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Experimenting with Magnetic Components
Blog Experimenting with Magnetic Components - DIY Inductance Meter
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  • Author Author: Jan Cumps
  • Date Created: 7 Nov 2021 6:13 PM Date Created
  • Views 5635 views
  • Likes 2 likes
  • Comments 7 comments
  • experimenting_with_magnetic_components
  • winners
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  • bourns
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Experimenting with Magnetic Components - DIY Inductance Meter

Jan Cumps
Jan Cumps
7 Nov 2021

I'm reviewing a set of inductors for the Experimenting with Magnetic Components design challenge.

In this post: make a simple and cheap inductance meter.

 

w2aew has made a video (included later in this post) on this little design. The original comes from n4tmi, published in 73 magazine, September 1990). I'm trying it out, only using things I have at home.

 

The Bourns inductors #2, #7 and #15 of this kit will be used, and the custom one I made in post #8.

image

 

Short story: this is an oscillator.

The inductor under test in parallel with a known capacitor will have a resonance frequency.

This circuit will oscillate at that frequency. You measure that frequency at the output, then use a formula to derive the inductance of your unknown inductor.

 

Schematic, Layout and BOM

 

image

image source: drawing based on n4tmi's article in 73 magazine

 

Strongly simplified explanation. Check w2aew's video and n4tmi's article for details.

The Inductor Under Test and C1 (150 pF) form a resonant tank circuit.

C2 provides positive feedback to keep the oscillator swinging.

R2 puts Q2 in the saturation region. Like w2aew, I used 1K instead of the 470R in the original design.

The output can be checked with a frequency counter or an oscilloscope.

 

image

image: layout prepared on a piece of paper

 

I used perf board (part of a shopping cart I won with Project14). I placed components on a piece of paper first.

 

image

image: the circuit built up on a piece of perfboard

 

You can see that the final layout on the perfboard is a translation of that. I placed C3 the wrong way, may fix that later.

In the left slot you can place the inductor. The upper 3 pins are input, the lower 4 ground. In the right slot you can put the reference cap (C1).

These slots are interchangeable because both sides' inputs and grounds are connecting to the same circuit node.

If you want to measure the frequency of an unknown resonant circuit, remove C1.

 

image

image source: BOM based on n4tmi's article in 73 magazine, except the JFET + drawing

 

Before continuing, let's enjoy w2aew's video:

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video source: w2aew video #199

 

Try It

 

The circuit starts to oscillate when VIN is just over 9 V. I got the best results when the voltage was above 12 V. You can use it with two 9 V batteries in series.

I've tested it with 3 inductors from the Bourns kit, and with one I made myself.

image

image: device in action, measuring frequency with a counter and an oscilloscope

 

I got reliable frequency measurements with an oscilloscope, a traditional frequency counter and with the EEVblog 121GW DMM.

The load at the output has little effect on the oscillator circuit, because the input impedance isn't affected by what you connect to the combined JJFET sources.

 

Before checking the individual tests, let's first check the formula to translate frequency into inductance.

image

image source: still from w2aew video #199

 

The original article uses IBM Basic. I used a spreadsheet (attached)

Formula: =1/(POWER((2*PI() * E9);2)*E8)

where E9 is the measured frequency and E8 the value of known capacitor C1.

image

 

Test 1: Inductor #2, RF Choke, Radial Lead, Radial Lead, 3.3 µHRF Choke, Radial Lead, Radial Lead, 3.3 µH.

image

image: Bourns datasheet

 

image

Frequency: 6.7 MHz

 

Result:

image

3.8 µH

 

Check with LCR meter at 100 kHz: 3.587 µH

 

Test 2: Inductor #7, RF Choke, 1.8 mHRF Choke, 1.8 mH.

image

image: Bourns datasheet

 

image

Frequency: 294.490 kHz

 

Result:

image

1.9 mH

 

Check with LCR meter at 100 kHz: 1.7349 mH.

 

Test 3: Inductor #15, Choke, 100mHChoke, 100mH.

image

image: Bourns datasheet

 

image

Frequency: 39.8 kHz

 

Result:

image

106 mH

 

Check with LCR meter at 120 Hz: 103.82 mH

 

Test 4: Own made, toroid 60 µH.

image

image: mine

 

image

Frequency: 2.4 MHz

 

Result: (the oscillator wasn't stable like with the Bourns devices)

image

 

30 μH (see Jon's comment below)

 

Check with LCR meter at 10 kHz: 60 µH

 

 

Related posts
1. Boost Converter part 1: Inductor and Calculations
2. Boost Converter part 2: Build
3. Boost Converter part 3: Measure the Inductor in action
4. LCR meter experiments
5. SMD transformers experiment gizmo part 1: Build
6. SMD transformers experiment gizmo part 2: Measure
7. Common Mode Choke
8. Make your own Inductor
9. Calculate your own Inductor
10. Boost Converter part 4: Efficiency
11. DIY Inductance Meter
Planar PCB Transformer: GaN Point of Load converter 48V to 1V 50A
Measure Unknown Inductor Value with Function Generator and Oscilloscope
Experimenting with Magnetic Components: About the Competition
Attachments:
calculator.zip
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  • Jan Cumps
    Jan Cumps over 3 years ago

    Another option if you don't have an LCR or L meter, from EEVBlog.
    Use the capacty mode of your DMM:

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  • Jan Cumps
    Jan Cumps over 3 years ago

    The challenge is finished, time to recycle the material

    image

     

    If you are interested to review the inductor kit, and you want to eat the mail charge, I can send it to you.

    Without the box, in a padded envelope, the shipping costs shouldn't be too high. Open for  swap too.

    Let me know if interested.

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  • Jan Cumps
    Jan Cumps over 3 years ago in reply to genebren

    genebren  wrote:

    ...The attached video was interesting and informative, thanks for sharing!

    He has the gift of explaining more advanced topics in an understandable way. The videos give you knowledge that can be used in practice.

    There's a lot of great subjects: bipolar transistors, various FETs, opamps 101, antennas, mixing/modulating signals, using oscilloscopes/VNAs, practical circuits, prototype techniques, history ...

    It's wintertime. Watch all 340 videos image.

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  • genebren
    genebren over 3 years ago

    Excellent blog/experiments.  The attached video was interesting and informative, thanks for sharing!

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  • Jan Cumps
    Jan Cumps over 3 years ago in reply to jc2048

    Thank you. Yes, these cores are intended to be put around wires and filter, not as proper toroid kernels.

    That would explain it - my LCR has a max test frequency of 100 kHz.

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