Parts List - First Revision

I just put together a full parts list for this board revision. I should have just ordered parts when I ordered the boards, but I thought I had everything I'd need. Oops. It turns out that I still need the battery holders, some LED's ( i have no idea where all my leftover LED's are...) and the NPN switches to go with my PNP current regulators. Heres a short form of the parts list for the first revision... this is sure to change in the future.

• 6x Schottky Diode (Fairchild MBR0520L)
• 2x NiMH Batteries (Panasonic HHR-70AAAB8)
• 2x AAA Battery Holder (Keystone 2466)
• 1x Boost Converter (ON Semi NCP1402SN50T1G)
• 1x 555Timer (Intersil ICM7555CBAZ-T)
• 3x Logic Inverter (Fairchild NC7S04P5X)
• 1x Logic "Or" (Fairchild NC7SZ32P5XCT-ND)
• 2x Current Regulator (NXP Semi PSSI2021SAY,115)
• 2x NPN Switch (NXP  Semi PDTC124XT,215)
• 3x SPDT Slide Switches (APEM Components NK236WH)
• 4x Red 1mA LEDs
• Assorted Resistors (1x 5.3KOhm, 1x 10KOhm, 2x 620Ohm)
• Assorted Capacitors (9x 10uF, 5x 1uF, 1x 0.56uF)
• 1x Inductor (47uH, Sampled from Coilcraft MSS6132 Series)

Thats about it. The LED's will probably change to a surface mount with a shiny backplane to reflect light, and the three slide switches will change to a single multi state push-button or a single slide switch in the next revision. In total, the parts I don't have should cost about $5.40 plus shipping. Ill order tomorrow morning, and hopefully finish building the prototype boards this week.

PCB's - First Revision

The new boards came today! they look OK, ill get to building and testing when i have a bit more time. I may build it all on this one PCB first, but really its 3 parts,

1. The main converter plus batteries. This is the large square on the right.
2. The coil board, this is the small square with a notch in the lower left.
3. The LED board, this is in the upper left, and has pads for just 4 LED's

I'm running low on some parts so ill have to wait a bit to finish populating this, but i should be able to test the coils and blink logic without much trouble.

Layout - First Revision

I finally finished the board layout for my first revision. It's split into three small boards. The first (lower left in the picture) is for the coils and the rectifier. It has some capacitance as well just  to smooth things out. I wanted to have the option of placing this circuitry next to the coils, instead of up with the batteries, as  it seems like a better idea to have a DC voltage travel from the rim to the seat rather than a pulsating voltage. The main portion, including the boost, the batteries, and the blink logic is on the right side of the image. One battery is on the top, with the other on the bottom. This gives a total area of 0.825"x2.0" or so for the main board. This small form factor isn't really needed for this first revision, but i wanted to see how easy this was going to be to layout the board small enough to fit in a seat post. The last board just has places for the four diodes. this is so i can test different cable lengths between the main board and the LEDs. Ill send this out for fabrication, and hopefully have a prototype soon.

Schematic - First Revision

Ive come up with a first draft of the schematic. It uses mostly left over parts from other projects, so shouldn't cost that much to finally build. Unless i find a better way to build the first two stages of this generator, ill probably lay this out and get a PCB fabricated. Not all parts are defined (ill post a full parts list before i send out the PCB)  but some of the more important ones include

• Boost Converter - NCP1402. sort of a random choice, this was the first reasonable boost IC that i found. If i decide to place the boost before the batteries again, ill design my own with the correct input impedance to get maximum power, based off of the plots i posted earlier.
• Timer - ICM7555. This is an improved version of the regular 555 timer that people have been using for years. It has lower power draw, and most importantly i have some sitting around the lab.
• Schottky Diodes - MBR0520L. Just a fast, low on voltage diode. One of the recommendations i got from the energy harvesting people.
• Current Source - PSSI2021. A really simple chip that provides a constant current source. It implements one of the usual PNP current source circuits.
• LED on/off switches - PDTC124XU. These are simple NPN switches, with some built in resistors. A recommended part from the current source datasheet. Eventually i may redo the LED drive circuitry to be a bit better, but for now this is OK. Eventually ill remove this and the previous part, and just design my own stage here. But for now these two IC's are easier to use.

Hopefully ill be able to get the layout done this weekend, and have a full working prototype by the end of next week. Then i can refine the power stage, find some better LED drive solutions if i need, and get a bike ready design within a few weeks.

Slight Redesign, Batteries, and a Boost

I decided to redesign the input stage. Ive placed the batteries in parallel, and the boost after the batteries instead of before them. The new battery voltage is 1.2v, which is fine for the boost and works well for direct charging off of the full bridge rectifier. I bought two 700mAh NiMH cells. If these are trickle charged at below 1/10 their max rate, (so 70mA), they can be continuously charged without hurting them or severely reducing their lifespans. With my input stage, 3mA is about max at this voltage (see the results from my last post), so i think this will work fine. I hooked up the boost after the batteries as well, and added a switch to turn it on and off.

Here the coils are seen charging the batteries (green waveform is battery current, positive is into the battery), then the boost comes on, (yellow waveform) and then switches back off. Blue is the battery voltage. Ill add a larger input filter to the boost later. Ive now got a decent 5v supply, that runs off of rechargeable batteries. These batteries are charged with a coil through a full bridge and filter. Now i just have to design the timing and logic circuits for the LEDs. Ill probably make a few modifications to the coil/rectifier circuit (Stage 1) and the boost (Stage 2) as well, just to increase efficiency, but what I have now is enough to build the rest of the circuit off of.

