# Charge / Voltage

Discussion in 'General Electronics Chat' started by Alex009, Jul 29, 2014.

1. ### Alex009 Thread Starter Member

Jul 2, 2014
30
0
Hey guys,
I'm kinda confused now. (And i thought the following was always clear to me..)
a) Can you explain me charge again? I'm not even kidding. Since I m confused about the following
b) You all know the piezoelectric effect i guess. So if i got a pressure sensor and change the pressure there is a change concerning the charge right? So how do i figure out the exact number of Q ? I would just use an OPAmp and look at the voltage. But somehow it doesnt work like that. Why? Isnt chare proportional to voltage and do i always have a certain amount of voltage if there is any charge?
and finally
c) How does the charge act on a capacitor if i got AC Voltage. (For different Frequenzies)
Cheers for your help.
BR
Alex

2. ### AlphaDesign888 New Member

Jul 27, 2014
189
10
You don't make any sense is the honest truth. Sorry.

3. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
if yuou want to measure electrical charge on a piezo sensor, you need an extremely high input resistance meter, sometimes called an electrometer. the current from a piezo sensor isnt very great and will bleed off through normal test equipment.

4. ### AlphaDesign888 New Member

Jul 27, 2014
189
10
You can easily lose the plot with electronics.

Why are you asking these questions? Is there an objective?

Sep 20, 2012
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6. ### Alex009 Thread Starter Member

Jul 2, 2014
30
0
Okay..cheers for that. I ll take a look at it. But I m not sure if it covers the topic i m actually interessted in (/confused about). As soon as i m home i m trying to give you a circuit to explain it better.

7. ### AlphaDesign888 New Member

Jul 27, 2014
189
10
Without reactance or resistance charge is instantaneous. There is nothing to measure.

Take it easy huh. This stuf is enough to send you round the twist.

8. ### wayneh Expert

Sep 9, 2010
12,373
3,225
Mechanical strain applied to a piezoelectric device does work on it, adding energy to it, and that energy goes partly into charge separation. (Most is lost as kinetic energy, heat.) The energy captured in the charge separation can be recovered by a circuit that allows the charge to drain so that there is no longer any separation. It's like you've carried a bucket of water up a hill and now you want to get that energy out by pouring the water back down the hill.

Except with a piezo, it's like a thimble of water. It will quickly evaporate on its own and only specialized equipment can recover such a small quantity.

a) Can you explain me charge again?

It's like the water, a quantity of "+" separated from an equal number of "-". The available energy depends on the amount, and the distance.

b) You all know the piezoelectric effect i guess. So if i got a pressure sensor and change the pressure there is a change concerning the charge right?

Yes, you separate a small number of "+" and "-".

b2) So how do i figure out the exact number of Q ?

Do you mean "figure out" or "measure"? The former might be estimated from the data sheet for the device you are using. The latter requires high precision, specialized electronics if you really want an accurate answer. If you just want an estimate, you can tell from the behavior of the circuitry that responds to it. The circuitry is somewhat predictable, so you can infer how much charge must have been made available.

b3) I would just use an OPAmp and look at the voltage. But somehow it doesnt work like that. Why?

Voltage alone won't tell you the whole story. It's just the height of the hill. You also need to know how much water was in the bucket, how much charge was separated. Unless you know the capacitance of the device holding the charge, I think you have to let the charge flow out in order to measure it as it goes by.

b3) Isnt chare proportional to voltage [In a capacitor, yes] and do i always have a certain amount of voltage if there is any charge? [I don't get that question.]

and finally
c) How does the charge act on a capacitor if i got AC Voltage. (For different Frequenzies)

Charge is moving back and forth with each cycle just like current in a lightbulb. Instantaneous voltage is proportional to the amount of charge stored at that moment. That's why capacitors have ratings for current and frequency as well as voltage. They can overheat and blow up.

Alex009 likes this.
9. ### AlphaDesign888 New Member

Jul 27, 2014
189
10
There you go. You got an answer from a guy with an physics or science degree.

10. ### Alex009 Thread Starter Member

Jul 2, 2014
30
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That was really helpfull cheers. So if i want to "simulate" such a piezosensor (and measure it afterwards), I need to use a capacitor to be able to "let the charge flow"? Right? It wouldnt work to just use a very small voltage?

Edit:
What i mean by that (in a simple way):
It would work with:
Voltage supply -> capacitor -> measuring device
and not with
Voltage supply -> ressistance -> measuring device

(i know thats probably just a very simplyfied concept of a more complex circuit)

11. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
but the charge would only be there if you didnt let the pressure off the piezo, or it didnt leak off on its own. most op amps are too leaky to use for this test. you need specialized electrometer tubes or fets optomised for extremely low leakage. measuring is like a hole in the bucket, less leakage and the water is in the bucket for a while, a lot of leakage and it is gone fast.

12. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
there are several circuits and explanations on this site, http://www.techlib.com/
that use charge detectors to measure electrical charges in the air and for measuring radioactivity.

13. ### AlphaDesign888 New Member

Jul 27, 2014
189
10
Arh! Hunter. Let me put some thought into this too.

14. ### AlphaDesign888 New Member

Jul 27, 2014
189
10
All I can think of is to RUN for it guys with some experiments. So many possiblities to consider.

15. ### Alex009 Thread Starter Member

Jul 2, 2014
30
0
Ok cheers guys for answering. I m done for today. Gonna try some stuff now with these information.
Probably gonna reply tomorrow again. Have a nice evening. (or whatever - depending on your time zone)

16. ### AlphaDesign888 New Member

Jul 27, 2014
189
10
You're not missing much. It is all actually quite boring unless you are creative gifted at kinetic level. Bringing things to life. Saying I made that.

These blokes get paid to read through all these posts and answer?