Hello,
I am interested in measuring a Small change in Capacitance for instance nF change..
Can anyone suggest circuit ?

 

Hello,
I am interested in measuring a Small change in Capacitance for instance nF change..
Can anyone suggest circuit ?

Hello ...

It is uninspiring to design such an circuit given that you can buy an professional capacitance meter on eBay for something like USD50 inc ship. This meter will measure caps in the pF range. Most likely better than 1pF with accuracy of +/- 1%

 

1) Please clarify whether you want to detect a change in capacitance, for example, to operate a switch, or do you want to measure the capacitance of various capacitors?
2) Are you familiar with microcontrollers, or do you want a completely analog solution?

John

 

A 555 putting out known pulse width pulses, through a resistor to charge the cap, makes an integrator, voltage higher for more capacitance see "integrator"

 

1) Please clarify whether you want to detect a change in capacitance, for example, to operate a switch, or do you want to measure the capacitance of various capacitors?
2) Are you familiar with microcontrollers, or do you want a completely analog solution?

John

Hello,
thanks for your reply!!

I am familiar with Arduino microcontroller ..I have tried 555 timer circuit is not so fast for measuring the capacitance...I am open to any circuit but i want to measure the change in capacitance in nf

 

A 555 putting out known pulse width pulses, through a resistor to charge the cap, makes an integrator, voltage higher for more capacitance see "integrator"

Hello,

555 timer is measuring frequency but it is very slow in updating frequency .....I am interested to measure capacitance with sampling rate of 2 KHZ

 

1) Please clarify whether you want to detect a change in capacitance, for example, to operate a switch, or do you want to measure the capacitance of various capacitors?
2) Are you familiar with microcontrollers, or do you want a completely analog solution?

John

Hello,
I want to measure change in capacitance in nf ...The measured capacitance should be very fast with sampling rate of 2khz minimum

 

hot measuring the 555 frequency,, just measuring the voltage across the cap. a method used here to measure liquid levels in chemical tanks.

 

Another approach is to make the capacitor part of the timing components for an oscillator and then measure the frequency. But the quick and easy approach is to buy a meter as already suggested.

 

Your question needs more work. Is the capacitor in a circuit or on a shelf? Do you want the value displayed in a readout or can it be a change in the pitch of an audio tone? In other words, what are you trying to do, and why?

ak

 

...555 timer is measuring frequency but it is very slow in updating frequency .....I am interested to measure capacitance with sampling rate of 2 KHZ

Then make the 555 astable oscillator run at a nominal 2kHz (or higher) and then use the Arduino to measure the period of the square wave (time between two successive up-edges, for example). It is trivial to compute the freq by taking the reciprocal of the period.

 

I'm curious as to what you have that exhibits a 1nF (at least) change of capacitance?
Do you want to know the amount of the change, or simply detect that it has changed?
How will you signal/indicate the change?

 

Hello,

I would like to see the change of the capacitance for instance (fF) ..I am really sorry for saying nF in previous comments ..
My intention is to see the capacitance change in (fF)..

thanks in advance!!

 

That is an entirely different problem. It won't be easy. May we ask why femtofarads?

John

 

It would have been better if you had kept it to nF or pF.

fF?

Forget it. That is beyond the expertise of most people here on AAC and anywhere else.

 

Here's a discussion of fF measurement: http://www.eevblog.com/forum/testgear/sub-femtofarad-capacitance-measurement/ and a link to an old dissertation on sub-picofarad measurement that turned up on that search to give some feel for the problem: http://dspace.mit.edu/bitstream/handle/1721.1/34304/17455811.pdf

John

 

unless your able to spend very large amounts of money and effort, it will be like the 8 digit resisgtance meters, the last several digits just jumoing around. air currents will affect femto farad measurements. look up body capacitance too. anything conductive moving even a little in the area will mess up your measurements.

 

What is the capacitor you are trying to measure?

 

Here's a discussion of fF measurement: http://www.eevblog.com/forum/testgear/sub-femtofarad-capacitance-measurement/ and a link to an old dissertation on sub-picofarad measurement that turned up on that search to give some feel for the problem: http://dspace.mit.edu/bitstream/handle/1721.1/34304/17455811.pdf

John

Hello,

I have PCAP from Acam but they have limitations the updating rate is only 600Hz ..I would like to have 2KHZ ..
I have also spoke with the company (ACam )they are asking to write code for making it to 2KHZ updation but that not so easy it has lot parameters to load it ..
So can anyone done simple or complex setup with minimum of 2khz update rate ....

 

This instrument is in the USD $20K range according to the site I previously linked to: http://www.andeen-hagerling.com/ah2700a.htm It can apparently go to 2KHz.

John