Piezo Film Output

Status
Not open for further replies.
P

patpatpat

New Member
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.
Click to expand...
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
Click to expand...

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

Do you have part number or description of the film.?
 
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
Click to expand...
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.
 

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.
Click to expand...
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.
Click to expand...

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.
Click to expand...

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.
Click to expand...
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.
Click to expand...

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?
Click to expand...

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.?
 
Status
Not open for further replies.

Similar threads

AC/DC Transformer with 240V output
2 3 4
Replies
63
Views
7K
ThomsCircuit
S
  • Question
Flomec-qs100 Open collector output
Replies
7
Views
912
Nigel Goodwin
  • Question
Buck converter with transformer output
Replies
14
Views
2K
throbscottle
S
Device that Data Logs Generator Output?
Replies
5
Views
1K
Soul21
S
CD4017 max output question
Replies
19
Views
3K
Tony Stewart
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…