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What is the easiest way generate AC from a natural source, and then transform it into DC? Answered

I have a school project where I have to generate AC, it can be a small amount, using wind or whatever; and then I have to turn it into DC.
To be honest I'm a bit lost, I've read you can do it with an alternator, but I couldn't find one in my local electronic store. I've also read that with two magnets and a square shaped wire, but I don't know how plausible it is to do it this way.

Any help is welcome, also if you could help me with how it should be connected would be great.

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Jack A Lopez

2 years ago

In a previous answer here, I promised some pictures of homemade alternator toys, and I think these pictures plus some explanation of what they are can stand on their own as a top-level answer.

Sort of the focus of what I was saying before, is that the most important part of a homemade alternator toy, is a coil with a very large number of turns, and the most convenient way to obtain that coil is to scavenge it from something else.

The coil in the first two pictures, for the twangy, hacksaw blade alternator, came from the 2-step, stepper coil, from a battery powered clock. I actually built a bunch of other circuits for this one. It can actually power that calculator in the picture, provided I keep plucking the twangy hacksaw blade, every few seconds or so.

I built this thing several years ago, and I am kind of thankful I attached some documentation to it, so I don't have to remember the details of how I wired it. I think the two diodes in the doubler circuit are germanium, for extra low (0.3 V?)forward voltage drop, but ordinary silicon diodes (0.6 V) would probably work too.

The pancake-shaped coil in the other alternator toy, seen in the last three pics, that coil came from a dancing solar flower, or some other kind of dancing solar toy, bought from the local Dollartree(r), for 1 USD.

The permanent magnets used in both these toys came from a product called "magnetic thumbtacks", basically something for holding papers to the side of a fridge, or filing cabinet. The magnets themselves are NdFeB, cylinders, approximately 5 mm in diameter, and 5 mm high. The twangy alternator has two of these magnets, on the swinging hacksaw blade. The spinning alternator has eight of the same kind of magnet, equally spaced around the edge of that white disc, arranged so the coil sees north and south poles alternating.

Both these toys need my hands to put the magnets in motion.

For the spinning disc alternator, I was planning on putting some wind catching cups, similar to an annemometer, but I never got around to it, and that is what the little arms made of copper wire are for.

twangy-alternator-toy-01.jpgtwangy-alternator-toy-02.jpgspinning-alternator-toy-01.jpgspinning-alternator-toy-02.jpgspinning-alternator-toy-03.jpg
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Jack A Lopez

2 years ago

These two instructables:

https://www.instructables.com/id/Windbelt-Redux--21...

https://www.instructables.com/id/Windbelt-from-hard...

show how to make an alternator, a source of AC electricity, using a "voice coil" from an old computer hard drive.

This, voice coil, is the part that moves the read-write head back and forth over the surface of the spinning disc. I think it is called a voice coil because the principles that make it work are similar to those for a speaker. Both are essentially a coil of wire placed in magnetic field of a permanent magnet. The main difference is the kind of motion. The voice coil in a speaker pushes a paper cone back and forth. The voice coil in hard drive swings an arm back and forth.

The prime mover for the these "windbelt" instructables linked above, is a long piece of ribbon, that vibrates, or flutters, when wind blows over it. One end of the ribbon is attached to the voice coil, and it swings it back and forth.

Also I built something similar, but different, using the stator from the tiny two-step stepper from a battery powered clock, with the moving magnetic field provided by some magnets attached to one end of a vibrating hacksaw blade.

I should upload a picture of that thing, because a picture would describe better than words can.

By the way, I think the secret to making little toys like this, actually work, lies in using a coil (or coils) with very large number of turns. The reason for this, is a consequence of Faraday's Law. For a single coil of wire, the induced voltage in this coil is:

Vinduced = N*(dPhi/dt)

where N is the number of turns, and (dPhi/dt) is the time rate of change of the magnetic flux through the coil.

So, from that equation it is apparent, one way to make Vinduced larger, is simply to make N larger.

Making (dPhi/dt) larger requires using stronger magnets, or faster motion between magnets and coil, or both.

