where I probably didn't make a lot of speed, it went to 35V.

And thanks, [MENTION=197614]fietsbob[/MENTION] for the e-bike light reference. I think I'll toss out this whole idea. There are perfectly fine battery lights, and there are more and more all the time, and they're getting better, too. I could figure out how to bolt them on well if I really need to.

It doesn't! There is a Busch & Müller tail light (Busch & Müller) that monitors the frequency of the AC from the dynohub. When it detects a sudden decrease in the frequency, it increases the brightness of the light... a brake light without a switch or any other connection to the brakes themselves(!).

LEDs, being diodes themselves, can rectify an alternating current. They just don't produce light when they are "reverse-biased". Unless the frequency of the AC is very low, the intermittent light emission is not detectable by the human eye.
But that's not the reason most lights designed to operate off of a dynohub won't work properly on DC. If the light has different levels of brightness, there is a more complicated circuit that provides pulses of DC to the LED array. See "Pulse Width Modulation". That circuitry is designed to work off of AC in most lights intended for use with dynohubs. Many of these lights are also made in a slightly different format to operate on DC (for e-bikes) as noted earlier.

DC may be considered a zero-frequency limit of AC. I have been testing any possible dynamo light I got my hands on with a DC power supply set to 6V and I never had any problems aside from occasional quirks tied to polarity. The set did not include Supernova but I doubt an e-bike's circuitry runs on 6V.

hmm

An acquaintance of mine told me he hooked up a 5V battery to his dynamo lights, and it worked fine.