So I am testing my PZT Piezoelectric material's Voltage output across a Capacitor of 1000uF after converting AC to DC using an Oscilloscope. Oscilloscope model: Gwinstek GDS-2102.

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The diagram is my model for testing the Voltage across the capacitor.

I am trying to see the output across the capacitor when I tap the PZT. this follwing waveform is from my AC to DC converter.

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So as you can see, my AC to DC works perfectly. But now the Voltage across the capacitor is shown in the next waveform.

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So as you can see it is not a DC rectified waveform.

Could anyone please let me know what is going on here?

  • 2
    \$\begingroup\$ Scope set to AC coupling? \$\endgroup\$ – Olin Lathrop Mar 9 '16 at 14:42
  • 1
    \$\begingroup\$ If you are trying to measure low voltage signals a diode bridge isn't ideal: the diodes drop 0.7 V when conducting, and there are always two of them in series with the load in a full wave rectifier. You wouldn't see anything at the output when the input signal is less than 1.4 V peak to peak. \$\endgroup\$ – jms Mar 9 '16 at 14:48
  • \$\begingroup\$ @OlinLathrop Its the same with DC coupling ive checked it. \$\endgroup\$ – imad Mar 9 '16 at 15:04
  • \$\begingroup\$ @jms I am using Schottky diode for this exact same reason and can clearly see an output there. \$\endgroup\$ – imad Mar 9 '16 at 15:04
  • \$\begingroup\$ Which schottky diodes? Although they switch fast and drop just 0.2 V, they are known to have a significant reverse leakage current, sometimes many milliamps. Had you disclosed these details when asking the question, you could have had an answer faster. \$\endgroup\$ – jms Mar 9 '16 at 15:11

Your capacitor is too large to allow the PZ to charge it. Try 1 uF or 0.01 uF.

Use 1N914 or 1N4148 diodes. If you use 1N4001-type, their leakage and parasitic capacitance may also degrade your output.

In any case, the scope input resistance will discharge the capacitor relatively quickly and the signal you get will actually be quite small.

  • \$\begingroup\$ 10MΩ from the probe is going to discharge the capacitor quickly? RC time constant is... \$\endgroup\$ – Voltage Spike Mar 9 '16 at 18:50

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