Piezo Sensor Voltage Problems- DAQ

[cf8989]

Hello,

I am trying to set up a piezo sensor (e.g. Maplin YU82D) with Labview (USB 6251) but am having some problems.

When nothing is hitting the piezo, there is usually a constant output of -10.5V and sometimes 10.5V. When the piezo is struck, a jump in voltage is seen, but this doesn't always drop back to the same level.

I am unsure how to solve the problems. I have looked through the forum and seen others having similar problems but am still unsure what to do. Any help would be greatly appreciated. Something tells me I need to add in some resistors.

(All my set up contains is a piezo with the positive going into an analogue input and the negative going into an analogue ground. I have tested with a multimeter and get sensible readings.)

Thanks in advance.

[johnsold]

Both the piezo device and the DAQ inputs have capacitance. Any bias current or leakage in the input circuit of the DAQ will charge that capacitance, possibly to out of range values such as your +/-10.5 V readings. The multimeter probably has a 10 megohm input resistance and any stored charge is discharged through that. 

Try adding a 10 megohm resistor in parallel with your piezo device.

Lynn

[cf8989]

Hi,

Thanks for the suggestion. Just tried it there and it's all working now.

Thanks again!

[Henrik_Volkers]

Some more words about piezo sensors:

You said hitting the sensor...   Make shure you don't exceed the valid maximum input voltage range of your analog input.

Piezos easely generate very high voltage peaks.... as can be seen in a ligther.

[cf8989]

Thanks- will keep that in mind.