Bicycle Dynamos

[Roger Ramjet]

I would guess most on the forum would remember bicycle dynamo's. Most popular were the"bottle", shaped one's than ran of the tyre but I always preferred the one built into the wheel hub.

Of course the lights only lit when the bike was moving & I often wondered if anyone had invented a rechargeable battery standby pack - powered off the dynamo to keep the lights on when not peddling.

I remember a chap in our village who converted a hub dynamo to a wind generator by removing the wheel bit & fitting fan blades in place of the spokes.

In hindsight, I would have been interested to see what waveform presented on a scope, especially the hub which had a circular segmented magnet rotating around the stationary coil.

Rog

[stevehertz]

I'm sure there was some kind of 'solution' to power the lights when the bike was stationery. But as it is something that I too have thought about in the past, then maybe it is just that - a thought!

[duncanlowe]

The ones in the hubs were called Dynahubs (unsurprisingly). I had one of the bottle shaped ones, and the front lamp had 'dip and main'. I'll be honest I paid a lit for it and was really dissapointed in the performance. Went back to battery lights.

These days, with LEDs the light output might be better. And with the improvement in battery technology rechargeables are now worth considering. But that said, LED and modern batteries mean the way to go is simply LED and Lithium rechargeables. Oh, and of course when you get to eBikes with an onboard battery, there is plenty of battery to go round.

[Red to black]

these were simple alternators as I recall, in that they produced AC, no need to rectify to DC when only powering incandescent lamps. moving magnet core and stationary field coils no commutator or slip rings required.

[Techman]

Originally Posted by Roger Ramjet

Of course the lights only lit when the bike was moving & I often wondered if anyone had invented a rechargeable battery standby pack - powered off the dynamo to keep the lights on when not peddling.

It's already been done many decades ago.

My dads old 50s roadster that I inherited and ride round the village sometimes, has a backup battery pack fitted as standard equipment. There must be hundreds, or even thousands of these old bikes still around out there. There's some sort of metal rectifier in the top of the battery tube, although I've never actually taken it to bits to find out and it's not shown as such in the circuit diagram.

Pictures and circuit below:-

[duncanlowe]

Originally Posted by Red to black

these were simple alternators as I recall, in that they produced AC, no need to rectify to DC when only powering incandescent lamps. moving magnet core and stationary field coils no commutator or slip rings required.

I'd certainly assume they were moving magnet rotors. You could feel the distinct 'steps' as you turned the drive, a bit like when you manually turn a stepper.

[Cobaltblue]

One post already deleted for Automotive content.

This thread will close pretty quick if there is any more discussion of eBikes or motor bikes.

Push bikes are also not a topic for this forum so keep it to the Dynamos.

Cheers

Mike T

[stevehertz]

So did that battery backup system kick in automatically when you stopped, or did you have to manually turn it on?

Now I think about it, I do remember those backup batteries. Always on dark green bikes!

[hamid_1]

Back in the 1980s I sometimes used to visit an old radio and cycle shop on Muller Road, Bristol. It's long gone now. Even then, the elderly owner kept the shop open but didn't actually sell anything. It was full of old bicycles, radios and 405-line televisions from the 1950s plus spare parts. It was like a museum - fascinating. I remember the old chap talking about a bicycle dynamo system with a gel-type lead-acid battery recharged by the dynamo that kept the lights running when stopped at a road junction, for example. He didn't say whether a metal rectifier was used or the dynamo was a DC (magneto) type instead of AC. It would have been old, even then. The guy didn't do modern (80s) technology.

[duncanlowe]

Originally Posted by Cobaltblue

One post already deleted for Automotive content.

This thread will close pretty quick if there is any more discussion of eBikes or motor bikes.

Push bikes are also not a topic for this forum so keep it to the Dynamos.

Cheers

Mike T

Sorry.

[Uncle Bulgaria]

The late, great Sheldon Brown has a good page on Dynohubs. I always thought they were a pretty neat idea. https://sheldonbrown.com/dynohubs.html

[Roger Ramjet]

Originally Posted by Uncle Bulgaria

The late, great Sheldon Brown has a good page on Dynohubs. I always thought they were a pretty neat idea. https://sheldonbrown.com/dynohubs.html

Thanks Uncle B, that article was a very interesting read,

Rog

[cmjones01]

Trying not to stray too far off topic for the forum, I did once design and market a range of bicycle lights, the most popular of which was powered by a hub dynamo. Hub dynamos are interesting because they're very simple, with effectively no moving parts: there are magnets in the wheel's hub, and a single coil winding attached to the axle. As the wheel rotates, alternating magnetic poles (approximately 20 or so per revolution) pass the coil pole pieces and a roughly sine-wave-shaped waveform comes out.

