Measuring the Exact Capacitance Value of a Capacitor

[khan303]

Well maybe not the exact value. Before asking my question, I want to emphasize that I did some research on the topic and came up with empty hands. I've found some very useful info here in this website https://www.pupman.com/listarchives/1998/April/msg00625.html but my question is a bit different from the methods on the page. What I want to know is if there is a way to calculate the capacitance of a real-world capacitor. That is, for example, I know that a 10 nF capacitor does not actually have this capacitance value due to internal resistance or leekage although the label on it says so. And I know that a real-capacitor can be thought as an ideal capacitor with a resistor parallel to it. So, I need to find a way to measure two unknown variables, one is the ideal capacitance and the other is the resistance parallel to it. I am able to use any circuit components and construct any circuit that I want. But the only thing is I don't have any LCR meter and multimeters are useless for this particular purpose. And finally, it would be better if the method is really implementable. I mean I want to see the results on my oscilloscope.

I forget to mention above that the internal resistor in a capacitor is frequency dependent. But since I am going to measure the capacitance within the frequency range of 20MHz-30MHz with a signal generator, it can be considered as a constant once it is found for any frequency on this range. If you provide me with even a simple clue or direction, I will be very glad. Thanks in advance and sorry for the long post!

 

[FvM]

As you deduced, a real capcitor has a frequency dependent and complex capacitance. To determine how you can measure it with limited instrument means, you should get a rough idea of the expectable values. A capacitor datasheet is the right place to look for.

You'll notice that the real impedance component, which can be either represented by a series or a parallel resistor in the equivalent cicrcuit, will be usually specified as loss factor or tan D value. You'll surely be able to convert it into an equivalent series or parallel resistor.

You should be also aware of the existence of a series inductance and a respective series resonance frequency. If you are measuring capacitors at 20 or 30 MHz, you might even catch the resonance.

Before modern LCR meters were developed, bridge circuits had been used for component measurements, e.g a Wien bridge. They can be still used today.