Inductor testing

Thread Starter

Jonlate

Joined Dec 21, 2017
110
Hi folks,
I am building a nixie clock and am using two different value inductors a 100uh and a 33uh.
Neither have any markings on them and look about the same dimensions size wise.

But I have a problem. I have mixed them up and can’t tell which one is which.

Is there a simple way to test them to find out? I have tried google, but it seems to involve lots of maths and preferably the use of an oscilloscope that I don’t have.

So can I make up a SIMPLE circuit using things most people have on hand, to test them with a multimeter, that doesn’t involve any maths? (Resistors, breadboard, etc)
What’s the best way to find out which is which and even if they work?

Suggestions greatfully received!!
 

Ylli

Joined Nov 13, 2015
1,058
If they are of similar construction, you *may* be able to tell the difference with an ohmmeter. Assuming the same core material, the 100 uH will have more turns and thus a higher resistance than the 33 uH.

Post pictures and perhaps an excerpt from the schematic as to where they are used.
 

wayneh

Joined Sep 9, 2010
17,167
An oscilloscope would obviously be the quick choice but I don't have one either.

Well, actually I used a sound-card based o-scope to diagnose a problem I was having once. I had built an inductor coil ring-tester. It drives a signal into the coil under test, in parallel with a capacitor, and then it counts the number of times it rings. But a really big inductor would ring slowly and not appear to be good whereas a really small inductor might ring several times before the counter was online. My point is that all of this was a piece of cake to sort out once I could see the waveforms.

If the resistance test suggest above doesn't work, your cheapest way out is to simply buy new ones and don't mix them up this time. As @spinnaker noted, I've seen a lot of praise in these forums for the component testers you can get for <$20.
 

Thread Starter

Jonlate

Joined Dec 21, 2017
110
So no simple way to do it tonight with a multimeter!!

I feel sorry for the company I use for my parts, Farnell element 14, they do free postage regardless of what you order in Ireland, but I feel bad ordering two inductors that are then delivered by DHL!

Why don’t they print the values on them or have a code like resistors do?

Oh well, I can either guess the right way around, after all I might get it right!

https://www.nixieclock.biz/Manuals.html Classic 5 is the one have.

Thanks for the help
 

DickCappels

Joined Aug 21, 2008
8,831
Or feed some high frequency pulses into the inductor (with the other end grounded) then average the voltage across the inductor with a resistor and capacitor. You don't need calibration -you just want to know which inductors have a significantly higher average voltage across them.

upload_2019-2-12_1-15-9.png
 

MisterBill2

Joined Jan 23, 2018
12,427
If you have a means to measure the AC voltage across the inductors, connect a small capacitor across them, one at a time, and find the resonant frequency. The lower inductance will have the higher resonant frequency. All you need is some way to get a frequency, and that could even be whistling into a microphone feeding an amplifier. No math at all.
 

crutschow

Joined Mar 14, 2008
30,116
So no simple way to do it tonight with a multimeter!
Did you try measuring their resistance?

If the resistance is too low to give a reliable reading on your Ohm's scale, put them in series and run a small dc current though them until you can a readable voltage drop across both and determine which has the higher drop.
 

BR-549

Joined Sep 22, 2013
4,928
Have you got a low voltage AC source?

And you can always guess and install. You might need to switch them. Big deal.
 

SLK001

Joined Nov 29, 2011
1,548
Using an ohmmeter is the way to go. Even at the lowest setting, the 33μH will have a lower resistance than the 100μH. I'm assuming that the 33μH is part of an oscillator tank and the 100μH is a VCC choke.
 

MisterBill2

Joined Jan 23, 2018
12,427
Have you got a low voltage AC source?

And you can always guess and install. You might need to switch them. Big deal.
Both of the inductors are in switcher power supplies, the 100 is in the 170 volt supply while the 33 is in the 5 volt supply. So it may not be trivial to get them wrong. I checked the schematic in the link. Interesting circuit. With any decent DMM the low ohms range ought to give a clue. Some inductances are color coded, the 33 should have some orange, the 100 brown and black. Or maybe not.
 

Thread Starter

Jonlate

Joined Dec 21, 2017
110
Thanks all for your comments and answers.

I don’t have a way to put in or measure resonant frequency.
I don’t have a way to put in a small AC voltage, unless 240 is okay!!
I looked up colptts oscillolater but that went straight over my head!

So, I will try crutschow way and put a small voltage through them, as I can’t read the resistance on my meter at the moment, and that seems a simple way to find out the difference.

I willlet you know how I get on.
 

MisterBill2

Joined Jan 23, 2018
12,427
Thanks all for your comments and answers.

I don’t have a way to put in or measure resonant frequency.
I don’t have a way to put in a small AC voltage, unless 240 is okay!!
I looked up colptts oscillolater but that went straight over my head!

So, I will try crutschow way and put a small voltage through them, as I can’t read the resistance on my meter at the moment, and that seems a simple way to find out the difference.

I willlet you know how I get on.
The only thing that you need to use is a digital meter to read the voltage across the tuned circuit, which consists of a parallel connection of the inductor and a suitable capacitor. If you have a computer with a sound card then all you need is a cord with clips to plug into the line out jack on the sound card. The sound card can generate the variable frequency signal. You may also need a resistor, probably about 1000 ohms, (1 K), brown, black, red color code. Or some value between 1000 and 10,000 ohms. Somebody else can draw the circuit, I don't have a drawing ability on this computer.
 

MrSoftware

Joined Oct 29, 2013
2,025
Using an ohmmeter is the way to go. Even at the lowest setting, the 33μH will have a lower resistance than the 100μH. I'm assuming that the 33μH is part of an oscillator tank and the 100μH is a VCC choke.
Along these lines, check the data sheets for both inductors. The data sheet will list the DC resistance for the inductors, compare to see if there is a significant difference. If there is, then you can use your ohm meter to identify which is which. If the DC resistance is the same, or very close, then you'll need a different method.
 
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