Using a PIC micro device to measuring LC

Using a PIC micro device to measuring LC

This tip demonstrate how to use a PIC micro devices measure L/C with internal comparator module .

Theory of Series and Parallel resonance

Figure 1.

in figure 1. use a LC parallel circuit to generate resonance frequency and we can obtain the frequency with following equation.

--------------------(1)

Measure inductor(L)

Figure 2.

To measure an unknown inductor(Lx), Lx is connected in series with L1 as figure 2 and oscillate the new resonance frequency(fx) and we can obtain a new fx (lower than f1) by :

----------------(2)

from Equation 1 and 2 ,Then Lx can obtain from :

--------------------(3)

Measure capacitor(C)

Figure 3.

To measure an unknown Cx we connect Cx in parallel with C1 as figure 3,new resonance frequency(fx) calculate from :

-----------(4)

from equation 1 and 4 we obtain Cx from :

-----------------(5)

Measure Resonance frequency

When we known how the resonance freqency cause by L and C we can use any frequency counter to measure it but in the article we want to use a PIC microcontroller to be this function.
To measure frequency we use the circuit as in figure 4, the
comparator with positive feedback to make a parallel LC oscillator with digital output and take in to 8-bit or 16-bit counter input to count number of pulse per second.

Figure 4.



Figure 5.

Comment

In practice, the circuit traces on the PCB, the switches , and the test leads all contribute a small amount of "Stray" inductance (Ls) and capacitance (Cs).These stray values add to the values of Lx or Cx. To avoid these stray, you should be implement a "Zero" function in your hardware and software . For an inductor mode, just short the leads and store measured Ls in memory. For a capacitor, zero it with the leads open and store measured Cs in memory when we measure Lx or Cx we must subtracts measured values from stored values of stray inductance or capacitance in memory.