How to Charge Any USB Device by Riding Your Bike

Step 6 The Actual Circuit!

Tackling the circuitry was the most difficult challenge of the process. Electricity from the motor first travels through a voltage regulator which will allow up to a continuous five amp current; a larger current than other regulators would pass. From there the voltage is stepped down to 2.5 volts which is the maximum the BOOSTCAP can store and safely handle. Once the BOOSTCAP attains 1.2 volts, it has enough power to allow the MintyBoost to provide a 5 volt source for the device being charged.

On the input wires we attached a 5A diode so that we don't get an "assisted-start effect," where the motor would start to spin by using the stored electricity.

We used the 2200uF capacitor to even out the power flow to the voltage regulator.

The voltage regulator that we used, an LM338, is adjustable depending on how you set it, as seen in our circuit diagram. For our purposes, the comparison of two resistors, 120ohm and 135 ohm, connected to the regulator determines the output voltage. We use it to reduce the voltage from ~6 volts to 2.5 volts.

We then take the 2.5 volts and use it to charge our ultracapacitor, a 140 farad, 2.5 volt BOOSTCAP made by Maxwell Technologies. We chose the BOOSTCAP because its high capacitance will allow us to hold a charge even if the bike is stopped at a red light.

The next part of this circuit is something I'm sure you are all familiar with, the Adafruit MintyBoost. We used it to take the 2.5 volts from the ultracapacitor and step it up to a stable 5 volts, the USB standard. It uses a MAX756, 5 volt boost converter coupled with a 22uH inductor. Once we get 1.2 volts across the ultracapacitor, the MintyBoost will begin to output the 5 volts.


Our circuit 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

[ctonks]

How do you handle surplus power?

[name101]

That's What the 140F capacitor was used for. In the graph explains that the surplus power is stored in the Ultra capacitor ~Name101

[ac-dc]

Such a capacitor would store power, but will not "handle" surplus. When the load current drops the input voltage will rise and the linear regulator will be dropping more voltage, resulting in a higher heat level. You'd need far more than a capacitor to store the energy, something like a battery of a capacity high enough that it never reaches full charge, and yet this would be an additional drag on the bike, it should not be used because the whole circuit is wrong in the first place.

[BC-45]

would a 3000 farad cap work?

[PCfreak]

7.2v RC Battery is Lightweight and ive used one in many applications

[name101]

The way I see this design, The Supercapacitor was not meant to store "Large" amounts of energy but enough to keep the power consistent if the rider was to slow down or stop for a short period of time.. I agree a battery would be able to store a lot more energy than a Capacitor. but the capacitor is short term. Correct me if I'm wrong. ~Name101

[ac-dc]

Of course, a supercap is short term. The problem is, a supercap doesn't have enough capacity to *buffer* for regular riding. That means that once the supercap is charged the voltage in the system rises and creates even more heat and loss with the linear regulator.

Let me put it another way. I don't revel in wasting energy but it's not so much a concern when something is AC wall powered. When you are peddling on the other hand, and a design has multiple forms of loss, energy conservation is really good, worth the time to do it right.

The problem was they didn't look at how to get from point A to point B, point A being a human being producing linear movement of a wheel, and point B, producing the desired charge, then finding the best way to get there.

Instead, they reused a design not just suboptimal for the purpose but contraindicated for the input and output.

The way this is set up it would be far better to just strap a battery pack onto a bike to recharge something, or of course to use a proper bike generator, a switching supply circuit that accepts (uses) input over the voltage variations that result from a bike generator, and and output with current regulation and the associated charge control chip complimentary to the battery type being recharged.

I appreciate this is beyond the ability of someone starting out, but at the same time this is what is great about technology today that we have ready-made ICs to do things difficult or lengthly to do with discrete parts. It is good to experiment but it is also good to see when it is redundant work, that each part of the problem can be seen modularly as how to get from point A to point B and that today we have great custom ICs to do these jobs (since when broken down into units, none of the things being done are new electronically).

I think it's great if they had no hands-on experience, to learn from building something like this, BUT to put it out there for others as an example of how to get something done, it is a poor one.

[name101]

Well Personally am new to electronics.

I see that stepping down then stepping the voltage up again is a waste of energy. but when the power supply is so close to a LDO linear regulator I thought Providing consistent energy would be a problem.

You said "supercap is charged the voltage in the system rises and creates even more heat and loss with the linear regulator."

How is this so? I would like to learn from this experience.

I would like to learn how to make this as efficient as possible.

(I'm sorry about the short comment I wrote a big one but pressed cancel instead of post =( )

~Regards
Name101

[ac-dc]

To make it as efficient as possible, to start you would approximate an efficiency, perhaps 80% pulling numbers out of air. Next determine the required charge current for the device, and divide that by 80% or 0.8. Next determine the minimum dropout voltage for the regulation stage, some working familiarity with switching regulator designs helps a lot here, but just to pick a number a run with it until something more is known experimentally let's say 3V dropout across regulation stage. With this much info we know the minimum output voltage a generator will need, and can begin testing generators loaded to the desired output power (current times pre-dropout voltage) to see if they are sufficient. Once one at least capable of this output power is selected, the max and minimum voltage during riding with it powering the same load level will tell us the voltage range the switching subcircuit will have to accept. Next go to a website like Digikey if you don't have a regulation controller in mind yet. Browse through their offerings for a regulation chip capable of the input voltage variations and either fixed output for the device charging, or variable output with components you add to select the target voltage. Study the regulators example datasheet circuits as they provide the basic topology as well as reading any datasheet notes about precautions and requirements for it's rated functionality. Then you are ready to convert a rough sketch of circuit logic blocks into a schematic of electronic component values, and generate a PCB layout. Prior to that you probably also want to have selected a chassis for the circuit board, a vibration resistant mounting, and have therefore determined the dimensions of the PCB and keep-out zones for mounting hardware so you can plan the circuit layout around these dimensions. Obviously there's a bit more to it than described, but it is a start.

