Hi All, I wonder if there is anyone on here willing to assist me with what I think (Hope ) is a simple problem.
Simply put, I'm an older gent who has fond childhood memories of riding his bike and when the sun set it was just a matter of flicking the lever on the dynamo and the bike had a head light and tail light with no concerns about keeping charged batteries "just in case" they were needed.

Fast forward 45 years and I'm back riding for exercise (as opposed to transport) and although I have rechargeable LED lights I find it annoying to have to keep them charged when I rarely use them.
I expected that with low power consumption/high light output LED technology, dynamos would be popular again but it seems they are not.

I have bought a 12V headlight https://www.ebay.com.au/itm/403707240131
(Rated at 5W according to the listing on ebay)
and a 12V tail light https://www.ebay.com.au/itm/272491849491.
(No power rating stated in the listing but I messaged the supplier and his reply stated "Watt: .5W")
(Both came in 2 in each pack but I believe I can only run one of each with the rating of the dynamo.
This is the dynamo I'm planning on buying.
https://www.ebay.com.au/itm/224327658490?hash=item343af86bfa:g:u3QAAOSwuRpgDj0T
The 12V output is rated at 5.5W
All of that is simple enough but I'm wondering if it is possible to have a capacitor be charged my the dynamo that would continue to run the lights for approx 3 minutes when I'm stopped at a traffic light?
Pondering 2 also would it be difficult to incorporate an optional flashing facility to the lights to assist me to be noticed in daylight.
I'm quite surprised that I seem to have not been able to find any discussion on this or any other forums or even a pre-manufactured option.
Perhaps my whole idea is flawed in some way.
The lights were quite inexpensive so I bought them to see if they were going to be bright enough and they are.
I have not purchased the Dynamo yet but I plan to do so soon.
Kind regards,
Paul

 

I would consider a small lead-acid or lithium battery which could be charged by the dynamo. An original bicycle dynamo is actually an alternator, where the current is limited by its inductance. I’m not sure what voltage you would get out of it, it should be proportional to speed.

 

LED lights use DC. The 12V lights probably are dim or do not work below 10V and they burn out soon of the voltage is above 14V.
Dynamos for bicycles produce AC at a voltage that depends on the speed. Maybe their voltage is limited by the very high current used by old incandescent lights.

You can rectify then filter the dynamo output to produce a smooth voltage that is too high then use a 12VDC voltage regulator to power the LED lights. You still need a rechargeable battery for traffic lights plus a battery charger circuit.

I think producing the 5.5W will be like pedaling uphill all the time. A guy tried using a fan as a generator but discovered that it was hard work like pulling an open umbrella around behind him.

 

and although I have rechargeable LED lights I find it annoying to have to keep them charged when I rarely use them.

Why do you need to keep them charged? A decent LED light with lithium-ion battery can have 10-20-30+ hours of runtime. If you rarely use them, you could get a riding season worth of light on a single charge.

 

Ok thanks for the replies.
It seems that as I said my idea does appear to be flawed.
I guess I'll have to keep putting the batteries that I use for the existing lights on charge once a week just in case I need them.
I probably get 10 hrs run time out of mine.
But I have been caught out a couple of times where I had the head light turn itself off about after about 15 minutes.
I turned it back on but it went off again after 5 minutes.
This happened after about a couple of weeks with intermittant use.
Maybe 8 hours usage.
I doubt it was 10 hrs.
I also had one instance where the tail light was off when I got home but I know I turned it on when I started my ride.
After I discovered it off I turned it back on but it stopped again after 15 minutes.
Placing the lights on charge in each instance seemed to correct the issue.
I have started just putting the lights on charge once a week to avoid being in the position of having no lights.
I don't really need the lights to see.
Where I ride the ambient light from street lights is sufficient.
I want them so that there is more chance of the traffic seeing me.

 

Sadly a capacitor isn't going to work.... based on the charge formula C = I ΔT/ΔV, your 5w LED takes about 0.4A on 12v, so for a ΔT of 180sec and a max voltage drop ΔV of 3v, C = 24F That's supercapacitor territory. A 15v rated SC at 30F is ~ 250Au$

I've assumed that the 5W spec for LED is constant output, not flashing or other fancy power saving.

A typical club cyclist generates about 250 - 300W on a training run, but 'for exercise' you'd be generating probably no more than 100 - 120W so your dynamo will be sucking 10W or 10% out of your efforts (5W out @ 50% efficiency maybe if you're lucky).

10W, as AG suggests, is roughly like continuously climbing a 5% gradient at 10km/hr (6mph) (100kg bike & rider).

As you say - a small flaw in the thinking... but not an uncommon one!

(yeah, i know this post is a bit late to the party, but I wrote it then got called away having thought I'd hit 'post reply' but my mouse decided otherwise )

 

Thanks for the reply Irving.
Well at least I know the answer.
I never noticed any "load" when I was 12 or 14 when the dynamo was on or not.
But I guess I was fitter then.
The only issue I ever had was the incandecent lights were not very bright.
I was hoping that the LEDS would be better, but I'll just go back to what I have in place already.
Cheers.
Paul

 

The cheap No-Name-Brand lights from ebay might simply produce 4.5W or more of heat and a little bit of light.
5 true Watts from real LED headlight is too bright for a bicycle. I have a 1W LED flashlight (torch?) that is also too bright for a bicycle.

 

I was hoping that the LEDS would be better, but I'll just go back to what I have in place already.

They will be but the cheap ones aren't great. I had a CATEYE Volt front light that would last the whole week on 1 charge riding 25miles each way to and from work on 'flash' and you couldn't miss it (unless you were a London cab driver!). But it would only last around 5h when used to see where I was going (250km/h night rides) so I had a secondary battery pack strapped to the frame.

@Audioguru again Hi ag, there's no such thing as too bright when dealing with London traffic!

 

Here's an LED headlight that's designed for use with generators, has a beam pattern that's compliant with the German StVZO regulations, and has some magic that can keep it lit at 20% brightness for 4 minutes when you are stopped at a traffic light. I just found it by searching for stvzo.
https://www.ebay.com/itm/324728104241
https://www.bikeradar.com/advice/buyers-guides/stvzo-bike-lights/

 

Looks good.
Thanks for showing me.
This doesn't come up on searches of the Australian Ebay site, but it seems like it ships to Australia.