How to Charge Any USB Device by Riding Your Bike

Step 3 Design a circuit

To start off, we had to design a circuit that could take the ~6 volts from the motor, store it, and then convert it to the 5 volts that we needed for the USB device.

The circuit we designed complements the function of the MintyBoost USB charger, originally developed by Limor Fried, of Adafruit Industries. The MintyBoost uses AA batteries to charge portable electronic devices. Our independently constructed circuit replaces the AA batteries and supplies power to the MintyBoost. This circuit reduces the ~6 volts from the motor to 2.5 volts. This allows the motor to charge the BoostCap (140 F), which in turn supplies power to the MintyBoost circuitry. The ultracapacitor stores energy to continuously charge the USB device even while the bike is not in motion.

Comments

[emihackr97]

I don't understand, why regulate down the voltage to then boost it up, it's nonsense!

[jenninstructables]

I've got the added task of recifying the output of the source, as it's actually an AC alternator, as opposed to a DC generator, or dynamo - which seems to be the subject of this instructable.

At least a couple of companies (Shimano being one of them) make these alternators which are built into the hub of the front wheel. Mine puts out a fairly steady 6vac at speed, but is slews to 0v at stop.

Has anyone addressed this previously?

[dirty_valentine]

I really like this instructable, but the circuit is not optimal. Stepping down from 6V to 2.5V with a linear regulator such as the LM338 is inefficient. In this case only about 42% of the power from the generator will make it to the Minty Boost circuit. The Boost circuit will have further losses lowering the efficiency down even more. The circuit will no doubt work but it is not optimal.

If the generator really puts out 6V, then a simple 5V output Low Drop-Out (LDO) regulator such as the Linear Technology LT1963A (http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1040,C1055,P1879) could be used to drive pin 1 of the USB directly. In this case about 83% of the generator power would make it to USB device. The USB device would charge twice as fast, and the circuit cost and complexity is significantly reduced.

If the generator typically puts out less than 5V under load, a 5V output Buck-Boost converter could be used. This would provide for further efficiency improvement. I don't feel the added complexity and cost is justified if the generator can put out more than 5V under load though.

[rpvanpatt]

 I actually was thinking the same type of thing, but i am a newbie with all this stuff, I was wondering if a capacitor would be good tho, so that if the bike comes to a stop, the charging wont stop, i feel like that may hurt the battery to stop and go, is it?

[dirty_valentine]

I=C*dV/dt is the equation for a capacitor where I=current, C=capacitance and dV/dt is the change in voltage with time. rearranging the equation you get: C=I*dt/dV. If you are riding along and getting 6V out of the generator then suddenly stop, your capacitor will be charged to 6V. If a linear regulator is used as I suggested above It will start dropping out around 5V, and most LiIon chargers will stop charging around 4V. Thus best case you can only discharge the input capacitor from 6V to 4V (dV=2V). As for dt, that depends on how long you plan to be stopped. Lets assume you want to stop for 10seconds (dt=10s). Most LiIon chargers charge at 100mA or 500mA. I'll assume the best case (I=100mA). With this information we can figure out what size cap we need: C=100mA*10s/2V=0.5F. 0.5F is a REALLY big cap and would only buy you a best case stopping time of 10seconds.

Most likely any hand held electronics you buy today (cell phone, media player, etc) will have a LiIon type battery. LiIon batteries do not like to be overcharged. If overcharged they will start to loose capacity (best case) or can even burst into flames (worst case). The good news is that most quality electronics have built in protection circuitry that keep from overcharging the battery. I have not heard of any issues with starting and stopping charging. In fact some quality chargers do just that to measure the Equivelant Series Resistance (ESR) of the battery during the charging process.  Hope that helps. 

[rpvanpatt]

 thanks for the help...another idea...  if you havent already seen the 'contactless dynamo' bike light instructable, I was wondering if it would be possible to use that idea and your info to create a USB charger instead of with a motor...im not sure how much output voltage the contactless magent generator creates though, but I think this would be a better idea

[dirty_valentine]

Contactless induction can certainly be made to work. There are a lot of variables and I do not have much experience with this type of generator so I am not sure if the 'contactless dynamo' instructable will work in this application. My feeling is that the magnets turning at the rate of the wheel will be too slow and too far away from the coils to generate enough power.  FYI - it is really power that you are interested in and not voltage. My guess is that the peak voltage is probably high enough open circuit, but as soon as a load is applied (i.e. while charging) the voltage will collapse and charging will stop. To really make this work you will probably need an inductorless motor geared up off the wheel or chain to produce enough power to sustain charging.

[rpvanpatt]

gotcha...would they be good with just led lights tho? and maybe a stepper motor to charge?

[dirty_valentine]

Not sure, as I said I do not have much experience with generators, just a basic understanding of how they work. You would have to experiment or seek out knowledge elsewhere. Good luck.

[Ohnanka]

Yey, constructive criticism! : ) would've been nice to have some from ac-dc.

[advante]

I need 4.9 volt for my Sony Ericsson phone. Does it matter if over-voltage 0.1 volt? Cause 0.1 volt is matter for computer's processor. Please give a reply on [e-mail]

[Jodex]

You CAN always charge with a lot higher voltage than your battery needs. BUT if you use enough big voltage your battery MAY explode. But couple of volts doesn't matter.. Except LiPos may be bit different.. Sorry for the words written with BIG letters : D

[jules15]

no man, thats fine

[Alexbretherton]

complicated

[capricorn1]

agreed. highly inefficient. the key is though that you don't get 6V the whole time. I think you should just be using a switch mode design that can take any voltage and output 5V. Any as in ~1-12, these circuits are easy to find online. This way, you would be at about 85-90% efficiency.