Simple Method to Measure Unknown Inductors
A simple and quick way to measure the inductance of an unknown power inductor
(provided you have a function generator and oscilloscope).
Ronald Dekker





Contents:

Introduction
2. method Step-by-Step
3. How it works & some theory
4. Including Series Resistance
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1. Introduction
Whenever I can I always salvage (power) inductors from old PCBs and switched
mode power supplies. A good assortment of different value inductors always
comes in handy during experiments, especially with boost converters and the
like. Now, I am sure that there must be a system by which manufacturers of
these inductors mark them with the inductance value, but so far I have not
been able to discover it. Some inductors have some numbers printed on them,
while others are marked with colored dots which are a disaster anyway
because I am color blind. To quickly sort out the inductance value of these
inductors I use a simple method which I am sure will interest other inductor
ignorami. The tools you need are a 0-100 kHz function generator WITH 50 OHM
OUTPUT, and an oscilloscope.  to top of page 	back to homepage



2. The Method Step-by-Step
Since most people will be more interested in the method rather than in the
theory behind it, let¿s start with a step-by-step description:


Connect the 50 ohm output of the function generator to the oscilloscope,
and select a sine-wave signal.
Adjust the frequency of the generator to approximately 20 kHz.
Adjust the output voltage of the generator to 1 V peak-peak.
Connect the unknown inductor parallel to the oscilloscope (Fig. 2.1).
Doing so will decrease the amplitude of the signal.
Now adjust only the frequency of the generator in such a way that the
amplitude on the oscilloscope is exactly half the original value (0.5V pp).

    The way I execute steps 3 to 5 is as follows: In step 3 I first set the
vertical sensitivity of the scope to 0.2 V/div. Then I adjust the amplitude
of the signal generator so that the sine wave exactly fits between the 25%
and 75% markings on the screen (Fig. 2.1A). The amplitude is now exactly 1V.
Next I connect the inductor (step 4), and increase the vertical sensitivity
to 0.1 V/div. In step 5 I now adjust the frequency so that the sine wave
again exactly fits in between the 25% and 75% markings (Fig. 2.1B). The
amplitude of the sine wave is now 0.5 V.

Finally, read out the frequency, and calculate the inductance from
L=4.57/f. With L in Henry and f in Hz. You may also prefer L=4570/f with L
in uH and f in kHz. 

	
	Figure 2.1 Measuring unknown inductors.

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3. How it Works & some Theory
The inductor, in combination with the internal series resistance in the
generator form a voltage divider circuit (Fig. 3.1). Without the inductor
connected, the voltage drop over the 50 ohm resistor is negligible and the
oscilloscope displays the ¿internal¿ voltage of the generator. With the
inductor connected, the current through the inductor will cause a voltage
drop over the 50 ohm resistor causing the amplitude of the signal on the
screen of the scope to drop. The current through the inductor is a function
of both the frequency as well as the inductance. For a DC signals (0 Hz) the
inductor represents a short circuit. For very high frequencies to current
through the inductor is negligible. Furthermore, for a given frequency, the
higher the inductance, the lower the current.

	
Figure 3.1 The ¿circuit diagram¿

The exact ratio between the internal generator voltage and the voltage measured 
by the scope can be calculated with a bit of straightforward network theory:
	

In this formula L represent the inductance, R the resistance (50 ohm), and
omega the radial frequency ( = 2*pi*f with f in Hz).
The question now is for what frequency (Vscope/Vgen) = 0.5:
	

So finally:

In which L is the inductance in Henry, and f the frequency in Hz.
This method only works well for inductors with a low series resistance,
and an inductance in the range of say 10 to several hundreds of uH.



4. Including Series Resistance. 
The nice thing about a website is that people from time to time make very
useful contributions. Karen Orton (UK) improved the method proposed above
for inductors which have a significant resistance. Simply DC measure the
resistance first and use it in the formula below. Otherwise the procedure is
exactly as described above.
	

Here is the math in Karen¿s own hand: