OT - generating electricity on a bicycle

[NY]

I'd never thought about this. For lighting it doesn't matter at all whether you generate AC or DC - it's only when you need to power electronic equipment (eg USB charger) that you need some form of rectification and smoothing.

Do modern dynamos that can power USB devices still generate AC, or do they have the commutator rings wired to reverse the phase of the negative cycles to give full-wave rectification? I suppose it's about as easy to modify the generator as it is to add a couple of extra diodes as part of all the other electronics needed to provide a smoothed and regulated 5V for USB, together with a bit of storage in a battery to cover times when you are going slower or when you stop at lights.

[The Natural Philosopher]

Not all dynamos have commutator rings. IIRC hub dynamos are simply rotating magnets and a series of coils.

[charles]

I remember seeing some dynamo systems which had a battery container attached to the frame. The advantage of batteries was that when you stopped moving - eg at traffic lights, your cycle lights stayed on. But nowadays cyclists never seem to stop for traffic lights, so behaps a back-up battery isn't needed. ;=)

[tabbypurr]

It's not, that's the biggest problem with dynamoes. Stop at a junction & your lights go out. Dangerous & illegal.

NT

[The Natural Philosopher]

Yep. pack of nicads clamped to the vertical tube under the saddle and a hub dynamo.

Long time ago that was.

[Theo]

Modern LED dynamo lights have a capacitor that keep the light on for a few minutes after you've stopped. But many also have a button to discharge the capacitor - saves having to explain to people who tell you that you left your lights on when parked.

Theo

[Theo]

I haven't seen the USB kind, but it seems unlikely they would make big changes to the dynamo mechanics based on the application when they could send a few pennies on some diodes. The standard lighting systems (be they hub or bottle) are all AC, with diodes inside the LED lamps to rectify. It would be quite a bit of effort to make a non-standard system. Most are 6v but there are some 12v systems around. I suspect most of the goal is USB charging rather than USB power, so it's expected that cutouts will happen from time to time.

Theo

[Syd Rumpo]

Not illegal if you stop on the left.

Cheers

[Syd Rumpo]

Public hire bikes.

Cheers

[Syd Rumpo]

Make your bridge rectifier out of LEDs.

Cheers

[The Natural Philosopher]

Not a great idea. They are not designed to take much reverse voltage.

I think the reverse breakdown is around 5V.

What I would do for a quality bike light solution is first of all select a rugged rechargeable. LiFePO or NiCd.

Then get some kind of hub based alternator featuring Neodymium magnets and as lot of iron and copper with a bridge rectifier and some seriously large capacitor to drive a smart inverter that would provide a stabilised battery charge and light driving circuit coupled to some decent high intensity LEDS with or without strobing.

Normal day riding would recharge the battery, and at night one would hope that the alternator would be enough to at least drive the lights except when stopped.

[damduck-egg]

Bugger, I used to know the bloke who was fairly high up the ladder in commissioning and then running the Fawley plant a Mr L White so he may have had a hand in that. Passed on a while ago now as have some of his colleagues so I can't chat to them about it. Site has been sold now though for years the place was only occasionally used, it had the misfortune to be built and commissioned as an oil fired plant just a couple of years before the oil price shot up in 73.

G.Harman

[Martin Brown]

These days with 10W LEDs being so bright it is probably easier to invest in some rechargeable 10Ah D cells and charge them offline.

IME Bicycle dynamoes invariably develop expensive faults. It isn't a kind environment with vibration, mud, salt, grit and wet around.

Regards, Martin Brown

[Clive George]

All bike dynamos still generate AC, the rectifiers are in the lights or the USB adapter bit.

A good reason for this is that there is a german standard for bike dynamos - 6v, 0.5A AC, must generate that at 10kph or so.

[Clive George]

Not for over ten years now - all decent dynamo bike lights have standlights these days.

BTW it's not illegal, the law on lights understands that dynamo lights did stop in those situations and allows for it.

[newshound]

Not used on any later sites, though either in UK or globally AFAIK. Presumably the capital cost and/or reliability of the cryogenic plant was the problem? Although superconducting magnets are now (relatively) commonplace in MRI scanners. (I still want to call them NMR imagers).

[Clive George]

Bottle or hub? Hub dynamos are clear of a lot of that, and are very reliable IME. I fitted my first in 1997 - the first one Shimano made. It finally died of bearings (same as any bike hub) about 15 years later. There are now 6 bikes with hub dynamos in our house, and I'm happy with all of them.

I had a brief dalliance a couple of years back with battery lights, but finally replaced the reason for that (filament front bulb on one of the bikes, replaced with a decent LED), and have gone back to not having to worry about charging.

[Syd Rumpo]

You have a four LED bridge, with one or two LEDs across the 'DC' output, often with a standlight capacitor. The rectifier LEDs won't see more reverse voltage than the forward drop of the 'DC' LEDs plus one rectifier LED. This works particularly well as a rear light arrangement in combination with a battery front light.

Before good white LEDs, my front light was a 12V 20W halogen spot run from a SLA battery, nowadays, I use the cheap rechargeable lights from Lidl and keep a spare set charged.

Cheers

[damduck-egg]

My other had a back up system on her Dawes bike back in the forties, apparently quite an upmarket option back then. There was no charging involved , I don't actually know if the set up switched automatically to the dry batteries which would have been the std zinc carbon or if it had to be done manually by a selector switch as a approached an area that stopping was to be anticipated.

G.Harman

[Tim+]

Then you are a man of very little imagination.

Tim