Thursday, February 21, 2013

Zero-friction bicycle dynamo


I wanted this non-friction dynamo for my bike that you can buy, but then I thought, why don't I just make it myself?

I've always preferred dynamos above battery powered lights. There are several advantages:
- No need to change battery
- No concern with turning it off when not cycling
- Most of the time you can turn it on with your foot while cycling
- The superior feeling of satisfaction, using your muscles to produce light

My experience is, however, that normal dynamos often fail. Sometimes they simply stop working without any hope of repairing them, which has happened to me a couple of times. Less serious, but still not acceptable, is that they rarely work when it's raining, or even less likely, when it's snowing. Sometimes the position of the dynamo changes slightly and has to be readjusted. Another annoying fact is of course that it becomes significantly heavier to cycle.

My idea was to instead use magnets, coils and LEDs to make a dynamo. In that way you should be able to make it more robust, and hopefully solve all the problems I mentioned above.

As always, I searched the Internet to find out what other people have done before (see links at the end of the post). I saw one solution where one magnet was put to motion by other magnets on the wheel, but I preferred another one, where the magnets on the wheel simply pass close by a coil with an iron core. No moving parts at all in other words, besides the wheel.

This is the circuit I used:



I got a hold of two good coils, so I decided to create one complete system each, as seen above, for front and back lights. Very simple, as you can see. I chose to add a rectifier bridge to make use of both the positive and negative voltage peaks. Each diode has a forward voltage, which will increase the amount of voltage needed before current is flowing, so I used schottky diodes (they have a lower forward voltage).

Most people seem to use voltage protection on their circuit (with a zener diode). I'm not sure it's needed. If you measure the voltage of the open circuit you will probably see a higher value than the LED data sheet recommends, but the coil will not act as a perfect voltage source. The voltage will drop to the forward voltage of the LED, and the current will be saturated to a maximum level. If you don't have an oscilloscope it will of course be difficult to know what this maximum level is. In my case, since I use three LEDs in parallel I figured the current wouldn't be too high for each diode to damage them. They have been working fine for over a year now.

To find a good coil, look for an old relay (I found a couple in a heap of electronic trash at my fathers place). Some work better than others, so you may want to temporarily connect it to the LED and pass a magnet over it to test it, before attaching everything to the bike.


I used shrink tube to protect the windings, as seen below.


I bought a rectifier bridge from the local electronics shop, and placed it straight on top of the coil. To know what wire is what, I used red shrink tube on the positive, and black on the negative. To keep it all in place, I again used shrink tube, covering both the rectifier bridge and the coil (not seen in the picture).


As a final step for the coil, I used even bigger shrink tube encapsulate the thing (I know, it's starting to sound like an obsession). I made holes for the iron core (which conveniently was threaded at the end) and the two wires, and sealed the ends of the shrink tube with electrical tape. To prevent water from getting in through the hole for wire, I covered it with silicon.

There was already a metal clip on my bike, probably for use with bicycle bags, so I used that to mount the coil. As with most other dynamos, I connected ground to the whole body of the bike (by simply sticking it in between the nut and the metal when mounting). Since I had two coils, I placed an extra metal piece and mounted the second coil at the end of it.


My first plan was to use magnets from an old hard drive to put on the spokes, but by coincidence I found two reelight magnets on a trashed bike. Besides the extra effort needed to mount hard drive magnets to the spokes, they should be equally good, or better. The hard drive magnets are without doubt stronger. If interested, see the links at the end of the post. I placed my magnets pretty close to the nave and for me that gave enough brightness. The further away from the nave they are, higher currents and therefore higher brightness you will get.



I got the LEDs from old battery lights. I had to scratch some PCB traces on the backside, and solder a bit together.



As a final step, I used the cover of an old 2.5 inch usb hard drive to protect the coils, and to mount a switch (the switch finally broke after turning it on with my feet too many times, so now the lights are always on)



And the result!


(It looks like it's blinking slower than it actually is because many blinks come between the frames of the video)
Possible improvements:
  • I'm using two magnets. I could add at least two more to increase the blinking frequency.
  • The magnets I use are not very strong. If I use some hard-drive magnets, the intensity should go up.
  • The intensity of the light can simply be increased by placing the coil and magnets further away from the nave. At the same time, you might be need to add some protection in your circuit.
  • If you don't want the light to blink (in some countries it's illegal), a capacitor can be placed in parallel with the LEDs. I've even read of examples where a capacitor big enough was used, to keep the light on when not biking for a minute or so. In that case it will, however, take a while to charge it up.

13 comments:

  1. Pretty Usefull Post.

    when i was using my bottle dynamo, it felt like climbing a mountain, now I have almos the same light (with 8 white leds) but without the extra effort.

    Thank You so much!

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  2. Nice one! But cant see you video.

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    1. yes, would be great if you could repost vid. thx!

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    2. Sorry about that. Tried to fix it through blogger but it didn't work, so I uploaded it to youtube instead. Now all should work!

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  3. Can you make the electricity this dynamo produces to charge a phone?? I've read in other pages a normal dynamo can charge a phone. For doing that you connect the normal dynamo to a bridge rectifier and then to a micro USB cable. ANd when u cycle, u can charge the phone. Is it possible with that type of dynamo??

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    1. Hi there. The principle can certainly be used to charge a phone, but it becomes a bit more tricky. First of all, the current I generate with my dynamo most certainly isn't enough to make it useful. With some clever and careful design it should work though. There are front wheel hubs you can buy which uses the same principle, but has a lot of coils and magnets and places them very close to each other. You could do something similar, but again, that will take quite some effort. Secondly, you would need to make sure the voltage is at a certain level. Probably a voltage regulator would do, combined with a capacitor to make sure the input voltage stays above zero throughout the ride.

      A traditional dynamo would indeed probably generate enough, since they are made for incandescent lights which require much more energy. The downside is of course that you loose a lot of energy from friction, which makes a significant impact on your biking.

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  4. Hi, can you tell me the resistance and characteristics of that coil?
    I also salvaged some coils but unable to get any useful voltage. I just tested with a multimeter and a magnet in hand.

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    1. Hi! I think I measured the resistance at some point, but I'm afraid I didn't write it down... I think it was a few hundred ohms. The more resistance the better, I would say, since they are usually designed for more current than you need for the LEDs (more resistance means more windings and higher voltage, but also limits the current).

      My suggestion would be to simply connect it directly to an LED and see if it lights up when you pass a magnet next to the coil. If you're not able to generate any light by hand, it's not good enough.

      Also remember to keep something ferromagnetic in the core of the coil, otherwise it probably won't work.

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  7. Hi,

    I saw many hand-cranked mobile charges on different sites that use same dynamo principle. I do not know much about electronic stuff. Though i want to know how much voltage is required to charge a mobile phone and can a dynamo made with simple equipment provide that much power supply.

    Also in that case which will be better - a magnet and coil powered dynamo or a mechanical shaft powered one.

    Would be great if you enlighten.

    Thanks in advance.

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