Bicycle Generator Help

I think you mean capacitor, not compactor. I personally prefer cazapitor, which describes what happens when you short the terminals.

Attach a clutch and flywheel to the drive shaft on the generator. When the bicycle wheel is driving the idler wheel faster than the generator, the clutch is engaged and it drives the generator. When the bicycle wheel speed is slower than that of the flywheel, the clutch disengages and the flywheel continues to drive the generator. However, this is really crude and not very elegant.

Obviously, some calculations and measurements will need to be made to see if this will work. "Some LED's" is not a very good basis for calculations. A car alternator is far too big for this application. Look around for a small permanent magnet motor.

You can do as well using an energy storage super-cap instead of the flywheel. The mechanism is the same, except instead of a clutch and flywheel, you have the supercapacitor to store the energy. "Ultracapacitor powers bicycle light"

Now, if you're really devious, you can use discard the clutch and use regenerative braking to charge the capacitor only when you apply the brakes. No added friction when pedaling and perhaps easier stopping. I built something like this in 1974 by replacing the rear brake pads with rollers that spun two small hobby motors. It rapidly destroyed the rubber idler wheels, but basically worked. A hub-motor would have been easier. I also have some better ideas on how to do this today, but wanna keep those secret.

Good luck.

OK here is the thing that all of those seemilgly batteryless devices use. It is called a super capacitor currently typically .25 to 1 farad 5v. These are the settings backup battery in your VCR and many audio devices.

Currently there are higher voltage versions out there. Here is a source of information on them.

If you want to experiment there are many surplus sources of the small ones for less than $2 each. But remember that stacking them does reduce capacity. It will take some experimenting to determine how many it will take for your time requirements. One thing to remember if using these is that you will find a compleatly discharged bank of capacitors act as a near short circuit when beginning to charge so a current limiting resistor will be needed to protect your generator.

I believe that your best solution would be to NIMH batteries instead they are less of a problem to assemble with a better discharge time life in your application.

Gnack

Yes I meant capacitor. I was planning on building a light board. The amount of LED lights I use would be determined on how much power I can generate and store. How can I determine how big of capacitor I will need to run X number of watts for an X number of minutes? I understand that the size of the alternator would play a big role in how fast the capacitor charges. In theory (please understand I do not really know what I am talking about due to limited knowledge of electronic, but I am learning) I would imagine that if I use a large capacitor I could store enough power to run the lights for a longer period of time. However, I am not sure how much pedaling would be required to fully charge whatever size of capacitor I would use.

Yes I meant capacitor. I was planning on building a light board. The amount of LED lights I use would be determined on how much power I can generate and store. How can I determine how big of capacitor I will need to run X number of watts for an X number of minutes? I understand that the size of the alternator would play a big role in how fast the capacitor charges. In theory (please understand I do not really know what I am talking about due to limited knowledge of electronic, but I am learning) I would imagine that if I use a large capacitor I could store enough power to run the lights for a longer period of time. However, I am not sure how much pedaling would be required to fully charge whatever size of capacitor I would use.

I still prefer cazapitor.

I'll assume that you have a constant current switching regulator (DC to DC) between the power source and your LED's to keep them from changing intensity as the power supply voltage goes up and down. The basic formulas are at:

Please give up on the idea of using an alternator. What you want is a small permanent magnet motor (or generator). Your bicycle idea does not have enough power available to run the field winding of a typical alternator.

Welcome to bicycle science, and back of the envelope guesswork. (Incidentally, your questions would be more appropriate for sci.electonics.design).

There are plenty of articles online on using a bicycle as a generator to charge a battery, run a TV, or do something useful with the electricity generated. That should give you a clue as to how much energy can be generated. For example, this contraption claims 300 watts. Yeah, sure:

"The typical adult will create 100 to over 320 watts of power depending on the rider's strength. The average person can create between 125 to 215 watts. Those in better shape that are stronger can create between 225 and 320 watts or more." The problem with this is that you're using all your energy to power the generator, leaving nothing to move the bicycles down the road, up hills, and in the generally forward direction. My guess(tm) is that you would not want more than about 10% of the energy generated to go to charging the generator. That's about 20 watts. That's also optimistic, because unlike the stationary generator, your mobile generator is running at wildly variable speeds. My guess(tm) is that cuts your generated electricity budget down to 10 watts. The actual power delivered is totally dependent on your selection of PM motor.