[ac-dc]

With a variable input voltage (6V is not constant) an LDO will be a problem, but even if kept above the LDO regulated voltage it's still a very lossy circuit. With the supercap you are improving charging, but putting more drag on the rider to generate that charge, and by keeping the voltage higher it is continual drag instead of the voltage being allowed to drop below the critical level needed to power the rest of the circuit so the rider has no moments of relief. So it does improve the circuit but it does not meet a different goal in use on a bicycle, plus it adds a lot of weight. With a battery on the other hand, or really I mean a pack of series cells, they serve as a crude but effective enough form of regulation to keep the voltage at roughly the sum of the cells. There is some loss in doing this too, but not as much as use of an LDO. One thing that is not really clear is why either would be needed, charging doesn't have to continue at all moments necessarily. It could be good to even tweak the circuit such that applying the brakes bypasses a current limiter right after the motor so that during braking a higher % of power is produced and more motor drag, then less when not, but it would be a secondary issue, the primary one being that the better solution is none of these supercaps or batteries, to just use a wide input voltage, regulated output switching circuit. I'm not suggesting no capacitors are needed for that, but rather less capacitance, a smaller design with only the parts the regulation stage needs, also built more rugged (leaded parts like capactors don't like vibration much as you'd see on a bike, you can cement/glue/etc them in place or you can pick parts more immune to the environment).

[name101]

Personally. I want to charge my device a constant as possible(GPS device). Once the Supercap is fully charged the Circuit will only draw the current needed to maintain the charge on the device. Correct? When I was doing the Research with my basic knowledge Something similar to this was perfect. I was aware of the energy loss through heat and converting the voltages 2 times is excess but the way I saw it was the voltages(form power source to USB device) are too close to use the LDO linear regulator. IF the hub generator produced 12v for example having a single Linear regulator would be perfect. Simplifying the circuit drastically. If the supercap was replaced with a battery pack for example would the circuit charge the batteries and would the batteries act like a capacitor? From my understanding capacitors have the advantage of being able to charge or discharge incredibly fast while if a battery had the same current draw the battery it self would be damaged. I completely forgot about the environment with my research. I must thank you so much with helping me here. ~Name101

[ac-dc]

The generator voltage is not "close" to the LDO regulator necessarily, it depends on the load on it and of course gearing or friction wheel diameter, how fast it is spinning. A genuine 6V bike dynamo goes quite a bit above and below 6V even being designed for the task. That's why it's normally it's normally rated for wattage like 6W or 12W. I'm not saying it's terribly bad to have the supercap except it puts more drag on the bike at the worst possible time, getting started from a dead stop after having discharged. A constant charge isn't really needed, you only need to limit the max voltage and current within what the battery will accept. For example, suppose 20% of the time it wasn't charging, but the rest of the time it was charging at a high enough rate to compensate. The point that I'm attempting to keep drifting back towards is this needs to be seen as an accessory to bicycling, not the primary detail that matters so much as what factors are present while riding. Yes the batteries would charge like a capacitor, the point would mainly be that you can either: A) Have already charged them before the bike ride so they are a renewable portable power pack. B) Shut off the charging circuit before it drains them too much, then turn on again after having gotten up to speed so they aren't putting a high early drag on acceleration. You really don't want a linear regulator, not even an LDO. The voltage from a motor varies quite a lot, to always have it charging you will have to use something larger and create a great deal of heat as well as extra drag riding. The only way to have a small drop before the linear regulator is if you already had a wide input range switching regulator, but if you did then you wouldn't need the linear regulation stage at all. I have already outlined a much better way to do it, for good reason, that it should not be done a different way for bike powered use.

[name101]

Oh. I'm starting to understand. Haha. I suppose I'll keep this in mind when I'm designing future circuits. I did throw myself into the deep end by starting with this projects with very little prior knowledge. Ill Look into what I can do.

Seriously, Thank you so much. =]

~Regards
Name101

[lmaclean]

I have looked at the circuit and I can't see where to put the 140f supercapacitor. Can someone please help me?

[kerembasaran]

A 140F capacitor is just crazy.

[ReCreate]

i have seen 300 Farad caps

[uberdum05]

I've seen a 3000F supercapacitor on http://www.wima.com/EN/supercap_r_2.htm

[agent harmsy]

add a transformer, hook it up to a coilgun and watch it blow up a HOUSE! :D

[jevoncarlson]

The supercap post ride charge is a fun idea but who would actually leave their ipod on their bike while not riding? security issues.

[Algag]

 if you are talking about how it charges even if you are not moving thats so if you are stopped it still charges, not to leave it on there when you are done

[jenninstructables]

.... this supercap is hard to find... does anyone know of a source that will sell ONE ultracap at a time?

[harpleblues]

 I found the capcitor available in quantites of one at www.tecategroup.com/store/index.php

[jenninstructables]

man... SO long it took me to locate these

BCAP0150 - 150F ultra-capacitor Mouser Electronics - yay!

i'll post more later.