# Power/Voltage question

Discussion in 'General Electronics Chat' started by codemastercool, Mar 8, 2012.

1. ### codemastercool Thread Starter New Member

Jan 23, 2012
9
0
Hey everyone,

I have a doubt regarding some basic concepts for a project I'm working on.
I'm doing a project on energy harvesting using piezos. Now, the theoretical values show an output peak to peak voltage of 2.2V, and with a load of 10MegaOhms, an output power of .2uW.

Now, what I want to do is try and portray another idea to my classmates, and that is - using the energy harvested, I want to give them an idea of the quantity by using day-to-day gadgets they use, for example, "With this circuit, you can charge an iPhones/iPods battery in X amount of time".

Could someone just give me a basic theory explanation of how to do this?

Some more background info:

The paper from which we're referencing uses the piezo to generate AC voltage, but I want to put in a bridge rectifier to change that to DC.
So am I right in saying -
1. I calculate the voltage output of the rectifier, with an AC input of 2.2 Vpp.
2. Try and find out the gadgets battery specs (perhaps the resistance) so that I can calculate the current flowing through? And with that, relate to charging capacity or something?

I'm not sure as to how to go about the second part of step 2 (if that's right).
I did a Google search, and one replacement battery for the iPhone 4 shows specs of 3.7V and 1420mAh.

I would really appreciate if someone could help me out here.

2. ### Audioguru New Member

Dec 20, 2007
9,411
896
What activates the piezo? If it is very loud continuous sound then the rectified output will be 1.1V peak minus 1.4V for a fullwave rectifier leaving you with absolutely nothing to charge a battery with.

Why not connect six piezos in series so their peak voltage is 6.6V? Then it can be fullwave rectified into 4.9V and can charge a cell phone.

A cell phone has a 1420mAh battery. The output current from the piezos is almost nothing which is about 10nA. It will take about 142,000 hours to charge the battery. That is 16.2 years.

3. ### codemastercool Thread Starter New Member

Jan 23, 2012
9
0
It is activated by air flow. Now, what we're doing is demonstrating the energy harvesting properties of the piezo, we're using a pretty small piece as is.
We are not physically charging any battery - I just want to do some of the math to give them an idea of *how* much current/power is being generated. This class consists of a majority of mechanical and civil engineering students, so I don't want to just say " Our setup presents us with x amount of current", I want to say "The setup gives us this much current, which could charge up an AA battery in so many hours, or light up an LED for so long".