Thruhiker 

I put shimano alfine DH s501 dynamo hub on a dt tk540 rim. The hub produces 6 volts 3 watts .It's the second one I have set up. The first one is on my fuji touring bike. For it I bought a supernova light and sinewave charger its great. I love the whole set up. This second set up is real low budget just for pleasure riding. I used a 1000 lumen flashlight that ran on 4 aa batteries. I soldered my wires to the terminals. It works. However there is a very high speed flicker wich is good for traffic dureing the day but not so much for night riding and there is no stand light. It goes off as soon as I stop. I was wondering if I could run my wires to a group of 4 rechargeable aa batteries and then the light wires from the batteries. Would the dynamo charge them? The batteries would then keep the lights on at stops. My experience with wiring pretty much stops at residential stuff. I web searched but quickly became confused. Do I need a capacitator or some other part? If so can I scavenge it from something like an old radio or pc? Part of this project is to be as cheap as possible

dsbrantjr

The hub likely produces 6 volts AC and the LED only lights on alternate half-cycles which is why it flickers. You cannot charge batteries with AC. You might be able to hook up a bridge rectifier to convert the AC to DC which would double the flicker frequency to where you might not notice it, or you could put a capacitor (minimum 15V, correct polarity) across the bridge output to filter the pulsations and perhaps store enough energy to keep the light lit for a short time after stopping, the so-called standlight function. I would start with a 1000 microfarad, 15V capacitor; how long it would keep the light lit depends upon how much current the LED draws.

sweeks

The hub *does* produce AC. It's probably not a good idea to run a DC light off an AC power source; LEDs have circuitry to drive them, unlike incandescent bulbs. You would be better off getting an inexpensive light intended to run off a dynohub.

Thruhiker 

Thanks for the answers! I, for some reason thought it was dc. Lol

Dan Burkhart 

You could be excused for thinking that, given that the term dyno actually suggests direct current, and is a misnomer.

Bill Kapaun

A lot of vagueness there.
Incandescent lights are used AC or DC. Houses & cars.
Running AC to an LED means it's turned OFF more than 50% of the time. It's a DIODE!
It's off when the current flow is negative.
It's off when the current flow is positive UNTIL the voltage reaches the "turn on" point.
You can use a bridge rectifier to turn AC to DC, but each diode involved is going to cause a nominal 0.2 to 0.4V drop. You can't afford a lot of those drops on a 6V circuit.

Moe Zhoost

Dyno hubs can produce well over 100 volts AC. To be useful, this needs to be rectified to DC and limited to the desired voltage. The link here may be helpful to you.

sweeks

Yeah, I guess so. I was trying to be brief. There's a lot more inside the LED lights that I've seen than LEDs and rectifiers. The electronics can be complicated.

EDIT: Here's why I said the electronics can be complicated. Attached are images of the inside of a Tern "Valo 2" headlight designed to run off of a dynohub. Attaching a light like this to a DC source *might* work, but also might damage the light. There is a DC version of this light for e-bikes.

Here's the inside of the light showing the circuitboard and the LED element (barely visible).



The LED element is just above the pop-rivet in the red circle.

sweeks

I've seen one of these dynohubs, unloaded, in a bench vise produce a little over 10 volts with the wheel spun as fast as possible by hand. 100 volts seems a bit of a stretch.

gilesa

OP's original plan should work with the addition of a rectifier. The simplest scheme would be to place a 1N4002 between the dynamo and battery positive, with the other dynamo wire to battery negative. The end with the stripe (cathode) connects to the battery. This is a very cheap part, and also easily scavenged. The problem with scavenging is to recognise the part you need, but you could try posting a photo of your junk's internals.

The batteries need to be capable of absorbing the dynamo current even when charged. 6V/3W/half-wave gives 250mA, so high-capacity AA NiMh cells (2200 mAH capacity) should do it. With a nominal 1000lm light connected, the batteries will not charge with the lamp on. A bridge rectifier would double the output, requiring bigger batteries for prolonged riding with lamps off.

I have not tested these rectifier/battery arrangements myself.

Moe Zhoost

Voltage output is not quite linear with rpm but if you spin them fast enough you can get pretty high voltages. Do they spin fast enough on a bike to achieve 100 volts? Not likely, but on the bench they can. See here for an account of high voltages with a Schmidt hub. I recall that I measured close to 50v with a Shimano hub (not mounted in a wheel) as a demonstration for sale, but memory being what it is, I decided this morning to do some measurements on my current hub/wheel so I could measure both no load voltage and speed (measured with my cyclometer). Hub is Shutter Precision on a 650b wheel. I put the bike up on a stand and spun the wheel with a rubber drum on my electric drill. I found that I could get about 13 volts for each 10 mph. At max drill speed I was getting 32 volts at 25 mph. Hubs on smaller wheels spin faster so this same hub on my 20" trike would give over 40 volts at this same speed.

vane171

You can get those bike light that have micro usb port for charging, which is 5V DC, so putting a diode on the 6V dynamo might work to trickle charge to keep it at full charge all the time. But I fail to see its utility, unless maybe you bike across country on wild back roads or offroads without access to AC main.