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Piezo Film Output

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patpatpat

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Hi,

I got these piezo films that output from 0 - 140V when I punch it. I want to scale it down to 0 - 2V for the microcontroller ADC. The signal itself is 4ms long.

I first used a voltage divider: 70MegaOhm : 1MegaOhm. I put the scope across the the 1Mega and no voltage drop across that. The scope itself has a 1MegaOhm impedance.

Please help, thanks!
 
patpatpat said:
Hi,

I got these piezo films that output from 0 - 140V when I punch it. I want to scale it down to 0 - 2V for the microcontroller ADC. The signal itself is 4ms long.

I first used a voltage divider: 70MegaOhm : 1MegaOhm. I put the scope across the the 1Mega and no voltage drop across that. The scope itself has a 1MegaOhm impedance.

Please help, thanks!
hi,
The PIC's ADC expects the impedance [resistance] driving it to be 10K0 or less.!!!
 
Well, I have yet to hook it up to the ADC. I am trying to scale it down first, but I dont see it happening on the scope.
 
patpatpat said:
Well, I have yet to hook it up to the ADC. I am trying to scale it down first, but I dont see it happening on the scope.
hi,
Do you know what the output impedance of the 'film' is.?

From that you could calculate a suitable resistive divider.
 
On the scope, the waveform generated across the 71MegaOhm is the same as the one generated over the 1MegaOhm. This is what I am confused about :(
 
patpatpat said:
On the scope, the waveform generated across the 71MegaOhm is the same as the one generated over the 1MegaOhm. This is what I am confused about :(

hi,
I would like to help but I need more details.

Do you have part number or description of the film.?
 
patpatpat said:
My previous post linked to the application note. Here is the manufacturer's site:
http://www.meas-spec.com/myMeas/sensors/filmElements.asp

hi,
Looked thru the link specification.

It says the film gives 500mV output when hit by a 800lb force a 55mph, how can you be getting 140Volts output.?

Most of the applications in the datasheet require an high impedance input amplifier, not attenuation.
 
patpatpat said:
On the scope, the waveform generated across the 71MegaOhm is the same as the one generated over the 1MegaOhm. This is what I am confused about :(
Since the film looks like a capacitor, the voltage you see may be determined by the capacitance of the scope input (probably about 50pF), not the resistance. Thus a capactor may be a better voltage divider than a resistor for your purposes. Experiment with different value capacitors from the film output to ground (start with a 100pF or so) and see what effect that has on the voltage.
 
crutschow, the scope has 13pF. I will give your advice a try.

Eric, thanks for sticking around and helping out. On page 1 of 7 of the app notes, it says: "The
voltage source amplitude is equal to the open circuit voltage
of piezo film and varies from microvolts to 100's of volts,
depending on the excitation magnitude."

I put the 2 leads of the piezo film striaght to the scope and read the 140V that way.
 
patpatpat said:
crutschow, the scope has 13pF. I will give your advice a try.

Eric, thanks for sticking around and helping out. On page 1 of 7 of the app notes, it says: "The
voltage source amplitude is equal to the open circuit voltage
of piezo film and varies from microvolts to 100's of volts,
depending on the excitation magnitude."

I put the 2 leads of the piezo film striaght to the scope and read the 140V that way.

hi,
But thats the point I am making, if you use the Hi-Z amplifiers or the 'single impact' detector shown in the datasheet, you dont need to use a resistive divider..:)

EDIT: the output from this circuit could be made to drive a PICs input.
 
Last edited:
patpatpat said:
crutschow, the scope has 13pF. I will give your advice a try.
The reason I said about 50pF was, you stated the scope had a 1 megohm input, which implies no 10:1 probe. 13pF is more typical of a 10:1 probe which has a 10 megohm impedance.
 
crutschow said:
The reason I said about 50pF was, you stated the scope had a 1 megohm input, which implies no 10:1 probe. 13pF is more typical of a 10:1 probe which has a 10 megohm impedance.

I didnt mean to be rude, sorry. I just wanted to inform you about the scope's specs.
 
patpatpat said:
I didnt mean to be rude, sorry. I just wanted to inform you about the scope's specs.

hi,
Look at the edited post, added the extract from the datasheet.
 
patpatpat said:
I didnt mean to be rude, sorry. I just wanted to inform you about the scope's specs.
Didn't think you were being rude. Just wanted to clarify whether you were using a probe or not.
 
thats another thing we are trying to avoid: powering a component. I just noticed a 7mega:6mega ratio worked for some reason. This is getting a bit strange. I am going to keep playing with it.
 
patpatpat said:
thats another thing we are trying to avoid: powering a component. I just noticed a 7mega:6mega ratio worked for some reason. This is getting a bit strange. I am going to keep playing with it.

hi,
The ADC input on the PIC will NOT work correctly with a multimeg ohm source impedance.!
 
patpatpat said:
I see. What is the typical range that it would work under?

hi,
The PIC datasheet says, <= 10K0. ADC input source impedance.

I am puzzled that you say you want to avoid 'powering' a component, surely you are powering the PIC.?

The HEF4013 or HiZ OPA will only require microamps.

As you are using the ADC input for the signal, why are you considering a divider that gives a 2Volt output.?

If you are looking for a voltage and ADC conversion thats proportional to the 'punch' power, you should consider the Hi-Z OPA's shown in the datasheet.

The reason the 6M0, 7M0 are reducing the divider voltage compared to the 71M0, is because you are loading the 'film's output, its an Hi-Z output.

Do you follow.?
 
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