Of those three choices, {larger number of turns, stronger magnets, faster motion}, a coil with large N number of turns is, in my experience, the easier way to go, for building a homemade, toy, alternator.

Moreover, it is nice if you can find that coil with large N, in some existing piece of junk.

I've already mentioned voice coil from hard drive, and stator coil from two-step clock motor. Another good source for a coil with very large N number of turns, is those dancing solar flowers and similar toys.

My town has a Dollartree(r), and for where I live, that's the easiest place to find dancing solar toys, and at 1 USD per, they're cheap enough to buy just to take apart for the parts they contain.

Somewhere around here I have some pictures of a homemade, toy, alternator, made from the pancake-shaped coil, removed from a dancing solar toy.

Regarding your question of how to change AC into DC, the usual way to do this is using some kind of rectifier circuit, made from one, or two, or four, diodes, and the Wikipedia page for "rectifier" has some explanations for these, here:

https://en.wikipedia.org/wiki/Rectifier#Single-pha...

The only really tricky part about rectifier circuits, is they don't work at all if the peak voltage of your AC is less than a minimum forward voltage, that is a characteristic of the diode itself. The ubiquitous, garden variety, silicon diodes have a forward drop of around 0.6 volts. There are diodes out there with smaller forward drop, around 0.4 volts, or 0.2, but these are somewhat harder to find. The voltage drop across light emitting diodes (also called LEDs) is much larger, but not huge, typically 1.5 to 3.5 volts, depending on the color, with more energetic colors having larger voltage drop; i.e. infrared<red<yellow<green<blue<uv, and that is something to consider if you want to use your alternator to drive some LEDs directly; i.e make your rectifier circuit out of LEDs.

Actually that kind of looks like what ToolUsingAnimal, author of the'ibles I linked to above, was doing with his windbelt alternator, from the video he made of it, here:

I will try to try to find some pictures of similar stuff I built. I've got them around here somewhere.

Oh, final thing. Instead of using a rectifier circuit made of diodes, it is possible to do the same thing with carefully timed switching. If that switching is done using mechanical means, the thing that does the switching is called a "commutator"

https://en.wikipedia.org/wiki/Commutator_%28electr...

A permanent magnet DC motor can be used as a DC generator, sometimes called a "dynamo"

https://en.wikipedia.org/wiki/Dynamo

Because this machine has a commutator, it magically makes DC, without a rectifier circuit, without diodes, although the DC it makes is kind of, uh, lumpy and bumpy. If smooth DC is desired, a capacitor, and maybe just one diode, could be used for to make this output voltage more smooth.

For this one, the trick of using permanent magnet DC motor as DC generator, I actually do have some pictures of my own I can link to right now, here:

https://www.instructables.com/id/Generator-Demonstr...


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Vyger

2 years ago

I remember seeing a flashlight that you charged up by shaking it back and forth. Apparently it had a coil with magnet that would slide back and forth in the middle and generate a small amount of power. Pretty simple but in the dark it would keep you busy.

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rickharrisVyger

Answer 2 years ago

And a bad reputation if anyone looks through the window. there are a few instructables to make a shake flash light.

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Josehf Murchison

2 years ago

You get an alternators from a specialty store or an automotive store, you can buy one off the net, most electronics stores won't carry them. An automotive alternator has a built in rectifier to convert the AC it generates to DC.

Google "axial flux generator" that might impress you teacher more than an alternator.

Then convert the axial flux generators AC to DC with a 3 phase rectifier. as long as the voltage is right you do not need a transformer.

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rickharris

2 years ago

Turn a magnet close to a coil of wire and out of the wire comes AC. Look up some of the projects to rectify it from the related column on the right >>>>>>>

This will get you started.

I expect thanks or I will not help a student again.

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iceng

2 years ago

What will you learn if every problem is solved for you and later you cannot hold a job because you do not have the ability to think about how to engineer...

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Downunder35m

2 years ago

Apart from a lot of possible Instructables and Google there are plenty of websites showing solutions for small generators.
From as small as a palm sizes windmill up to big unit powering an entire house.
Guess you need to spend more time using Google and finding the correct search terms ;)