# Piezoelectric Voltage and current measurement across a 3.3kohm resistor.

I am testing a PZT 43 material with tapping sequence. The measurement of the material is: 10mm in length and 0.8CM in diameter. I am tapping the material with a plastic and holding it down with a fat-glove to ensure that no other materials interfere with the reading across the oscilloscope. I have enabled noise rejection plus any Low frequency rejection.

This image is my output seen across the oscilloscope: 1. can anyone confirm if this reading is reasonable?

2. can you suggest me a way to find the averge of this waveform (this wavefrom was kept on Trigger mode)

3. by using the ohms law across the resistor, will my current value be accurately measured?

Note: this is channel 1 and my 1st question here.

• (1) Plausible, yes. Correct? Not enough information to know. (2) Average will be 0 as it's an AC signal. (That may be the wrong question). (3) Ohms law works quite well with resistors. – Brian Drummond Mar 7 '16 at 12:30
• @BrianDrummond What other information you require? – imad Mar 7 '16 at 12:36
• Maybe you should add the sensor's datasheet – MaximGi Mar 7 '16 at 12:42
• Would it be helpful if I paste my reading after rectifying the AC to DC? – imad Mar 7 '16 at 12:56
• The waveform certainly looks plausible - it is about the same shape (for amplitude and envelope) as I am seeing from an ultrasonic receiver attached to a piezo sensor being used on one of my projects. – Peter Smith Mar 7 '16 at 13:06

The waveform looks to be correct.

As a sensor

If you want to use this as a sensor, treat the piezo as a voltage source with a big series resistance. E.g. you can put a 100 kOhm resistor across the piezo output, and you will see that the generated voltage is much lower. For sensor usage, ensure that a high mechanical input will not produce a voltage which kills your processing device (e.g. a microcontroller) by using a series resistor of a few hundred ohms.

As a power source

If you want to use this as a power source, well, that is not a shiny way. You can get out very little electric power from a piezo. In order to get the most out of it, you have to have a mechanical input which is having the same frequency as the resonant frequency of the piezo (i.e. tapping with a pole won't work). For more info, refer to http://www.piezo.com/tech3faq.html#app4

You can measure how much power this produces by rectifying the output, and connect it to a resistor of known value. Measure output of the piezo, and current in this circuit, multiply it and you get a (very little) wattage. This is a non-periodic signal, so you may wish to connect the measurement to an excel sheet where you can mmultiply voltage and current at each measurement point and calculate some average. Alternatively you can feed your measurement to a buffer, and drive an averaging circuit.

Example measurement setup to measure generated power

A good mechanically triggered piezo shall produce a voltage of 4V and over. I'd suggest starting with 1k-2k loading resistor, which simulates a load. If you have a 2 ch digital oscilloscope, do the followings:

• measure the voltage across the pins of the piezo. One pin will be GND, another pin CH1.
• split the load resistor into two, i.e. have two 1k in series. The split point will be CH2.

Now you have a voltage signal on CH1, and a current signal on CH2. Make a measurement, feed these to Excel. Use I=U/R to convert CH2 measurements to current, i.e. 1V on CH2 will mean 1mA (current flowing trough 1k resistor between CH2 and GND, right?)

Once you have the CH1 and CH2 points in excel, simply multiply them at each data point to get a power at the given point. Note that even sometimes you have negative voltage on CH1, that also means power, since also the current flows in the opposite direction, this way you will always get positive values for power.

Now you have a column telling power (energy) generated in the timeline. Note that this will be bigger at the hit, and will be slower at the end. You need to average this out to get an average power.

If you want to use it for serious energy harvesting, you shall try to give a continuous mechanical input (i.e. use a vibrating motor to hit the piezo, or something like that during your tests). Otherwise it is very hard to measure anything if you can not repeat the measurement reliably.

• Sir, I have been stuck at finding the Power for a quite a long time, Using excel to calculate power was and is my right option but my BIGGEST concern is this: 1)yes I can use a KNOWN resistor value, BUT what is the RIGHT resistor value? if I keep increasing the resistor value reaching to the point where ill get more and more Voltage as it nears infinite resistance and hence Max voltage. So Please help me in suggesting what is my ACTUAL power which comes out of ADC to the Capacitor? – imad Mar 16 '16 at 22:25
• Aha, so you want the POWER :) You know what? It is really all the same how much power comes out from the piezo in theory. Anyhow you need to rectify that voltage, smooth it out, and if you want to do anything useful, you also need a regulator. These are very much affecting the efficiency of the whole stuff. So first, pick your energy harvesting IC, such as cds.linear.com/docs/en/datasheet/35881fc.pdf which takes care of the dirty work. Then ensure to have continuous mechanical input on the piezo, preferrably at the resonant frequency of the piezo. – Gee Bee Mar 16 '16 at 23:45
• Oh, about the right resistor value, how much power would you like to draw from such a thing? I'd go with 1k, that roughly simulates a load of 4mA. Note that you shall see about above 4V spikes at a good mechanical trigger - so probably you shall hit the piezo more, and hit a bigger surface. – Gee Bee Mar 16 '16 at 23:50
• My PZT is rather very small, so when you say 4V i can't help but be a little pessimistic because after ADC I have already come to mV range. and I do not understand how to find my resonant frequency of the material, I asked the manufacturer to give me the details. – imad Mar 17 '16 at 0:03
• Also, I would really appreciate your feedback on my other question as well. Thank you electronics.stackexchange.com/questions/223080/… – imad Mar 17 '16 at 0:04