Measuring coil inductance on the cheap.

[registereduser]

How did old school hams/experimenters determine random scrounged coil inductance without a multi thousand $$ layout on oscilloscopes, frequency counters and a half a dozen other specialty items? Suppose all you have is a cheap multimeter that does measure capacitance but not frequency?

Is this a legit method?

https://www.instructables.com/Measuring-Inductance-With-a-Multimeter-and-a-Resis/

If not can you point to a good write up on a method that is and is written so anyone can understand it.

I have a multimeter, some caps, resistors, diodes, transistors and a couple of quartz crystals (don't know if the crystals are any good) for building test circuits.

[TimFox]

Old-school hams would use an inexpensive Grid Dip Oscillator with a calibrated frequency dial, loosely coupled to the coil that was connected to a known-value capacitor.
You can do the same with an inexpensive RF generator and an oscilloscope or RF voltmeter.
To see the resonance clearly, the source must couple loosely to the L-C circuit.
The voltmeter or oscilloscope must present a high impedance to the parallel L-C circuit, and the capacitance of the probe must be included in the calibration.

See  https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/Servicing/How-to-use-Grid-Dip-Oscillators-Rufus-P-Turner-1960.pdf

[Benta]

All methods using basic physics are legit.
But you need to specify what you want to measure.
Trying to find the inductance of an IF coil in a radio receiver is completely different to finding the primary inductance of a mains transformer.

[TimFox]

For RF purposes, a commercial grid-dip oscillator had a set of different coils for different inductance values.
Audio- and mains-frequency measurements are easy to do with an audio signal generator and an appropriate known capacitor value.
Old-school experimenters did not use their Simpson 260 to measure inductance.

[donlisms]

And then for the big-budget players, there were impedance bridges, e.g. the ESI 250DA or similar.  I don't know what they cost back in the day, as a percentage of, say, the cost of a loaf of bread, or a new house, but they don't seem like they'd be THAT expensive for someone who needed it often.  They're sure not very expensive now!

[Electro Fan]

And then for the big-budget players, there were impedance bridges, e.g. the ESI 250DA or similar.  I don't know what they cost back in the day, as a percentage of, say, the cost of a loaf of bread, or a new house, but they don't seem like they'd be THAT expensive for someone who needed it often.  They're sure not very expensive now!

https://www.ebay.com/p/1018862844

[radiolistener]

if you have known capacitor, you can connect it in parallel to get LC circuit and use it for to generate carrier, then use some receiver that has frequency indication and find that carrier, then you can calculate inductance from known frequency and capacitance. So, all what you needs is just receiver, capacitor, solder iron and a couple of transistors and resistors 

Such way has some error due to parasite capacitance and inductance of your circuit, but it give you good enough result.

The same way was used in olde LC meters.

[MathWizard]

Make a test jig with a few Collpitt's oscillators, covering a few ranges. Sometimes I'll use DIP sockets to plug capacitors into, on a PCB/protoboard.

Do you have an oscilloscope tho ? A common DMM would have trouble as the frequency goes up, and Oscillator circuits can be pretty annoying to get working sometimes, especially without a scope.

There are well enough designed Collpitt's osc. circuits out there, that you should be able to get working, and have a large enough Vrms for a DMM, at least for under a few MHz I would think.


Cheap $20 multi-part-testers like on Ebay/Amazon will read common RF inductors no problem.

[Berni]

Does your multimeter have a decent frequency measurement feature?

If it works well into the 10s of KHz then you can build an LC oscillator circuit that pairs the unknown inductor with a known capacitor value, then measure the resulting frequency.

It all depends on the range of inductance you want to measure. For the mH range the 50/60Hz method you linked works well. For uH range inductors the LC oscillator mentioned above works well. For nH inductors things get tricky since things go into RF territory, so something like an network analyzer is the easiest.

Tho these cheep chinese multi component testers are actually really good:
https://aliexpress.com/item/1005005497975495.html

[Circlotron]

Tried to measure the inductance of a powdered iron toroid by putting a known capacitor across it and seeing what the resonant frequency was. Turns out the inductance is very level sensitive. Inductance easily varies by 50%

[TimFox]

And then for the big-budget players, there were impedance bridges, e.g. the ESI 250DA or similar.  I don't know what they cost back in the day, as a percentage of, say, the cost of a loaf of bread, or a new house, but they don't seem like they'd be THAT expensive for someone who needed it often.  They're sure not very expensive now!

https://www.ebay.com/p/1018862844

Also, people with good budgets bought "Q meters", originally from Boonton and then continued in production by Hewlett Packard.
https://www.angelfire.com/electronic/funwithtubes/Q_meter.html

[RoGeorge]

How did old school hams/experimenters determine random scrounged coil inductance
...
Is this a legit method?
https://www.instructables.com/Measuring-Inductance-With-a-Multimeter-and-a-Resis/

That could be a method, but most probably they were rather using an RLC bridge and a null detector, something like this (random search result, I didn't test it):  https://www.eeeguide.com/lcr-bridge/

[Berni]

Tried to measure the inductance of a powdered iron toroid by putting a known capacitor across it and seeing what the resonant frequency was. Turns out the inductance is very level sensitive. Inductance easily varies by 50%

Yeah ferite materials are a whole nother kettle of fish.

Those are trickier to characterize quickly on a bench. Some ferrite are low permeability while handling high field strength, some are high permeability but only low field strength, some are really lossy everywhere on purpose, some are only lossy at high frequency. Some are even vary a lot with temperature. Some are designed for RF use, some for energy storage, some are really lossy on purpose to be used for interference filtering etc.. Id imagine the ferite rings used in core memory were specifically formulated for the job to make them easily magnetizable in a consistent way.

But yeah my method for testing ferite rings is to pair it with a known capacitor, attach a scope, then briefly tough a PSU to it to get it ringing. The way it rings can tell you a few things about it. But in general i avoid using unknown ferrites