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Precision Inductance and Capacitance Meter
This circuit for measurement of inductance and capacitance can be used to test whether the values of inductors and capacitors quoted by the manufacturer are correct. The principle used in the circuit is based on the transient voltages produced across inductors and capacitors connected as series R-L and R-C networks, respectively, across a constant voltage source. The time constant for R-C and R-L networks is given by the relationships t=RxC and L/R, respectively, where resistance R is in ohms, capacitance C in Farads, inductance L in Henries, and time t in seconds.
The voltage across capacitor in R-C network rises exponentially to 0.632 of the applied voltage and voltage across inductor in R-L network degrades exponentially to 0.368 of the applied voltage in one RxC and one L/R time (referred to as time constant T of the combination), respectively. When the inductor/capacitor under test is connected across terminals A and B shown in the circuit, it is discharged through the normally-closed contacts of two-way push-to-on/off switch S1.
When switch S1 is pushed, the capacitor’s voltage begins to grow (or the inductor’s voltage begins to drop). Simultaneously, the output of timer 555 IC, which is wired as an astable multivibrator, is passed through NOR gates N1 and N2 and applied to the counter circuit. When the time constant (one CxR or one L/R, as the case may be) reaches, gate N2 is inhibited as its pin 2 goes high and the counter circuit freezes. Mode switch S2 is to be kept in position ‘a1’ for capacitance measurement and in position ‘a2’ for inductance measurement. As series resistance R1 is 1 kilo-ohm, the capacitance value is given by the relationship C=Tx10–3 while the inductor value is given by the relationship L=Tx103 .
The time period (1/frequency) of timer 555 (IC2) is adjusted for 1 ms and 1 µs in ‘b1’ and ‘b2’ positions, respectively, of the range switch. The values of capacitors and inductors covered in each range, together with displayed values, are shown in the table. From the table it is obvious that this circuit can measure capacitance from 1 nF to 9,999 µF and inductance from 1 mH to 9999 H. While presets VR1 and VR2 are to be adjusted for the in-circuit value of 1.717 kilo-ohm each, the in-circuit value of preset VR3 is close to 4.7 kilo-ohm. If a regulated +5V is not used, the measurement of capacitance and inductance will be imprecise.
Given below are some important points to be taken care of:
- The position of mode-select switch S2 and range-select switch S3 should be changed before switch S1 is pressed.
- If the circuit is allowed to function until it displays a constant value, the maximum time taken for measurement will be 10 seconds.
- When mode-select switch S2 is in position a1, capacitances can be measured, and when it is in position a2, inductances can be measured.
- When range-select switch S3 is in position ‘b1’, the output of 555 IC will have a time period of 1 ms (frequency = 1 kHz), and when it is in position ‘b2’, the output of 555 IC will have a time period of 1 µs. (EFY lab note. The guaranteed frequency of NE555 is limited to 500 kHz, and hence it may not be possible to get 1µs period. One may therefore use a 2nF capacitor to get a period of 2 µs and multiply the displayed value by 2, in b2 range.)
- Use a breadboard for connecting inductors or capacitors across terminals A and B.
- Using both the ranges for measuring an inductor or capacitor enables one to obtain the accurate value. For example, a 4.7µF capacitor will display only 4 µF when measured in range b1 , while in b2 range it will display 4700 nF (or 4.7 µF).
- Don’t press switch S1 before inserting the capacitor or the inductor between terminals A and B.