Output Power, First Tests

I took some first output power measurements today, at the suggestion of one of my lab mates. Things look fairly good i think. I took two sets of data, one with 4 magnets (magnet passes the coil every 27ms), and one with 8 magnets (magnet passes the coil every 13.5ms). As expected, this just increases the output power. Also interestingly, it seems like i may want to load the circuit heavier than i was going to, as i get better output power at about 2mA load. perhaps ill regulate to this current. Ill redo these plots for the final coil and rectifier design, so that i can achieve the maximum power point on my final design. On that note, i asked about the diodes that the energy harvesting people use, and they recommended some good ones. Ill order some, and see how much better they are.  These are good results! Its nice to know that this can actually drive a load.

Magnets

I'm waiting on some parts right now to build the next stage, so i thought id go over the magnets that i'm using while i wait. I'm using Neodymium disk magnets (Neodymium Iron Boron, generally Ne2Fe14B, to be precise). They are slightly larger than a dime, and quite strong. Neodymium was chosen both because is stronger than most (all) other magnets and because this type of magnet doesn't loose its strength when it gets knocked as easily as some other types.  This is the same type of magnet in most hard-drives.  They are also fairly light, and although relatively speaking they loose their magnetism at a low temperature with respect to other magnets, that temperature is still fairly high. (the Curie Temp is 590deg-F for mine. So probably high enough. Max operating temp of 176deg-F)

The 'Grade N42' number on the package tells us what material the magnets are made of, with higher grades generally corresponding to stronger magnets. The place i bought these from sells N35-N52 (in the 'normal' temperature range type), so Grade N42 (the one i got) is sort of middle of the road. This grade has a "Residual Flux Density" (another name for Remanence i believe), commonly Br,  of 1.3-1.32 Tesla.  Using this we can approximate the field density (or magnetic flux density) as we move away from the magnet, seen below. The surface field here (at distance = 0mm) matches nicely with the manufacturers data, which is comforting.

As we need large changes in magnetic field to generate power, from this plot we obviously want to keep the magnet as close to the coils as possible. Currently, i'm using a spacing of 1-2mm, which seems reasonable on a bike wheel. As the coils i'm using are not perfectly flat, (some windings are further away than others) ill use 7mm as my 'average' distance.

If we take one of the plots of the voltage generated by the coil passing a magnet,

we can integrate to get an idea of what the field strength (generally denoted by a capital 'Phi', sub 'b', measured in Webers (Wb), or magnetic flux) looks like to the coil as it passes.

This is sort interesting, and shows a max field strength of about  3.15uWb in each turn. The coils i'm using have a diameter between 1cm and1.5cm, so the magnetic flux density seen can be calculated to be about  30mT. This is a bit lower than what we would have expected from the ideal equations (about 50mT or so), but in the right area so i think its OK.

In any case, none of this is that important. The magnetic field plot is sort of neat but not that meaningful. You should just buy the strongest magnets you can, and then place your coils as close to them as possible.

A Voltage Source!

Just got a chance to throw a full wave rectifier and some caps onto the coils from before. This results in a nice 2v source at about 18mph. woot! startup transient seen above (yellow is cap voltage, 1v/div, 2s/div). The output voltage is proportional to the speed, so at about 25mph it hits 3 volts or so. this is OK. Ill add a protection Zener anyway, but i think these are good levels. Ill add a boost to this stage to get the voltage high enough, and then a current regulator to charge the batteries.

Some Initial Experiments

I took the first experimental results today. Using an old DC motor that we usually twist litz wire on, and a piece of scrap material i built a scale wheel. it has 4 magnets around the edge, oriented north-south-north-south. The coils used for these waveforms have 300 windings on the old PQ bobbins. There are two coils in series, which is sort of odd since they don't get excited by the magnetic field at the same time. But it seems to work. Each magnet is 24cm apart, so the first waveform is analogous to a 12mph ride, while the second is a 21mph ride, roughly. These numbers are mostly just important for the amplitude of the voltage spike. more magnets can be placed for more frequent spikes, but the velocity of the magnet as it passes the coil is (sort of) proportional to the spike amplitude (Faraday's law again.) Ill optimize for a 17-18mph ride, adjusting the magnet spacing and number of turns to suit my power output needs.

Coil Prototypes

Found some old bobbins and coil formers to create the windings. The wire is #32 AWG. This is OK for the prototype, but for the final piece ill use much thinner wire and a better coil former (something that has a bit of a lower profile). Im using the larger wire for now as anything smaller than #32 is a pain to wrap, it just breaks to easily.

I wrapped a couple of test coils on the small coil formers. These have 50, 100, and 150 turns respectively. The voltage spikes caused by the coil passing the magnet should be proportional to the number of turns (Faradays Law -- special case with multiple coils of wire, Electromotive Force is equal to the number of turns times the time derivative of the magnetic flux. so... more turns is proportionally more EMF, if we keep the speed and distance between the coils and magnets the same. Electromagnetic Induction. ) These three coils should let me show this relation. Then i'll extrapolate the number of turns i need for the voltage spike amplitude i want. I think it will be quite a few more than 150 turns in the end.

 I built two prototypes, both with full bridge rectifiers on the outputs, and just a resistor (100Ohm) to load the circuit. The left prototype uses the 150 turn coil, while the right most one uses a new 300 turn coil made on an old PQ bobbin i found.  Im a bit worried about the voltage drops on the diodes, but ill test it tomorrow to see if this will work. If not, ill just use a single diode half wave rectifier. (The full bridge rectifier catches both positive and negative voltage spikes, but causes two diode forward voltage drops. As i'm dealing with low voltages, this may not be ok. A better option at this level, may be to just use a single diode. This will loose half the energy from the coils as only positive spikes can be harvested, but may be better overall. We'll see. Full Wave Rectification. ) The energy harvesting guys will be in on monday as well, so ill ask them if they have leftover rectifier diodes for low voltage applications. They should have just what i need.