Tuesday, October 8, 2013



Inductors and capacitors are general purpose components in AC circuits. The estimation of exact or approximate values of inductance can be found in much theoretical way with the relation that they make with their physical dimensions. But who knows what exactly the show in practice. Even the slight change in the pitch of turns in case of inductance can change the resultant value. That was the intention that leads me to the build one.

Well , This is not a Rocket Science as far as holy Google is there. After a long ride with google I got many examples and methods to estimate the freaky inductance and capacitance. As this are property related to metals I named this prototype as “de metal amigo” …

End of calibration display which appears soon after power on

The basic idea comes from the good old LC tank circuit. The frequency generated is taken in reference with a combination with known reference capacitance in case of inductance measurement and with a known inductance in case of capacitance measurement.

Basic tank circuit with resonance frequency equation


To make this even simpler for the controller a simplified equation can be deducted as far as we are using the reference capacitor Cr which is the heart of the measurement precision. The measurement method is in such a way that it controller determines the present values of capacitance and inductance in the circuit with reference to Cr . And the only parameter that controller reads the change in frequency in each stage of calibration as well as measurement.

Position of capacitance and inductance under measurement

Calibration of the meter takes place as the power supply is turned on. It measures the present resonance frequency formed by the 66uH inductor and 1000pF capacitor, stores it and then measures the frequency by adding the reference capacitor in parallel to this resonance circuit. From these two frequencies it estimates the value of present capacitance and inductance seen at the test terminals.

Equations for value of capacitance and inductance of the main LC circuit
The result of above equations let the controller know the values of the current capacitance and inductance at the measurement terminals in associated with the 66uH inductance and the 1000pF capacitance which may subject to change according to surroundings temperature.

Equation for inductance and capacitance to be measured
The measured capacitance is then converted to appropriate values in terms of uF,nF,pF etc. and then stored and displayed. The frequency meter is set at a prescalar of 1:1 which gives accuracy of 1Hz to the meter.

Frequency measurement is done by counter mode operation of timer0 module associated with PIC 16F877A which I used as controller. The frequency from LC Circuit is amplified using the LM311 which shoots it to T0CKI pin of controller. 


Circuit Diagram (click to enlarge)

Circuit Hardware with 16x1 LCD and power supply


I have tested with inductors locally available with tolerance ranging +/-5 % to +/-10% . Dint able to find a military tolerance type yet, anyway the results below shows the inductance measurement of 33uH  +/-10% inductor.
Initial display on selecting inductance measurement
 The above string is displayed on selecting inductance by pressing L/C switch.

Displaying calculated value of 33uH  +/-10% inductor

The above string on the display shows the measurement result of the inductance connected to measurement terminals.


In case of frequency measurement , i have tested up to 28.244Mhz which is the maximum available crystal with me, as per the datasheet of 16F877A it can recognize frequencies up to 50Mc/s without external or internal prescalar but for a good reason i have implemented a 1:64 prescalar for higher frequencies.


HEX file for 16x1 LCD display

HEX file for 16x2 LCD display

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