Converting a battery light to run from Dynamo Hub

[steveo_mcg]

After seeing my dynamo lighting set up a friend of mine has asked if it would be possible to retro fit a battery light to run on the dynamo.

I'm thinking is yes but you'd have to rip out the electronics, thoughts? Has any one done any thing similar, I can't seem to see any thing on the search.

 

[ianfield]

After seeing my dynamo lighting set up a friend of mine has asked if it would be possible to retro fit a battery light to run on the dynamo.

I'm thinking is yes but you'd have to rip out the electronics, thoughts? Has any one done any thing similar, I can't seem to see any thing on the search.

A few years back I had a design published in Elektor magazine for an LED lighting set, took 6V from a 1.2Ah SLA battery charged from a Sturmey Archer AG dynohub.

Each lamp unit had a current controlled flyback/boost converter to generate between 13 & 22V to drive a series string of LEDs (the rear light was a stripped out unit that had a "black blob" chip and ran from 2x AAA cells).

If you want to wire the off the shelf LED bike lights to run off a dynamo; you need some form of shunt regulator to give you the required 3V per lamp - and preferably backup so the lights stay on when you pull up to a stop.

As the plastic body lamps are isolated from the frame you can simply wire them in series with rechargeable batteries fitted and charge the batteries via a bridge rectifier from the dynamo - however there maybe a gotcha!, NiCd rechargeable batteries don't take kindly to freezing temperatures and I'm assuming NiMh cells probably have the same limitation.

A 6V SLA battery is the best option - the long wires from the battery will have resistance and inductance with the transient load of pulsed LEDs on the end - you will need to mount Electrolytic capacitor reservoit/smoothing locally at both lamps - find a couple small enough to fit in one of the battery spaces - you can get 6.3V very high capacitance types from scap PC motherboards.

 

[jdp298]

I like the sound of the circuits you've had published, but are there any links or do we need to buy the magazine?

 

[ianfield]

They'll probably ban me from having anymore projects published for telling you this, but various people illegally upload many of the magazines to sharing sites (Elektor being among the easiest to find).

There are various torrent sites, binsearch and on usenet: alt.binaries.e-book.technical although the guy who posts the electronics magazines rarely re-posts.

The blocking osc design was in the 0708 2011 summer double issue, the boost converter lighting set was years ago - it'll take a bit of searching.

Can I post attachments on this forum?

 

[Steve K]

After seeing my dynamo lighting set up a friend of mine has asked if it would be possible to retro fit a battery light to run on the dynamo.

the answer, as is typical, is "it depends". What's the light that you want to convert? If it uses no more than 0.5A at 6v, then it should be fairly simple. Otherwise, it's going to depend on the specifics of the current and volts at which it is designed to operate. Generally speaking, if it is running from a few AA or AAA cells, then it should be fairly easy. If it is running off of a good sized lithium battery and drawing 10 watts, then it is unlikely to be a good match for a dynamo.

regards,
Steve K.

 

[ianfield]

the answer, as is typical, is "it depends". What's the light that you want to convert? If it uses no more than 0.5A at 6v, then it should be fairly simple. Otherwise, it's going to depend on the specifics of the current and volts at which it is designed to operate. Generally speaking, if it is running from a few AA or AAA cells, then it should be fairly easy. If it is running off of a good sized lithium battery and drawing 10 watts, then it is unlikely to be a good match for a dynamo.

regards,
Steve K.

There seems to be a spread of power outputs, from the lowly Sturmey Archer GH6 hub at 1.8W to the Miller "bottle dynamo" at about double that - typical modern 'dynamos' (actually an alternator). Most generators use bulbs to overwhelm the output, as the speed increases the AC frequency also increases and the inductive reactance (Xl) progressively limits the current so the bulbs don't blow (in theory anyway!).

If you allow the generator to "stretch its legs" so to speak - that is reduce the current draw and the voltage will rise out of proportion to the reduction in current, plotting the new load curve you will find that under reduced current draw the generator is capable of higher power than stamped on the spec plate.

At the moment I'm using a rectifier from the generator to charge a 1.2Ah SLA battery and power my homebrew LED lighting set - ATM there is always plenty of charge to spare.

However; I would like to have the use for short periods of a 3W star LED "main beam" for navigating unlit cycle tracks. So even with only occasional use it would be desirable to 'cheat' a little more power from the generator.

A switch mode step-down converter will reduce current draw and downconvert the higher voltage, but there are a few gotcha's in the design process - I'm still scouring the web for research material but hope to put pen to paper before long.

 

[steveo_mcg]

Thanks guys. Its a pair of be seen lights, Cateye something or other. Having looked at them the biggest worry would be heat sinking the emitters, I wonder if something could be put in the battery box to provide cooling.

 

[Steve K]

So you are going to retrofit the lights with better LEDs?

I've converted a Vista light to LED, and it took a bit of work to add a decent heatsink:
http://www.flickr.com/photos/kurtsj00/sets/72157601241482040/with/1022557431/

I'm currently working on converting a Cateye Micro II to a Cree XR-E. I liked the light for its mounting system and the AA battery carrier. However, it is a *lot* of work to tap into the fittings that mate to the battery carrier, not to mention the trouble of fitting a heatsink and whatnot. I might just be making it more difficult that it needs to be??

For something as conceptually simple as a dynamo light (i.e. rectifier, LED, lens, housing), I'd recommend buying/scavenging a Cateye mount system and sticking the rectifier & LED in a housing made of aluminum extrusion. My approach to this is seen here:
http://www.flickr.com/photos/kurtsj00/sets/72157621965148305/

I think that my reason for converting old incandescent lights is that it bothers me to just throw the stuff out. Logically, the time spent on the conversions is just not warranted! Most of the time is spent hacking up the housing and the aluminum, etc., instead of making new circuits or other fun stuff.

Steve K.

 

[steveo_mcg]

Not sure, split between the two lights they should be able to cope with the 500ma. 250ma each probably isn't much different from what they draw at the moment on batteries.

I did a very similar thing with a trio of XR-E's that were bound for an off road light, because of the copper end caps its bloody heavy for what it is. Its not pretty but its functional.

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My current project is converting an old Union head lamp to led... just because. Its not like any of my bikes are vintage enough to warrant an old style lamp.