How do I convert a small AC current into DC current?

[Xkaliber]

I am constructing a small wind generator for a college project. I want the 0.1 V AC output of the generator to charge a battery. To do this, I need to convert the current into DC. I am familiar with things like bridge rectifier circuits, but the smallest diodes I can find have a forward "turn on" voltage of about 0.25 V, which will not even turn on for my small 0.1V signal. What are my options for getting the AC current output into the battery? Thanks

 

[vk6kro]

I am constructing a small wind generator for a college project. I want the 0.1 V AC output of the generator to charge a battery. To do this, I need to convert the current into DC. I am familiar with things like bridge rectifier circuits, but the smallest diodes I can find have a forward "turn on" voltage of about 0.25 V, which will not even turn on for my small 0.1V signal. What are my options for getting the AC current output into the battery? Thanks

Even if you found some perfect diodes, that isn't enough voltage to charge any type of battery.

Maybe you could modify your wind generator to give more voltage out? Even a simple "hobby" type motor used as a generator should generate a volt or two of DC.

 

[Proton Soup]

since you say the signal is AC, you could try leveraging the voltage up with a transformer.

 

[schip666!]

Or gearing up the drive between the blades and the {alt,gen}erator so it runs faster.

 

[sophiecentaur]

Your 0.1V output voltage is very low. Is this because the turbine is rotating very slowly (not much wind), the model is very small or because your generator is not a good design?
Is there, perhaps, a fault somewhere?
I think these things need to be addressed before going further with the project.

 

[Xkaliber]

Your 0.1V output voltage is very low. Is this because the turbine is rotating very slowly (not much wind), the model is very small or because your generator is not a good design?
Is there, perhaps, a fault somewhere?
I think these things need to be addressed before going further with the project.

Well, the project is to build our own wind flutter generator. (http://www.popularmechanics.com/science/energy/solar-wind/4224763)

The voltage is created by small magnets moving near a metal coil so it is not going to output a large value of power (or voltage). The output of 0.1 V was just a single run with a rather sloppy coil of 500 turns using 36 gauge wire. Our real coils are probably going to have at least 5x more turns. I was just trying to look at some of our options so we would have an idea in mind when it came to actually assemble the output circuitry.

In some other article or interview (not the one in the above link), the inventor says that he boosts the low voltage generator output with a device that can be found in any old radio. However, he never mentions what it was specifically. Does anyone have any idea what the device may be that he is referring to?

 

[vk6kro]

That explains the low output.
I think the author might be a bit optimistic about powering the world from such devices.

He may be referring to the ferrite rod used in the antennas of portable AM radios.

If you put some of this material above the top coil and below the bottom one, you might get an improvement in output.

You cam also get larger diameter, very thin Neodymium magnets. These might work better than the small button types.

http://www.dealextreme.com/details.dx/sku.13517
http://www.dealextreme.com/details.dx/sku.10305

 

[Mike_In_Plano]

Yep, this think looks more like conjecture than engineering - like those generators that were supposed to gather power at the sides of the road from the wind of cars racing by...
There's some pretty solid design work out there based on the good ol' propeller. From there, you can add a DC brushless motor, a motor from an exercise machine, or even an induction motor ( if you have a good design ).
Seems like there was an article out about a teenager in a third world country, building a generator out of car parts, PVC pipe and a wooden support. Just imagine what that was like! He probably didn't have so much as a decent drill.

 

[sophiecentaur]

Having seen your picture, I think I can see that the reason for such a low output voltage is the physical arrangement of coil, core and magnets. You need to have a very low reluctance magnetic circuit, with tiny gaps and a good coil so that you get a good dϕ/dt and lots of turns going through the field.
With your arrangement I should imagine it would be very possible to monitor the movement by amplifying the signal but it is another ball park to get useful power out.

 

[Xkaliber]

Hi all,

Just a quick update with some additional questions. We have improved our design so our output for a single belt is now a 2.5V AC output @ 8mA. This should be enough voltage to rectify with a full wave bridge rectifier using Schottky diodes, correct?

Also, the 2.5V and 8mA are only averages and are highly variable even at a constant wind speed. For instance, the output voltage can range from 0.5V to 3.5V over a period of 2-3 seconds. If I rectify this signal, then I will have a DC output with a similarly varying voltage, correct?

I am still wanting to store this rectified output in a battery (two AA's, possibly). Can a varying voltage DC current be delivered to a battery or does the voltage need to be constant? I guess I am wanting to know if the rectified generator output will need to be "cleaned up" somehow before sending it to a battery.

 

[vk6kro]

The battery will clean it up OK but you would take 250 hours (10 days!) to fully charge a 2000 mA/H NiMH battery if you could get 8 mA into it after rectifying the AC.

You would need to rectify it with a Schottky diode (BAT85 or similar) or a Germanium point contact diode (OA79 or similar) to keep the loss across the diode to a minimum.

You could try it with one battery first.

