OT: voltage regulator & battery for a bicycle dynamo hub

[rimcanyon]

With winter on the way I am adding a dynamo hub to my bike's front wheel. It is a Shimano Dynamo Hub DH 3N71 6V 2.4W Nexus (36 hole disk brake hub). I also have a Shimano LP-R600 Dynamo Hub front light.

What I would like to do is add a rechargeable battery pack to the system so that the light is powered when I am stopped at a corner or going slowly up a hill.

I have been unable to find a commercial product that will do this, so I was hoping that one of the electronics wizards that inhabit PM might be able to point me in the right direction. I have seen homebrew circuits discussed on the web, but don't have time for yet another project right now.

-Dave

 

[D.spencer]

well dave, if i am understanding you correctly,it sounds like you want the light on at all times(even when stopped or going uphill slowly). complicated circuitry does not sound warranted here. all that need to be done is a rechargeable battery(of the same voltage as the light) hooked up so the generator recharges it when in motion and a simple on/off switch to remove the battery from the circuit when not riding in order to keep the battery from going dead. you should be able to parallel the circuit from the generator to the light with the switch in series to the battery observing correct polarity( between the battery and the generator). of course if you stay stopped for a long time the battery could run down but if the battery is sized right it should not be an issue.(don't need no diesel engine sized battery-unless you have your heart set on that )

 

[gar]

060913-0650 EST USA

rimcanyon:

Put a diode between the generator and the battery.

A Schottky diode will provide a lower voltage drop. For example an International Rectifier 1N5820 rated 3 A 20 V PIV ( www.irf.com for data sheet, www.digikey.com or www.mouser.com for parts, there are a number of manufacturers). Put your on-off switch between the battery and light.

The positive output of the generator goes to the anode of the diode. The cathode of the diode, marked by a bar, goes to the positive terminal of the battery. The generator negative goes to the battery negative.

.

 

[Old Field Tech]

I knew GAR was good for something (Smile)

Rod

 

[jim rozen]

If the shimano unit is anything like the old
sturmey archer ones, the output from the
hub is AC.

He'll need a bridge rectifier to do a good
job if that's the case.

Jim

 

[Dave G.]

I have an old system (from 20 yrs ago?) that uses just 2 diodes to charge 3 "C" batteries. If you're building - Jim's idea is better, that is a 4 diode bridge. The new "AA" NIMH batteries have more amp-hours than "C" batts from 20 years ago.

 

[Kent David Redd]

The generator for bike light that I have seen are AC units. If you use a bridge( 4 diode) rectifier you will need to insulate the generator mount from the bike frame.

Kent

 

[Kent David Redd]

Forgot to add that a heavy duty zener diode could be used to limit the charge voltage of the battery. You would connect the zener across the battery. British motorcycles of the 60's used this system.

Kent

 

[sch]

Kent's point is a good one, over voltage from downhill runs can pop the bulb easily so adding zener across the bulb to limit the voltage the bulb sees is a nice safety factor there as well as for the battery.

 

[jim rozen]

"British motorcycles of the 60's used this system."

Hands-down the most *poorly* designed electrical
systems on the face of the planet.

It's unlikely that the batteries on the buss
would allow the voltage to rise very high, I
suspect.

At any rate if there were an issue there I'd
go with a 7806 series regulator, not a shunt
device. Anyone ever try to spin a dynohub with
the two terminals shorted together?

Jim

 

[Keith Krome]

I had a similar dilemma. I had a Sturmey-Archer dynohub, and wanted an LED light using the (at the time) new luxeon LEDs on the market. I used a full bridge rectifier, a voltage regulator and three 1.5 farad super capacitors in series for my stand light.

The super caps are really small (about the size of 3 stacked quarters each) and the circuit does what I need. I'm sure it could be better, but it was the first thing I came up with and I was pleased. I was thinking about some germainium diodes to make the rectifier from to make it a little more efficient, but right now I'm satisfied.

BTW, have you seen Peter White's page: Peterwhitecycles

He has some good lighting stuff.

Oh, and yes, the hub dynos are all permanent magnet alternators that put out AC. Sturmey, Shimano and Schmidt SON.

 

[gar]

060913-1048 EST USA

The battery itself is a relatively constant voltage device. Its terminal voltage will will be a function of its internal impedance and the charging current. There are also electro-chemical effects.

We have not heard back from rimcanyon yet so whether the above information will help is unknown.

If the generator (alternator) has one side grounded, then a single diode can be used. This would limit the energy flow rate to about 1/2 of the bridge rectifier.

