I am working on a project to convert energy dissipated during walking, to electrical energy. To convert the mechanical energy to electrical energy I made a linear electromagnetic generator which uses a change in the Area of the circuit to generate a voltage.

The device I have designed is outputting a fluctuating voltage of 20-50 mV AC. I would like to charge a small AAA battery with this but I'm not good with circuits and I know the battery requires a DC voltage of at least 1.5V.

I have done research and I realize I need a rectifier, but every example I run across uses much higher voltages and states that the rectifier loses 1V due to efficiency (this would kill my small voltage). I have considering an amplifier to increase the voltage or a capacitor to store the energy before, but I am having trouble figuring out what to do. Can someone help me figure out how to convert and store this small AC energy? I really appreciate any suggestions. thanks.

 

Can you change your design? You can use a peizo to convert the action into DC voltage. That will do away with the need for rectification.

 

The due date for my project is near so I can't afford to change my design. I just gotta find a way to make this darn thing work!

 

Well, the smallest voltage drop from a diode I know of is .6v

You could charge a capacitor (slowly) then dump the charged cap into the charger circuit.

 

Thanks, a problem that arises though is that because of the nature of the circuit (opening and closing) the current changes directions when the circuit is retracting so If all I have is a capacitor, it will drain out the energy that was just put in. I will definitely use a capacitor to store the energy before trying to charge the battery, but I think some sort of diodes must be used

 

This will eliminate the voltage drop across the diode:

http://en.wikipedia.org/wiki/Precision_rectifier

Full wave versions are around too.

Unfortunately you need to power the op amp and the output current is limited by the op amp.

 

For a precision rectifier I would have to supply an external voltage but because I am trying to design a 'green' device, using additional power to harvest small outputs of power doesn't make sense. What if I could get the voltage higher to 300 mV, would this broaden the chances of collecting it?
(btw thanks for that link)

 

Schottky rectifiers have the lowest Vf of commonly available rectifiers nowadays. But even at low currents, the Vf is still a good bit above your output.

Piezo transducer output still needs rectification, but its' output voltage is much higher.

There was an article in Electronic Design News (edn.com) a while back about using a piezo to harvest sound waves to charge a supercapacitor.

I'm afraid that unless you change your tactics, you won't have a viable project to turn in.

 

Googled: energy harvesting circuit
First hit: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.97.6865&rep=rep1&type=pdf
Looks like what you're doing.

ken

 

Thank you very much for that article. It looks pretty daunting with the pulse width modulation boost converter and feed forward and feedback control, but it looks to be a viable solution for harvesting smaller amounts of energy. I will discuss it with my professor to see if I have to build it or if I could possibly purchase a circuit which already does this. Thanks a lot! any other ideas certainly wouldn't hurt

 

Googled: energy harvesting circuit

If you look through the search results you might find more useful info.

Ken