Lcmeter

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PIC LCF METER
JOHN BECKER
Simple monitoring of inductance, capacitance and frequency values
HIS simple PIC-based unit was designed to measure and display the values of inductors and capacitors. As a by-product of the technique used, it can also display the frequency of an external 0V/+5V signal source. The ranges are approximately: Capacitance: 1pF to 6500mF Inductance: 1mH to 10H Frequency:0·05Hz to 5MHz

Constructional Project

Using this formula, if any two values are known, the third can be readily calculated. For instance, if C and F are known, then L can be calculated using the formula: 1 ( 2 × p × F) L= C
2

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OSCILLATOR CONCEPTS

Fig.1. A basic inductance and capacitance (LC) oscillator.
One technique for using an inductor in a CMOS oscillator circuit is that shownin Fig.1. Here the oscillation frequency is determined by the formula: 1 2 × p × Ö(L × C) where: F= F = frequency C= C1 × C2 C1 + C2

Similarly, using the capacitance-resistance oscillator configuration shown in Fig.2, the output frequency can be calculated for known values of R and C. Several formulae exist for this calculation and the one used in this application is: F= 1 p×R×C

The designis based upon the concept that oscillators can be constructed from CMOS NAND gates or inverters, and that their oscillation frequency depends on the values of inductance, capacitance and resistance in their feedback paths. The principles were discussed by George Hylton in his two-part series Logic Gate Inverter Oscillators of Sept/Oct ’02. Using a suitable microcontroller, such as one from thePIC16F62x or PIC16F87x families, software can read the frequency of an oscillator and calculate the value of an unknown component if the values of the other components are known. In this design, a PIC16F628 is used and the results are output to an alphanumeric liquid crystal display (l.c.d.).

from which the value for C can be calculated if R and F are known: C= 1 p×R×F

L = inductance p = 22/7Fig.2. A basic capacitance and resistance (CR) oscillator.

FREQUENCY TO PIC

Referring to the full circuit diagram for the PIC LCF Meter in Fig.3, two independent oscillators are used, one based on inductance and capacitance (LC) values, the other on capacitance and resistance (CR) values. The LC oscillator is formed around NAND gate IC3a. The inductance is provided by inductor L1, used inseries with the external inductor whose value needs to be measured. The external inductor is connected across probe clips P1 and P2 and switched into circuit by rotary switch S2 in position 1. The capacitance is provided by capacitors C5a, C5b, C6a and C6b. Two pairs of capacitors are used so that the value of C in the LC formula is simple for the software to process. The values for each of thefour capacitors is set at 10nF.

92

Everyday Practical Electronics, February 2004

Fig.3. Complete circuit diagram for the PIC LCF Meter.
Therefore the parallel value of C5a/b = C6a/b = 20nF (a value not obtainable in standard capacitor ranges). In the formula, as expanded above, the value for C is thus 10nF. A preset potentiometer, VR2, is also included in the feedback path. Its principalpurpose is to ensure that oscillation starts reliably and then continues in a stable manner. The effect is due to its relationship with capacitors C6a/b, which impose a more pronounced phase shift on the signal being fed back through the inductor than the output of the gate itself can allow. Potentiometer VR2 also has the side effect of providing a degree of frequency control, although this is notimportant in this application. In use, the inductor whose value is to be measured is placed in series with L1. The latter provides a minimum inductance value against which the circuit is “nulled” prior to taking measurements. Its use minimises the effect of stray fields within the physical circuit assembly. The CR oscillator is formed around NAND gates IC3b and IC3c. The maximum frequency at...
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