A bridge rectifier can used by floating the battery and lamp off of the bike chassis and leave the generator grounded.

If there is excessive voltage available from the generator, and you do not want the dynamic braking that jim rozen is describing, then a more complex circuit may be desirable.

Keep in mind that rimcanyon wants a battery to power his light any time the generator is providing insufficient energy to do that job.

.

 

[ken moss]

You need to talk with Brett but brett is doing ride suport for cycle Oregon Ken

 

[JST]

I did this about 30 years ago, with a clamp-on generator.

Used a full wave bridge, and insulated the clamp as suggested. Worked well enough. The light is dimmer when no charging is happening, and tends to be dimmer anyway due to diode drop.

The battery will hold the voltage down, and generally you won't over-charge anything, at least I never did. On a really long ride I suppose you might......... aside from the limited voltage, which may totally prevent overcharging.

However he said dynamo hub. IIRC, those may have only one wire and no way to isolate the thing.

If so, the single diode may work, although that will not give as good a light, due to receiving half the possible power. 6V is not a lot of voltage, 2.5W is a bit under a half amp, and at half wave it will average half of that.

I don't know what the effect of half-wave rectification is on the generator. Some do not like the unbalanced ampere-turns, with some it won't matter.


If it has two wires, he is fine, unless one is ground anyhow.

The other alternative is to lift the battery and lamp off "chassis ground". That works just as well, although it can be confusing, and can allow using a full wave rectifier.

With the power involved, it is less likely to need any sort of regulator system. I would suppose the light will do enough regulation by itself, thru "stealing current".

 

[jim rozen]

Doesn't get much simpler than a 780X series
regulator. Try pricing one of those gigantic
Lucas zener diodes....

Anyhow if his dynohup is single-ended (the
SA ones have both leads brought out) then
I would indeed float the low side of the
dc output to allow a bridge. One extra wire
equals twice the light, seems like a good deal
to me.

Do the shimano units weigh less then the SA ones?

They'd have to, those things were boat anchors.
Mostly lighter just to carry a full size car
battery with you...

Jim

 

[JST]

Actually, at that power level and voltage, about the only option is a "shunt" regulator like a zener.

The series regulators like 78xx nearly all take too much voltage drop across the pass element to operate decently in this system.

With 2.5 W, and a light bulb in the system as a load, I really doubt there would be a problem. The main problem might be getting enough spare energy to actually charge the battery!

Definitely Schottky diodes...... even the 0.3V drop is going to be noticeable in the light output.

 

[Motomoron]

I've got a few SA Dynohubs around as well as various 3 speed hubs, but for commuting this winter I'm gonna pop for an HID headlamp w/ LiPo batteries.

I'd be riding home on the pitch dark bike path last winter with my NiCad/halogen starting to dim and when it suddenly got light out, it meant I was being overtaken by someone with HID illumination. I'd jump on their way and hope they were going at least as far as I was.

Dynohub is a nice dim flicker, but that's what I keep my Austin Healey around for...

 

[rimcanyon]

Sorry for the lack of response, just had time to scan the responses and I'm off for a poker game. The Shimano hub has two leads. As I mentioned, I am looking for a simple solution, willing to purchase something already built if such a thing exists. Here is the url to the homebrew circuits I have found:

bike regulator

 

[JST]

two leads makes it easier.

However, I think the chargers in teh link, while interesting, are over-fancy.

I would do the charging relatively simply. A resistor will limit current to the battery, if that is deemed advisable.

A diode in parallel with the resistor will allow the battery to discharge through the light, without the resistor limiting current.

Therefore, I would suggest the generator hooked through the full-wave bridge, with the light across that.

A resistor of a value picked to charge the selected battery safely is connected from rectifier plus to the battery plus. Battery minus goes to rectifier minus.

A diode is connected in parallel with the resistor, poled to let battery-supplied current flow to the light.

A double-pole switch can be connected to disconnect the battery and light from the rectifierand remove load from teh generator.

Optionally, the switching could be set up to charge the battery with normal pedaling, without the light on.

And, a suitable zener could be put across the battery if it is desired to limit the maximum voltage supplied to it. Since with most types, the battery when under charge has a higher voltage than without charging, and the resistor limits charge current, that connection should be perfectly safe.

The above is essentially what I did, and it worked quite acceptably.

 

[jim rozen]

My guess is that any kind of regulation will
not be needed. Just hanging the batteries
on the buss will be enough.

I doubt they'll get overcharged.

If they do then simply put another lamp on
line....

Jim