# Need help with Relaxation Oscillator with Parasitic Capacitance

I am trying to build a capacitance meter using a simple relaxation oscillator with a fixed resistor R in series with a capacitor to be measured C. I am applying the formula where Freq=frequency and K is a constant.:

                             C = K / ( R * Freq )


I have however noticed parasitic capacitance in the circuit that affects the measured capacitance and frequency. I am therefore looking for a correct formula that takes into consideration of 2 extra parasitic phantom capacitors, C1 which is in parallel with R and C2 which is in parallel with C. The resulting formula should be in the form of:

                             C = f( R, C1, C2, K, Freq)


Looking forward to any assistance with this problem.

• And what about parasitic resistive effects of the capacitor to be measured? I am afraid, it is not possible to eliminate all parasitic effects. – LvW Jun 30 '15 at 18:34

## 1 Answer

Tomas,

You can measure the parasitic capacitance by removing the capacitor to be measured, leaving only parasitic capacitance in the circuit. This way, you will know the value of the parasitic capacitance.

Now, when you add in the capacitance to be measured and you obtain the result, you can subtract the parasitic capacitance. This will leave you with the correct value of the target capacitance