The magnetic circuit in the dynamo is designed to limit the output power so as not to burn out the lights when the bike is travelling fast. The output is roughly constant-current and sort of interacts with the characteristic of filament lamps to limit the power to about 3W at 6V, or about 500mA. Oddly, if you reduce the current drawn to about 350mA, the dynamo's voltage output can increase dramatically and actually generate considerably more power (6 watts is easy). A bit of electronics to keep the dynamo in this region of its operation and convert the power to drive LEDs was what gave my particular lights their edge in brightness over the competition!

Regarding backup batteries, this is well-established technology, and is known as a "Standlight" (a sort-of translation from the German term "Standlicht" which makes more sense, but only in German). These days bike lights are all LEDs, and so efficient that a memory backup capacitor (one of the 1 Farad 5.5V types once common in consumer electronics) can run them at good brightness for a couple of minutes, enough to stay lit when waiting at a junction, for example. Indeed, the "standlight" function of my front light would provide sufficient light for long enough to fix a puncture in the dark.

This is one of the few applications in which capacitors really have taken the place of batteries, and they did it more than a decade ago.

Chris

[turretslug]

For about 20 years now I've had a Busch + Müller "Toplight+" rear light on my pushbike that does just this- within it are SM bridge rectifier, steering diodes and transistors, supercap and a high-brightness red LED within a dome-shape Fresnel lens- it stays alight for several minutes when stationary. Since then, LED technology, not least white LED technology, has advanced to the point where such a scheme would be practical for front light as well. I replaced the original quite-bright red LED within for a far brighter and wider angle type a few years back with similar current consumption- yes, the technology advance here means you can have the cake and eat it. I suspect these sort of lights (front and back) would be more readily available on the Continent reflecting (sorry...) the utility as opposed to lifestyle nature of cycling.

I prefer the idea of a supercap over batteries here- less likely to turn into corrosive soup or inert salt whilst one's back is turned!

When I was a kid, I recall my dad's bike having dynohub, plus a tube attached to the downtube holding 4x D-cells with a changeover switch on the French-make headlamp- again off-the shelf items. No provision for charging from the dynohub, though- an idea I chewed over every now and then but, as outlined above, supercap plus LED strikes me as simpler/lighter/less problematic nowadays.

[Aub]

Originally Posted by duncanlowe

Sorry.

Me too

[emeritus]

I had one of the bottle dynamos (Miller) on the bike I had as a student in Cardiff. Going down the steep hills in the Cardiff suburbs at speed used to regularly blow the recommended rear bulb so I fitted a 6.3V 300mA radio scale illumination bulb and had no more trouble. I seem to recall that in the 1960's it was not obligatory to show a red light when stationary, that was why bikes had to be fitted with a red rear reflector, but I did also have one those Ever-Ready rear lights that used a single U2 battery and a 1.2V bulb.

[The Philpott]

My bottle dynamo slowed me down as if I had brake bind, mind you I was only 8 at the time. Never blew a single bulb, which surprised me. Front bulb had a micro-dimple surface, presumably to give an even image and make the focal length between filament and reflector less critical.

Dave

[Jonster]

The Sturmey Archer DBU (dry battery unit) was the device used on fifties roadsters to keep the lights working when stationary. There's some info here: https://hadland.files.wordpress.com/2012/07/sadbu.pdf

[kalee20]

Dynohubs are certainly AC output, and more-or-less constant-current. They're designed so that there is a significant series inductance.

As wheel speed rises, output voltage rises proportionally but so does frequency. The inductive reactance therefore rises, and the load voltage (into a resistive load) doesn't increase so much.

The characteristic has a 'knee,' easy to see if you assume constant inductance, constant resistive load, a supply where voltage is proportional to frequency, and you plot current against frequency.

It does mean (as others have said) that the load is fairly critical - often (but not always) 6V 0.5A in front, 6V 0.1A rear. If one bulb fails, or there's a connection fault, the other will have a short but bright life!

Originally Posted by cmjones01

Hub dynamos are interesting because they're very simple, with effectively no moving parts: there are magnets in the wheel's hub, and a single coil winding attached to the axle.

Sorry, there's one moving part - the magnet! There has to be movement in a generator... anyone who can make a dynamo with no moving parts has invented perpetual motion!

[Herald1360]

ISTR that typical bottle dynos ran with 0.5A front and 0.1A rear bulbs. SA Dynohubs were "weedier" and ran 0.2A front and 0.1A rear.

Dynohubs did have the advantage of being more efficient (less rolling resistance) though less effective (less power output) than bottle types.