 

[sophiecentaur]

However you use the electrical power your present design of generator is providing you with, you can do a lot better by improving your generator electrically. It's the heart of the system and should receive the maximum attention. The 'wind-belt' could well operate well as long as you get the tension right - that would be easy to adjust as you go. What is essential is that you get the magnetic circuit right. Modern button magnets can provide high flux but this needs to be linked to the coils well. Many turns of wire (hundreds, possibly?), wound round a 'C shaped' yoke of iron (transformer laminations would be suitable for this) with the magnet vibrating across the gap, should provide good flux linkage (I think it's called). If you can look at the emf generated with an oscilloscope, you can see the actual generated waveform and optimise it by adjusting the gap. In any sort of generator, it's essential to get a good range of alternating magnetic flux from max to min.
Re the link: if Frayne is right in saying that his micro-system is more efficient than turbines then I'm sure you should be able to get several Watts from something of the size of the system in his movie. It is not true that there are no low-power turbines available. They are used on boats to provide just a few watts in moderate winds (up to about 50W in a stiff breeze) so you should expect better performance than you are getting. Assuming that your ribbon is moving correctly, then you must be able to get more out by improving the electrical generating bits - not the rectifying circuit, which can only give you a tiny improvement.
Matching the generator, mechanically, to the oscillating ribbon can make a difference, too. The distance of the magnet from the end could make a big difference to the power delivered - too far out and it will damp the oscillations and too far in will not get enough power because the movement isn't enough.

 

[Xkaliber]

You guys are awesome! Keep the suggestions coming.

Right now the plan is to parallel at least 10 of these together for our final prototype so our total output current that goes to the battery will be more than what we have right now.

Just a question about our voltage and current measurements, though. When we are measuring the voltage and current of a single coil, we connect the multimeter to both of the open-circuited coil ends and measure both the AC voltage and AC current in that position. So it seems like we are measuring the open circuit voltage and the short circuit current produced by the coil. If we connect this coil to a 1.5V battery (after rectification), will our current going into the battery be much less than the short circuit current we measured with the multimeter?

Another question. Assume we take the AC output from the generator, rectify it, and then send it to the battery. Where does the linear wire resistance (ohms/meter) of the coil come into play during this process? Looking at my attached image of the simple circuit diagram, is R_s the resistance of the coil (all ~100ft of wire in the coil) or is it something else?

Regarding the design of the generator itself, I think there are some improvements we can make. We found out last week that our belt can withstand larger magnets so we will be testing those out later today to see how much our output improves.

Our magnetic circuit may be as good as we can make it right now. As you can see in the attached photos, we have tried to improve the magnetic circuit by attached a small spring to the underside of the belt along with a small magnet hanging from the spring. The magnet hangs into a cylindrical coil so it is always inside the coil with a very small air gap while it is moving. If we make the air gap any smaller, I am afraid that the magnet may start rubbing the inside of the tube while it is moving and impede its motion. Additionally, we have to wind the coil around something (a very thin plastic tube in our case) and finding something just barely smaller might be difficult. We thought about doing some sort of iron core but we realized that the main advantage of the windbelt is that it is dirt cheap. Adding iron to each coil would increase the cost and, unless the gains were quite significant, the power/$ ratio would probably decrease. We are still trying to optimize the generator by testing magnet placement and other variables. For the windbelt Shawn Frayne was using in his video, he was only able to generate 40mW with 10 MPH winds. We are using 8 MPH for our testing.

 

[sophiecentaur]

If you plan to add the contributions of several generators then the addition must be done after DC conversion of each output because the relative phases (and even frequencies) of all the AC contributions would be random and would not produce a useful waveform. There will be a lot of 'beating' between the outputs and frequent cancellations.

I can't see that the addition of an iron yoke would be a "significant" addition to the cost. Scrap transformers come free and many of them can be dismantled. "kit' transformers are not very pricey either. You should really try something, I think. But it all depends on how far you want to go with improvements.
You will definitely find the project an interesting one even though it is unlikely to produce seriously practical results without further work. You will get a feel for the subject - which is what it's all about.

 

[Xkaliber]

Question about the coil:

As you can see in the first attached picture of my previous post, this coil is basically two smaller coils glued together with a plastic washer in between. During testing, all four leads (two per coil) are accessible. We have to connect a lead from each coil together such that the generated emf from each coil adds together. If we connect them together incorrectly, the emf's from each coil will subract and we will get a poor output. Correct?

This has been our thinking thus far. Well, today we made a coil about the same height as the one in the picture seen above, but it was made out of one continuous wire strand (basically one big coil). We did not get very good results with it. Here is my thinking:

Let's assume the magnet is in the middle of the coil and is moving upward. For the top half of the coil (where the magnet is moving towards it), the flux is increasing with time, and creating a positive voltage in the top half of the coil. At the same time, a similar thing is happening in the bottom half of the coil except, since the magnet is moving away, a negative voltage is created here. Since the top and bottom half of the coil have opposite voltages, the total emf(voltage) of the coil is very low.

Is this what is happening and does it make sense?

 

[vk6kro]

When the magnet is in the middle of the coil, the two poles of the magnet generate equal and opposite voltages in the coil (although in different parts of it) and these would tend to cancel out at the terminals of the coil.

It is not really a case of the magnet "moving away" as the flux that is generating the voltages is that that leaves each pole of the magnet at right angles to the line from pole to pole.

Like this:

[PLAIN]http://dl.dropbox.com/u/4222062/magnet%20in%20coil.PNG