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I have a piezoelectric disk. I am trying to determine the sensitivity of this disk for the sake of making a charge model more accurate. The charge model in question:

enter image description here

I have attached the piezoelectric disk to a multimeter. So far, I have measured the output voltage of the disk by fastening it to a shaker. However, I am using a charge model instead of a voltage model, and would like to be able to determine the current.

So far, the PZT does not respond to AC or DC current detection, with no changes to the AC or DC current regardless of pressure placed on the PZT. This is different from voltage detection where a response is immediate.

enter image description here

I am aware that a PZT generates a charge instead of a current, but even when placed on the shaker when the pressure/acceleration is constantly changing (and the charge should be generating constantly), I am unable to detect any current from the PZT.

Please let me know if more information is required.

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  • \$\begingroup\$ You cannot expect 5 mA @ 10 Hz from such a small disk. I measure around 30 µA here at this kind of disks. \$\endgroup\$
    – Jens
    Feb 28 at 13:49
  • \$\begingroup\$ With the single-powered OpAmp you should shift the input levels, as in figure 2 of the manual. \$\endgroup\$
    – Franc
    Feb 28 at 14:14
  • \$\begingroup\$ What is C2 for? \$\endgroup\$ Feb 28 at 16:31
  • \$\begingroup\$ At 10Hz and sensible accelerations you will get almost no output current (the 19nF element capacitance has 1MΩ impedance roughly at 10Hz). If you mount it with a heavy mass you might be able get more without breaking it. I assume R2 is the model resistance at resonance. In simulation that will probably rail the amplifier because of the DC gain of 13,000+ and op-amp offset voltage. \$\endgroup\$ Feb 28 at 16:40

1 Answer 1

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Observation

You are using an op-amp as a TIA (transimpedance amplifier) to measure AC currents with the most negative supply pin for the op-amp tied to ground. The reference input (the non-inverting input) is also tied to ground. This means that the op-amp output is trying to keep the inverting input at 0 volts/GND.

Problem #1

If the piezo produces currents in excess of 10 mA and, the output is unable to drive below ground to keep the inverting input at 0 volts then, the input protection diode takes the full 10 mA. The absolute maximum as stated in the data sheet is 10 mA for the TL071A inputs: -

enter image description here

So, there's a distinct possibility that you may have destroyed the op-amp.

Problem #2

But, there's another more likely problem and that is the input range that the TL071A can handle: -

enter image description here

It clearly states that the lowest valid input has to be at least 4 volts higher than the negative supply rail so, if I were you I'd employ a negative rail of at least 5 volts and pray that the current you are measuring can always be dealt with properly by the op-amp output.

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  • \$\begingroup\$ Thank you very much. In this case, I was entirely unsure what the output amplitude of the disk could possibly be and assumed 5mA as a generic value. According to Jens, I should be expecting something far lower. I would attempt to measure the output current of the disk independently without circuitry but I feel that the low current generated would make the value hard to detect independently, and I am unsure whether this question is the best place to discuss this in detail. \$\endgroup\$ Feb 29 at 16:21
  • \$\begingroup\$ @HFOrangefish you still have Problem #2 to resolve so, maybe try and get that working to see what currents you do actually get from the piezo before asking a new question. I think the op-amp idea will work but, you need to add a negative rail. \$\endgroup\$
    – Andy aka
    Feb 29 at 16:24

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