I'm basically trying to determine everything about an unknow coil. I have a DC power supply, function generator, oscilloscope and various small electronics parts (resistors, capacitors and so on). I have zero information about this coil except that it has no core (air core). The diameter of the core is 7mm.

The coil itself is encased in plastic. I only have two wires available.

How can i calculate/measure the inductanse? How can i calculate/measure the number of coil windings?

Is it possible to calculate/measure the diameter of the wire, without measuring the wire itself?


  • \$\begingroup\$ Single layer, multilayer? Dimensions? info.ee.surrey.ac.uk/Workshop/advice/coils/air_coils.html \$\endgroup\$ Aug 25 '17 at 9:44
  • \$\begingroup\$ Can you access the central part of the coil i.e. the bit where a core could go but doesn't go because it is air cored? Can you see through where the core could go if it had one? \$\endgroup\$
    – Andy aka
    Aug 25 '17 at 10:23
  • \$\begingroup\$ Just updated the question with core diameter. \$\endgroup\$
    – HansEge
    Aug 25 '17 at 10:31
  • \$\begingroup\$ So, can you access the central part of the coil where a core could be inserted if necessary? \$\endgroup\$
    – Andy aka
    Aug 25 '17 at 11:18
  • \$\begingroup\$ @Andyaka yes, i can acces the central part of the coil. :) \$\endgroup\$
    – HansEge
    Aug 25 '17 at 11:21

If you can access the central part of the coil i.e. the bit that defines it as air-cored then you can calculate the number of turns. To do this you would fabricate a closed core using ferrite material and wind another coil of set number of turns around the fabricated core. It doesn't need to be a tight fit - the ferrite material should be chosen to have high permability so that magnetic lines of flux largely follow the path of the ferrite.

This allows you to couple about >95% of the flux produced by the added coil's windings through the unknown coil's windings (or vice versa). This turns it into a transformer and if you generate an AC voltage on one winding, the voltage on the other winding will be proportional to the turns ratio.

Better to choose high permeability core material to cut down on leakage flux i.e. flux that is produced by the driven coil not coupling to the measured coil. However, if you measure the coupling ratio in both directions you can more accurately calculate precise turns ratio.

You also should choose an AC frequency that is as high as possible but not so high that the core material losses play a role and produce inaccuracies. Something around 10 kHz is likely to be reasonable.

Inductance is easier - if you have a signal generator and a few capacitors you can drive the coil to resonance and, knowing F and C calculate inductance from the formula: -

F = \$\dfrac{1}{2\pi\sqrt{LC}}\$

  • \$\begingroup\$ Once you have the number of turns you can work out the length of wire from the mean diameter. You already know the resistance so with that information you should be able to look up a copper wire chart to find the gauge wire that will give that resistivity. +1 Andy for your usual highly practical approach to real world problems - in particular the reverse direction transformer test to work out the losses. \$\endgroup\$
    – Transistor
    Aug 25 '17 at 11:41

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