InductanceP - J E Patterson - 20210301

Description

This program calculates the inductance L (μH) of a coil using a 50 ohm output signal generator and an oscilloscope. See A Simple Method to Measure Unknown Inductors by Ronald Dekker. This version accounts for inductor resistance using a modified equation by Karen Orton.

The signal generator output amplitude is first measured at about 20,000 kHz using an oscilloscope. The unknown inductor is then connected across the signal generator output and the frequency f (kHz) is adjusted until the oscilloscope shows a reading of half the original amplitude.

On a digital oscilloscope I prefer to take rms readings as they are more stable than peak to peak readings.

The equation is quite simple: L =1000*√(21.11 + 0.84R - 0.025R2)/f.
If R is close to zero ohms the equation reduces to L = 4594/f.

R (ohms) ENTER
f (kHz) GSB A
The result is L (μH)

f is the frequency where the signal amplitude is halved when the inductance is bridged across the connection to a 50 ohm signal generator.

For best accuracy remove the inductor when the signal is at half-amplitude. Adjust the signal generator to the previous (convenient) full-amplitude value. Replace the inductor and trim the frequency until the half-amplitude setting is found. Use this frequency to calculate L.

The best measurement for R should subtract the resistance of the meter leads when they are shorted together.

Program Resources

Labels

Name Description
 A Calculate L from entered R and f

Storage Registers

Name Description
 1 Inductor DC resistance R
 2 Frequency f

Program

Line Display Key Sequence Line Display Key Sequence
000 016 40 +
001 42,21,11 f LBL A 017 48 .
002 44 2 STO 2 018 0 0
003 34 x↔y 019 2 2
004 44 1 STO 1 020 5 5
005 2 2 021 45 1 RCL 1
006 1 1 022 43 11 g
007 48 . 023 20 ×
008 1 1 024 30
009 1 1 025 11 √x̅
010 36 ENTER 026 26 EEX
011 48 . 027 3 3
012 8 8 028 20 ×
013 4 4 029 45 2 RCL 2
014 45 1 RCL 1 030 10 ÷
015 20 × 031 43 32 g RTN