How can I measure the inductance of an unmarked inductor?

Question

I have a few handfuls of scavenged inductors that I really want to know the inductance of. Some of the air-core inductors I've been able to somewhat successfully (worked in filters I made with them and etc) ballpark the value using formulas and programs from electronics sites. However, that really only works with the ones I am able to figure out the wire gauge and from there the number of layers, and the inner/outer circumference of the core. Also, I have a fair number of ferrous-core inductors that I can't measure either. I don't have the money or resources currently for an oscilloscope or signal generator, although I suppose I could just put together a quick oscillator for a signal to use...anyway, isn't it possible to just measure the current through the inductor (or maybe with a known value capacitor too) with 60-hertz line frequency with a known (obviously lower than line) voltage? Then calculate the combined reactance/impedance etc?

Answer 1

The above answers are both correct. I would add two things though:

(1) Unless you are dealing with large power inductors, their reactance at 60 Hz is probably going to be minimal. Most inductors you find will be in the millihenries or microhenries range (for example, to be used in radios and the like) and thus will appear as dead shorts when tested at 60 Hz. Most inductor testers operate at 1 KHz or above, which means building your own circuit (and usually needing an oscilloscope etc)

(2) One of the best projects you can build for yourself as you learn electronics is an LC meter. Check out this project:

http://my.integritynet.com.au/purdic/lc-meter-project.htm

or this one:

http://www.amqrp.org/kits/elsie/

Also, look about halfway down this page and you will see a set of links to different styles of inductor markings:
http://www.educypedia.be/electronics/datacomponent.htm

Inductors are probably the least-consistently marked components in electronics. About the only thing worse are some older military electronics assemblies where the component identifications are intentionally obfuscated (nothing but cryptic stock codes printed on all components, resistors and capacitors and inductors all look identical to make things more confusing, unusual packages are used so you can't tell if something is a transistor, diode, or what, etc.).

It really helps in electronics to get into the mindset that it is worth spending the time and money on good basic tools and test equipment. Look for bargains at local HAM radio swap meets, and always keep an eye out for kits and projects like the above ones, they are frequently published in magazines like Circuit Cellar. And although it is now out of print, your library may have copies of Electronics Now (aka Radio-Electronics, Popular Electronics, or Poptronics) and other electronics-related magazines.

And as you said, one way to approach this problem is to use the inductor and a resistor to control an oscillator, and read the output frequency -- but you will need a frequency meter for that. A frequency meter is another great early project in electronics, here are a couple examples:
http://www.invent-electronics.com/freq_Counter.htm
http://www.apogeekits.com/frequency_counter.htm
http://www.norcalqrp.org/fcc1.htm

The real beauty of building LC meters and frequency meters and the like as electronics projects, is that you not only gain experience building the things but you als have a great working tool when you are done.

Answer 2

I guess theoretically you can use the 60 Hz 120 AC Volts from you home wall socket as your source, but I DO NOT RECOMMEND THAT AT ALL. You cannot just measure the current in an inductor because the inductor has reactance against current while allowing voltage to vary across it. The only thing you can try to do graphically measure out the current curve, measure the rise time and approximate the inductance keeping in mind the frequency that way. The problem is that the current is going to reach its settle point a lot quicker than you can measure it unless you have specialized equipment.

My advice to you is to bite the bullet and visit your nearest community college/university with a physics/electrical engineering department and try to get a lab tech to help you. Let me know if you need any help calculating or setting up to measure the inductance.

My question is why do you need inductors anyway? It is one thing if you do not have an O-scope, but no generator? What good is the inductor?

Answer 3

Inductor Markings

Answer 4

It is theoretically possible to do what you say, but how much accuracy do you need? What purpose are you going to use this inductor for? How are you going to determine the current or volatge rating? If it is a high-frequency "choke" that doesn't matter or a tuned circuit, resonant circuit, etc it might. It would take so long and have not very good accuracy I would say just figure out what you need and buy it rather than mess with trying to figure out what you have there. Also ... are you sure it is unmarked, you might look for distinguishing features and try to determine by markings or codes? If you have the resources, just buy the one you want, unless you are in school and want to do this as an excercise ... in which case you need to figure it out yourself anyway! ;-)

Answer 5

The resistor is used to sense the current.