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I currently have a Supernova E3 Pro powered by a Shimano hub generator, and I would like to add the ability to charge USB-powered gadgets. My main concern, however, is that simply wiring up a charger in parallel to the lights would result in dimmer lighting at slower speeds (I already notice a decrease in brightness going up a steep hill when I'm tired).

Is there any way to wire things up so that the lights get priority, and charging only occurs when there is excess power available? Perhaps by using some circuit that would only provide power to the charger when the dynamo voltage exceeds a certain point?

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You either need a generator that's designed to do this or some electronic's skills. It's not simply a matter of splicing wires. –  Daniel R Hicks Apr 26 '12 at 0:02
    
How about a simple switch on or near the handlebars? Then you can only turn the charger on when you judge that the light is bright enough. Downside is that it's another thing to remember when you might be occupied with all that steering, braking and changing gears that people like to do on bikes. –  JamesBradbury Mar 20 '13 at 14:03
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3 Answers

The easiest way would be to use a low-dropout voltage regulator. You SHOULD regulate the voltage to a stable 5 volts anyway, because you can hurt some electronics by providing them lower voltage than the expected 5.

That being said, you can't just go get any old voltage regulator, because most have a dropout voltage of 2v - 2.5v above their target, meaning they would require 7v to 7.5v to give you a stable 5v.

The solution is a LDO, Low Dropout Voltage Regulator. The KA278 2A regulator is more than enough, and you don't connect anything to the adjustment pin to get a stable 5v. They dropout at 5.5v, meaning they don't put out any power when the voltage is lower than this. The Shimano DH-3N70 provides a nominal 6v, so you'd only be providing USB power at just under nomial voltage. Here's a link to 4 for about $5 on eBay.

KA278

This all being said, you may not end up providing that much charge from this setup, so be forewarned if your electronics don't seem to benefit from this setup. However, I'd strongly advise against hooking it directly either, some electronics are easily fried by throwing variable voltages through their USB (5v) charging port. If your light has an off switch, then by switching the light off you can make sure you get charging when you want it.

Also, be advised that most phones require between 500mA and 1A (iPhones won't charge with less than 1A), which at 5V = 5 Watts of power. Your dynamo only puts out 3 Watts of power, and at 6V = 0.5A. 0.5A * 5V = 2.5 Watts of effective 5V output, so don't be expecting to charge your iPhone even with the light off. Most Androids take between 500mA and 900mA, but will allow the charging to occur slower with less wattage, where iPhones will not charge at all.

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The ReeCharge unit below would be a much better overall solution, because it stores the excess into a battery for a better, more consistent charge. With this unit, you'd only have blips of charging when you're at full speed. –  Ehryk Apr 25 '12 at 22:43
    
I can make you a schematic and/or give more information if you like, just let me know via comment. –  Ehryk Apr 26 '12 at 3:24
    
I was always planning on using something to provide a steady current, such as this: pedalpower.com.au/page12.html . I am mainly interested in keeping it from stealing power from the light at low speeds. Would this solution work for that? If so, a schematic would be great. –  rkjnsn Apr 26 '12 at 18:14
    
That device will already have its own internal voltage regulator. Wire a switch before the PedalPower (unless it already has one), then make sure its switched off when you really need your light at full power. Otherwise, by default it will only pull current when the battery isn't charged. –  Ehryk Apr 26 '12 at 20:57
    
Or, if you're really determined to have the switching done automatically, just wire one of these in between the charging feed to the PedalPower. It has 4 pins: one's adjustment for non-5v applications, cut it off. The outer two will be power and ground from the dynamo, the middle will be regulated +5v ONLY when the dynamo's putting out +5.5v or more, which may never happen with this much load (light + battery unit). –  Ehryk Apr 26 '12 at 21:00
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I suspect you will find that most of the chargers that run off hub dynamos take this into account. For example, http://www.thinkbiologic.com/products/reecharge-power-pack in the (pdf) manual say:

The ReeCharge is charging when the green light on the side of the ReeCharge is lit. NOTE: The green light is only on when the wheel rotates fast enough to provide the ReeCharge with a current for charging.

In your position I would buy one of the charging setups and see how it works in practice. If there's a problem, you will need to add a voltage sensing switch. Either way, you need to buy the charger so there's no harm buying it.

A voltage sensing switch is easy enough to make, either using a zener diode or just a voltage divider into the gate of a transistor. http://www.instructables.com/id/Voltage-Controlled-Switch-1/ is one example (but without a bigger transistor on the output that will not handle the current your charger uses). I would go with the simpler approach of buying a power transistor that can handle the 500mA your dynamo puts out, the 15V or so maximum it's likely to put out. Use a variable resistor to work out what voltage divider ratio I need to make it work, then substitute a pair of resistors in the final version. That's 3 components and it should be easy enough to put them in heat-shrink or epoxy to make them weatherproof. You'll probably pay as much for the connectors as the components :)

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Alas, the ReeCharge alone does not appear to be sufficient. From the manual: "Do not charge the ReeCharge when using a light that is connected to the same dynamo hub. When using the light and charging the ReeCharge simultaneously, the power from the dynamo hub is split between the two devices, which lowers the brightness of the light, which can compromise safety at night." –  rkjnsn Apr 25 '12 at 23:35
    
One subtlety is that the dynamo acts as a current source not a voltage source, so it may be possible to run the light and charger in series. But that will require a (manual) switch to pull the charger out when there's not enough power to run both. You were looking for an automatic solution so I didn't suggest it, but if you're willing to have a switch that would work. –  Kohi May 2 '12 at 22:45
    
The voltage sensing switch will still work. So will the regulator @Ehryk suggests. –  Kohi May 2 '12 at 22:46
    
The problem with a both the zener diode solution and a transistor solution is that they'll both drain some of the current (more than a LDO). The zener will have a non negligible voltage drop (usually 0.6v) and require a small amount of current, as will the transistor (between the gate and collector), though it will have less drop (0.2V). Honestly though, I'd just put a switch on it and switch it off when the light is required. –  Ehryk May 9 '12 at 9:56
    
I have run both a ReeCharge and a Lumotec IQ Cyo T Senso Plus without much problem at night. The light is dimmed slightly, the degree of which lessens with increased speed and was barely noticed. The warning is likely to protect the company against any possible lawsuits. –  Rider_X Dec 28 '13 at 22:00
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http://www.bumm.de/produkte/dynamo-scheinwerfer/lumotec-iq2-luxos.html Light with an integrated USB socket for charging, but it's not clear from reviews how the USB charger and light will interact when using the output at the same time as the light once the cache battery is drained.

http://www.bumm.de/produkte/e-werk-usb-werk/usb-werk.html (warns that light output will be reduced while charging) http://www.cycletourer.co.uk/cycletouring/electrogadgets.shtml

More options on http://cyclingabout.com/index.php/2012/03/list-of-hub-dynamo-power-supplies-for-usb-devices/ (but mostly chargers that can be used when not running a light).

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