Assuming 10 watts stored in a 3V, with a 1 Farad super-cazapitor, you would get 4.5 joules of stored energy. Dump that into a typical 3 watt ultra-bright white LED, and you'll get about 1 second of light. This also assumes a DC to DC converter between the cazapitor and the LED. I'm going to be evil (and lazy) and have you work out the number from the above URL as a math exercise. Hint: Joules = Watts/sec Also: The energy stored in a cazapitor is proportional to the square of the voltage. However, finding super-caps over about 5VDC is difficult and expensive.

Everything else scales. If you want 2 seconds, double the size of the cazapitor. If you want 3 seconds, get a 1 watt LED.

This might give you some ideas: "Permanent magnet generator for bicycle light operation"

Also, follow some of the "citations" on this page for other bicycle lights and generators.

Good luck. Sounds like a fun project. You'll probably blow up a few LED's, buy the wrong generator, smoke the regulator, and possibly get irate at the effort involved in driving the generator. It's all part of Learn by Destroying(tm).

A super-cap is probably what you're looking for.

ta delenda est

I am not planning on riding the bike on the road. It will be stationary and used just to generate power to run lights. Thanks for the info. I actually started this project because I have a cabin w/o electricity. We stay there only 10-15 throughout the year. I am hoping to provide light for short periods of time to read or to just see. Since we are not at the cabin all the time, I am worried that batteries would go bad. I was thinking that a capacitor would have a longer shelf life than batteries. I am hoping that a bike generator could supply a decent amount of light for my needs, especially since gas prices are going up.

On 9/2/2009 8:24 PM dabedwe spake thus:

I guess it all depends on how much exercise you want or need to get pedaling that generator ...

If you just want to learn electronics, invest in test equipment and mess around building several systems that don't work before you figure it out...go for it.

If your objective is light, give it up. You don't say how much light you want for 15 minutes or how many 15 minutes's you want per trip. That info is critical. But, I bet, if you do the math, you'll find that a small number of batteries will do what you need. You can charge 'em with the bicycle generator if you want. Probably easier to take 'em home to make sure they're good when you next need 'em. Small solar panel like used to keep the battery in your RV from going flat from disuse is also an option.

Trying to use capacitors will dramatically increase your need for electronic knowledge...and the cost of the system.

The answer to your question is.... E = 1/2 C*v^2. Stored energy is related linearly to the capacitance. and the square of the voltage. The available BIG caps are VERY low voltage. You can stack a bunch of them in series, balance the voltages on charge and discharge, use high voltage techniques to convert to led-acceptable voltage over a WIDE range of input voltages. Or you can parallel them and try to get high-efficiency switching from low voltages. Stored energy is the same, but you have different kinds of difficulty using it. Yes, you can buy assembled caps with higher voltages, but you don't want to know how much they cost.

Bottom line is that it's tricky and expensive if you know what you're doing. If you don't.....

stationary and used just to generate power to run lights. Thanks for the info. I actually started this project because I have a cabin w/o electricity. We stay there only 10-15 throughout the year. I am hoping to provide light for short periods of time to read or to just see. Since we are not at the cabin all the time, I am worried that batteries would go bad. I was thinking that a capacitor would have a longer shelf life than batteries. I am hoping that a bike generator could supply a decent amount of light for my needs, especially since

My advise would be to get a second dual-purpose battery for the car. Drive up and pull out a line to hook-up to the spare battery and charge up as needed. If you are using LED's you might not have to.

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Why not use NIMH batteries or get some solar lights and gut them for the control circuit, batteries, and leds then recharge the batteries with the generator hooked up instead of the solar cells.

The only thing I can think of here is that you would need to limit the generator's voltage and current to keep from damaging the control circuits.

Unless you are wanting very bright lights these should give ample light to see by.

Gnack

You could try pumped storage! Large tank in the attic, pedal to pump up the water then let it run back through a turbine/generator to run your LED's. Am I joking? Sort of... :-) Cheers, Roger