Science Buddies: "Ask an Expert"

My research is now about using Rochelle salt as a piezoelectric material to power a small-scale house. Basically, we will use Rochelle salt powered by the pressure crated by the vehicles on the road to power a model of a house.

However, what I find troubling is converting AC to DC. As of now, I have the alternating current on wire and I plan to convert it to DC using a Bridge Full-Wave Rectifier. Is that possible? I have electricity in the wire and I will connect it to what, then? How will I exactly convert it?

Help would be very much appreciated. Most explanations online require knowledge of mechanics, which I do not have at this age. Some are too basic and does not elaborate specific details. Thank you for your time.

love_dust2 - There are several YouTube videos showing a Rochelle salt crystal being used as a piezoelectric generator, in some cases driving an LED as an output indicator. In one video, the demonstration shows an oscilloscope being used to measure the generated voltage. With a moderate tap, it produced only -2V on the negative side and 1.5V on the positive side. This was of very short duration, as the crystal was only struck once, and the length of the pulse was perhaps a millisecond. A continuous exposure to mechanical vibration would obviously produce a series of oscillations. For each oscillation, the crystal would have to return to it's relaxed position before the next mechanical force could generate another pulse. Constant pressure on the crystal will not produce constant output. I think there are a few issues with this type of crystal being used to generate any meaningful amount of power. A piezoelectric material, (ceramic, grown crystal or other material) has an inherently high internal resistance as a material. Therefore the current produced will be very low, suggesting that real, usable power cannot be generated. Although some piezoelectrics are capable of very high voltage (the gas ignitor in a range or butane lighter will produce thousands of volts) the current is probably in the order of a few microamps. Rochelle salts, with a very low voltage being generated (at least as I observed from the attached YouTube video), may have a much lower internal resistance and capable of higher current.

That may lead to your next issue of rectifying the output using a full wave bridge rectifier. In a FWB, two diodes are in series in each cycle so that both the negative and positive pulses are produced at the positive and negative terminals of the FWB. If, in fact, the Rochelle salt crystal only generates about 2 volts, the voltage drop (about 2 volts) across the two diodes in series will greatly limit any output current. If, in your experiment the Rochelle crystal can generate peak voltages say of over 6-8 volts, it may not be a problem The use of an LED may be better as a test vehicle because there are no diode losses to consider, although you will only capture half of the voltage cycle when the LED is being forward biased. Good luck.

http://www.youtube.com/watch?v=R9m79oWdOZI

Rick Marz