I'm trying to understand how an inductor works in an RL circuit. When I use an air core inductor, my estimation is fairly accurate to the test circuit but as soon as I use an inductor with a core, the calculations don't match the real circuit. Here are both examples values

V = 24V AC and f = 50Hz for both examples.

Example 1 - air core inductor R = 17 ohms (coil's resistance) and L = 20mH Calculations for XL, Z, I, etc match the measured values. No problem here.

Example 2 - 20mH inductor with core (https://au.rs-online.com/mobile/p/leaded-inductors/1633404/) R = 20 ohms (external resistance added because coil didn't seem to have much DC resistance) L = 20mH Calculations don't match the measurements at all. Infact, the inductor behaves as if it's not even in the circuit. Hardly any voltage drop across it and the circuit current seems to be controlled by the resistor.

I know there is something here that I don't understand, can anyone please help me identify what I'm missing and explain it? Why are the calculations not working as expected in the second example.


  • 1
    $\begingroup$ Your "inductor with core" is a transformer. What did you connect to the unused coil when you did your test? $\endgroup$ – The Photon May 9 at 21:24
  • $\begingroup$ Thanks for your comment @the Photon, I believe the component I used is a common mode choke, not a transformer. I didn't connect the other side of the inductor to anything, just used one coil. $\endgroup$ – Hus May 9 at 21:32
  • $\begingroup$ A common mode choke is one way to use a transformer. But your result is more consistent with having shorted the unused coil than with leaving it open. Do you have a photo showing how you hooked everything up? $\endgroup$ – The Photon May 9 at 21:38
  • $\begingroup$ Unfortunately, I don't a photo but I'm sure I left the other two legs open. Hopefully this might help - I tried to test this problem with a contactor coil, pulled it apart and used it as air core and it worked as expected. Then I took the cores out and put them in the coil, which increased the inductance and the calculations didn't match after that. In this experiment there was only one coil. $\endgroup$ – Hus May 9 at 21:43
  • $\begingroup$ Whatever went wrong, I don't think you've shared it yet. What is the source of the 24 VAC? What instrument did you use to measure the voltage across the coil? How did you make the connections (did you use a breadboard, veroboard, solder-tacking, ...)? Etc. I don't know your experience level, but if you're really a beginner then we need to know things like, do you know which sockets in a breadboard are connected (if you used a breadboard)? Did you use the right input sockets of your multimeter? Etc. A lot of this might be answered if you could take a photo of your setup and share it. $\endgroup$ – The Photon May 9 at 21:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.