I’m working on an old bike with an Enwell bottle dynamo. The bike is a 67 and I expect the dynamo is of the same vintage. However I am not getting the lights to work. What voltage should I expect from something like this? I’m getting 1.5 volts AC. Continuity is good from dynamo to bulb so I wanted to confirm expected voltage before condemning the dynamo.

I believe that the bulbs are rated at 6v, 2.4 watt headlamp if taillight used, taillight at 0.6 watts. But if no taillight, then 6v, 3 watt.

I know that I I did not answer your question, but I have no clue how much resistance you have in the circuit, if you have a higher wattage bulb then the voltage might not be very high. Look at your bulb(s) to see what they are rated at.

If you go fast down a hill you can blow out incandescent bulbs. I used to run zener diodes across it so that if voltage went above about 8 volts, the excess power would be drained off through the diodes.

It really depends on the load. If it's a dead short, zero volts, if it's open circuit, 100 volts.
If it's the correct size light bulb then I would expect around 6v.

Some dynamos have been low power, such as the Sturmey-Archer dyno-hub. It was only rated at 1.5 watts, IIRC. I think it was still intended to be 6VAC when connected to the intended bulbs.
The output voltage is related to both the speed and the resistance of the load. If you are going 10mph or more, I'd expect to see over 6VAC if there is no load connected. If the wrong bulb is installed, that might be pulling the voltage down (as mentioned earlier).
Or you might just be having trouble with a heavy layer of oxide on the dynamo connections? Some (well, a lot) of the old dynamos relied on the bike frame as the return path for the current, and this can easily be a very high resistance connection. Of course, some also use a little spring clip that could have a lot of resistance too...

How much current do you get from the dynamo when it is running at 10mph or so??

Steve in Peoria

Thank you. Downhill will I think be the only time this bike will go fast. It is an old Schwinn that weighs a ton.

Open circuit is how I was getting the 1.5 volts. Seems like this may be a dead dynamo.

I haven't measured current. When the lights didn't work I just went to voltage checks and confirming that I had good continuity from the output terminal to the contact at the end of the lamp as well as ground continuity from the dynamo to the case of the headlight. My open circuit voltage test producing 1.5 vac was done by spinning it up to an estimated RPM of what I would expect at the tire with a cordless drill. I didn't want to spin it at high RPM with the drill because I didn't want to tax the bearings or sleeves with whatever 54 year old lubrication that may be left.

Thanks all for all the ideas. I had one of these on a bike when I was a kid, but not since. I just always assumed these were DC generators and didn't realize they were AC.

Not that I know anything about bottle dynamos, but at this point, if it's dead already, then there is no downside.

Sounds like the internal brushes may be corroded. Age has a way of doing that. The good news is that contact cleaner can be sprayed in & the wheel spun to simulate in service use to clean the contacts.

Failing that the dynamo may be disassembled & a burnishing tool can be used to mechanically scrub the corrosion off.

If the brushes are worn to nothingness, Ace Hardware has a variety of electric motor brushes in stock in the assorted fastener aisle.

Could be a bad diode or capacitor if it has such a thing. A few minutes with a soldering iron isn't really a chore if you are so inclined.

Sadly, were disassembly an option, I would have already done the exploratory surgery. Contact cleaner I would imagine will wash away any lubrication to what I would imagine are sleeve bearings. The bike is going to be raffled off in a fundraiser most likely this fall, so I have time to explore options to resurrect it.

Are you setting the meter for a sensible range, i.e., 20VAC or so in this case? Are you measuring off the dynamo or farther away? Are you just spinning the wheel fast enough? Apologies if the questions sound insulting, but many people struggle using meters.

I do not think there are any brushes in a bottle dynamo. Just spins a permanent magnet inside of a non-rotating armature with the windings. Can't get any simpler than that. I have never disassembled one, so maybe I have that backwards and the windings spin in perminent magnets? But that would be the hard way to build one.

The bottle dynamos I had (Union brand from Germany) had no holes to shoot in any tuner cleaner, they were supposed to be sealed against the weather.

I have never heard of any capacitors or diodes in them.

Not insulting at all. Honest questions. But I'm using a Fluke 87V auto ranging multimeter which I'm adept at using. I'm just not experienced enough with these dynamos to know what I was to expect as far as voltage. I didn't feel 1.5 volts was going to be enough, but I thought I would ask to be sure.

