Yet Another Inductance Measuring Scheme | Hackaday

How do you measure the value of unknown inductance? If you have an LCR bridge or electric meter, you may want to use it. If not, you can use many different techniques. All of this relies on the same thing my algebra teacher, Mr. Harder, said in the 1970s: You must use what you know to get what you don't know.

[Ronald Dekker] has to think in the same way. He took

. He set the signal output to about 20kHz and adjusted the 1V peak-to-peak value on the oscilloscope. Then, he put the unknown inductor on both ends of the signal and adjusted the frequency (and only the frequency) to achieve 1/2 volt peak-to-peak output.

The idea is that the size of the inductive reactance in the midway part must be 50 ohms (a 50/50 voltage divider is formed with the source impedance). [Ronald] did a detailed mathematical derivation, but the result is that the inductor (unit is uH) is 4570/f, where f is the frequency in kHz. In fact, it is not necessary to set the 1V reference completely, but if you read the complete article, it will simplify the way he makes measurements.

Of course, there may be some stray resistance in the circuit, but it is not enough to make a big difference in most inductors. If you have reason to doubt, [Karen Orton] contributed mathematical principles and got a slightly more complicated expression that allows you to consider the DC resistance of the coil in the calculation.

Of course, this is not the only game in town. We like to use

. On the other hand, if

Where you are interested, please talk to Elliot.

I like reading articles about alternative methods and techniques for identifying such unknowns... awesome!

that's nice

I used to use the 1khz reference output of a standard oscilloscope, pass it through a known capacitor, and then connect it to the inductor ground. Use the same oscilloscope to measure the inductance and check the ringing frequency. Calculate from there.

The only tools needed are standard oscilloscopes, capacitors and some mathematical operations.

Can any arduino-ey board with a good enough ADC build a black box fast enough?

STM32F103 has dual 12-bit 1Msps ADC and many functions. You can get a plank for $2 from China. Use external DAC + driver as signal generator via SPI.

You can make an LC oscillator and use *duino to measure the frequency of a known capacitance, and then calculate the inductance. There are few online level gauges based on microcontrollers. Some of them use external or internal comparators to form LC oscillators.

There are also AD9833/AD9834, which can be used to measure complex impedance, and then can be used to calculate the parameters of components or networks.

Instead of trying to build an AC meter through ADC sampling, use an active rectifier and measure the DC value. Compared to buying a faster op amp, you can reach the bandwidth and sampling limits at the ADC frequency.

Check out this simple circuit for use with a multimeter (PDF in Polish):

Inductance is an interesting thing.

If they are small (physically or just very low values), to obtain accurate measurement values, the test leads must be compensated, etc.

If you run at a high enough frequency, you get capacitance between the turns of the coil

Not to mention the stripline effect, PCB traces with open ends will become inductive.

Inductors outside of Prius can be large:

In addition, the video shows the same measurement technique. ;)

The strange part to me is having such a good signal generator, but not having a simple LCR meter. I guess a person can assemble something with DDS chips. You will need an adjustable amplifier to bring the output to a known level. Even so, you will change the output level as the frequency changes. I guess you can split it into the second trace of the dual-track oscilloscope and continue to adjust the output as it moves.

Here is the poor ham scalar network analyzer (PHSNA). Using Arduino, drive the DDS chip and get the amplitude from the log amplifier/detector.

DDS chip tends to change output with frequency, so it can be calibrated by scanning frequency without inductance. Repeat the inductor across the terminals.

You can also set the frequency to about 10 Hz to get an approximation to the DC resistance.

I played this game a while ago, using a square wave generator using PIC16 instead of the smitt trigger used in the video:

Except that my edge transition is not fast enough (probably) and divide the resulting inductance by 3, all other methods work well. I used an Excel worksheet to combine the errors and give the correct values. I checked the known inductor and it seems to work. I use a potentiometer from 1khz to about 20khz to adjust the frequency. I replaced the 10pF capacitor with a 560pF capacitor and it gave me a stronger signal. I used a breadboard, but it still works at 4.7uH.

I don't believe that my super cheap function generator outputs a constant voltage when the frequency changes, nor that my voltmeter can accurately measure voltage at high frequencies. Therefore, I have always used the ratio measurement method to measure the two voltages across the inductor and the series resistor at several frequencies, and then fit the model to the data. see

E.g. (This method can handle not only inductors, but also more complex devices. I usually use it to characterize speakers that may have multiple mechanical resonances.)

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