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Like the question, i don't have an oscilliscope/Function Generator/LCR Meter. I do have a multimeter and a lot of "scrap inductors" that i would like to measure.

I have searched a lot on the web and everyone seems to include OSC or a Function Generator. I also have a lot of parts laying around so that i can build my own circuit if needed.

I don't really have any requirments, It doesn't have to be a 100% accurate reading, around 70% works fine for me. Thanks.

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  • \$\begingroup\$ This is a really broad question, and you don't have any requirements. It appears that you haven't done any research either. Please rewrite your question. Are you willing to build your own circuit? Have you looked for circuits that you might be able to build? \$\endgroup\$
    – Voltage Spike
    Mar 3, 2016 at 23:33
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    \$\begingroup\$ The specifications of an inductor are far more than just inductance. Unless you have very undemanding requirements or your scrap inductors were especially expensive ones, you may as well just throw them away and buy new ones (that you know what they are) if you need them. They aren't expensive, in most cases. \$\endgroup\$ Mar 3, 2016 at 23:35
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    \$\begingroup\$ @laptop2d Updated question. @ Oleksandr R. The reason why i keep them and not buying new ones is because i just like to play around with different circuits regarding switching regulators. And i have a LOT of inductors so i don't really feel like throwing them away. \$\endgroup\$
    – Xane
    Mar 3, 2016 at 23:39
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    \$\begingroup\$ Designing switching regulators with completely unspecced inductors seems like a massive waste of time. When your regulator misbehaves, how do you know it's not due to the entirely wrong flavour of inductor? \$\endgroup\$
    – uint128_t
    Mar 4, 2016 at 0:48
  • \$\begingroup\$ I do have a few inductors that has the value written on them, those are the ones i'm playing around with. \$\endgroup\$
    – Xane
    Mar 4, 2016 at 0:54

3 Answers 3

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Absolutely. You will need a variable-frequency oscillator, often called a function generator, though, and a meter of some sort (DMM or oscilloscope). Connect the oscillator (set up for sine waves, please), the inductor, and a resistor like this

schematic

simulate this circuit – Schematic created using CircuitLab

Start at very low frequency, and the voltage at the test point should be zero, or very close. Increase the frequency and the voltage at the test point will rise. Find the frequency at which the voltage at the test point is one half the voltage at the oscillator. Depending on the frequencies available, you may have to try different resistor values to find one which works. Call this frequency f. Then, since the impedance Z of the inductor can be written as $$Z = 2\pi f L $$ and for equal voltages across Z and R, Z = R $$R = 2\pi f L $$ and $$L = \frac{R}{2\pi f} $$ This will be true for what are called high-Q inductors, which have a low series resistance, and for low-Q resistors the problem becomes a good deal more complicated, but it's a good place to start. Also note that this value of L is only correct for this frequency. For something other than a high-Q inductor, the results will vary (a little or a lot) with frequency.

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  • \$\begingroup\$ This is exactly what i was looking for! Thank you. Even tho i need a function generator or similar for this, Do you think that i can use a "PC sound card" function generator? One that i plug into the DC jack? \$\endgroup\$
    – Xane
    Mar 4, 2016 at 0:57
  • \$\begingroup\$ @Xane - I expect that will work OK, but be aware that it's only going to work for relatively high values (like 1k or more) for the resistor. PC sound outputs aren't intended to drive low impedances. \$\endgroup\$ Mar 4, 2016 at 0:59
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Yes use mains frequency from an AC power supply.

Connect the AC supply via a resistor and measure the AC current and AC voltage at the inductor.

You can then get the inductive reactance using ohms law R = V / I.

The current and voltage on the meter will be RMS, but this doesn't matter as the reactance is the "resistance" of the inductor at that frequency.

Once you have the reactance you can use:

Reactance(X) = 2 x pi x Frequency x Inducance(L)

solving for L gives:

Inductance = Reactance / (2 x pi x Frequency)

You know the frequency as this is your domestic mains frequency, you worked out the reactance, so you can calculate the inductance.

Previous answers tell you to use a function generator, but the OP asked how to do without a function generator (FG)

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Regarding, "I also have a lot of parts laying around so that i can build my own circuit if needed."

Then you want to build "OSC or a Function Generator". Since you gave us zero information about the range of inductors you are trying to use we can only give you answer for what you should try to learn.

You can use an Arduino UNO as a function generator and you can use it as an oscilloscope. The "tone()" function of the Arduino works over audio frequencies. Do not connect the digital output pins directly to your inductor under test. You should buffer the output through an amplifier circuit. For messing around with audio frequencies learn about the audio amplifier ic lm386.

However this might work (I am speculating). Have your Arduino make a 60 Hz tone on an output pin. Measure that pin with your multi meter and make a note. Then put a series resistor of 1K from the Arduino output pin into your inductor. The other end of the inductor is connected to ground. Measure the voltage with the multimeter at the output of the Arduino again to see if it has been badly loaded and is no longer driving full amplitude (it should not be). Measure the output at the junction of the 1K and the inductor to ground (the other side of the inductor). This is the voltage across the inductor. It is likely to be a very small voltage. You have constructed a high pass filter by the way. The higher the voltage the higher the inductance you have. Learn about this high pass filter here: https://www.allaboutcircuits.com/textbook/alternating-current/chpt-8/high-pass-filters/#:~:text=A%20high%2Dpass%20filter%20allows,in%20parallel%20with%20the%20load.

Learn how to calculate your inductance from the equations there.

Using the Arduino as an oscilloscope in the same circuit is beyond the scope of an answer I am prepared to invent at this time. Use your multi meter.

If your multi meter works well at higher frequencies you might change the Arduino tone() function for 600 Hz or 6,000 Hz and repeat the experiment and may get better sensitivity.

I hope this helps.

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