Your assessment seems to be in line with what the googles say as far as construction. As far as the windings go, I'm beginning to guess that some of them may be bad, hence why I'm only getting a quarter of what it seems I should be getting. But that is only a SWAG.

You mentioned a electric drill, but how fast was the wheel really spinning. You probably need two revolutions per second to approximate 10 mph.

I would not worry about old lubricant for only a few minutes of experimentation.

I used a bottle dynamo on my errand bike because I wanted a front to-be-seen light in the evenings for short rides near home. But otherwise, bottle dynamos are a major inconvenience in my opinion. The LED light options out there for a low cost are much better options than the options that were available when bottle dynamos were one option. The noise, the poor light, the drag on the tire, the chance that a downhill run would blow out your bulbs, these were reasons to want something else. I think the drag was not that bad, but the noise made you think that the drag was much worse.

Is this a restoration project?

Now that I think about it, you are probably right.

I had forgot that "bottle dynamo" is a misnomer. "Magneto" is actually the correct term for what is used on bicycles.

I was thinking it may have had a commutator &/or may have had a rectifier for some reason. DC generators & dynamos have brushes & contacts to be cleaned & serviced. Whereas magnetos don't.

As to being sealed up tight: On bicycles they probably never saw enough age or use to justify maintenance access. And in, magneto there just isn't anything to "go bad." So there's no reason to consider maintenance at all.

On a resistive bulb electricity "type" would hardly have mattered, anyway...OP: Flip the AC meter to DC & see what it reads. That'd tell us what kind of device you have.

I would call it more of a preservation project than a restoration project. Everything is original except the tires, chain and spokes. The spokes were so dingy that nothing could be done for them so I replaced them with stainless steel. The Schwinn monogrammed seat is being touched-up by someone and should be done shortly. We tossed around the idea of repainting, but liked the idea of original paint. We are wanting to do the same with lighting.




Less than 500mv DC.

Paint looks very good for that era. Chrome looks like new. Looks like an old coaster brake single speed. Small womens model.

Since you are working to keep it original, then picking up another bottle dynamo out of a spares box at a bike coop or charity would be unlikely to provide the right brand and model. Plus, those usually were for the front wheel with built in light.

Headlamp looks like a battery model, not a dyno powered one. Dyno powered lights do not need a switch, you move it away from the tire to turn it off.

Nice project.

I've got an old Schwinn dynamo light set in my parts box, and maybe it can be a helpful reference.
It uses the dynamo body, brackets, and the bike frame for the "ground" connection, which is a potential source of troubles.
The other connection is through a weird terminal in the bottom of the dynamo that is shaped like a cup, and relies on shoving in wire and hoping it contacts it. I think perhaps you are supposed to wrap the bare copper wire around the little post in the plastic plug that gets shoved into the cup? As someone who has worked with electrical connectors for decades, it is hard to think of a worse way to make a connection.

a couple of pics....

the light set:


the bottom of the dynamo, where the connection is made:


If you've got a connection like this, check it and see what can be done to minimize the effects of oxidation or corrosion, and perhaps see if the wires can be made to fit tightly against the metal terminal?

Steve in Peoria

Thank you. It took some elbow grease, crumpled aluminum foil and quite a bit of Simple Green to get the rust off of the fenders, crank, handlebars and wheels. But it has been worth it. It is a yellow band Bendix 2 speed with coaster brake. It has a headlamp and tail lamp. I left the dynamo where it was originally to minimize any further damage to the paint. The switch for the headlamp is for high beam and low beam.

This has a slightly different but possibly better connection arrangement. I agree that overall though, it is not an optimal contact arrangement. I have about 36 years of electrical experience as well which is why I'm bothered that something as simple as this is kicking my keister. If I find time this weekend, I'm going to do some more experimentation which may involve spinning it with a cordless drill to RPMs greater than this old Breeze will ever give it and see what happens.

Sturmey-Archer hub magnetos are of course the 'Dynohub (tm)'. The classic GH6 was produced from 1946 through 1984. They were rated at 6V and until the early 1970s 2W, afterward 1.8W. Modern Dynohubs like the HDS12 are 6V, 2.4 or 3W.

Fun fact: Some modern Shimano hub dynamos, like the S701, are rated at 6V, 1.5W.