Sometimes you may be unable to find a particular inductor the market. This is actually a problem faced by most of the electronic hobbyists and the problem becomes more serious if your project is RF related. The inductors required for RF circuits (antenna, tuner, amplifier etc) are almost impossible to find in the market and the only solution is nothing other than home-brewing them.

how to make an air cored inductor

With a little practice and patience you can construct almost all air cored inductors at home. The inductance of an air cored inductor can be represented using the simplified formula shown below and to calculate the inductance of an air-core inductor, the same equation may be used.

L  = [d2 n2] / [18d + 40l]

  • Where’ L ‘  is the inductance in Micro Henries [µH]
  • ‘d’ is the diameter of the coil from one wire centre to another wire centre. It should be specifies in inches.
  • ‘l’ is the length of the coil specified in inches.
  • ‘n’ is the number of turns.

Notes :

  • The length of the coil used in the inductor should be equal to or 0.4 times the diameter of the coil.
  • As shown in the equation, inductance of the air-core inductor varies as the square of the number of turns.  Thus the value ‘l’ is multiplied four times if the value of ‘n’ is doubled.  The value of  ‘l’ is multiplied by two if the value of ‘n’ is increased up to 40%.

Winding the coil.

  • The coil must be first wounded on a plastic former of the adequate diameter (equal to the required core diameter).
  • The winding must be tight and adjacent turns must be as close as possible.
  • After the winding is complete, slowly withdraw the core without disturbing the coil.
  • Now apply a thin layer of epoxy over the coil surface for mechanical support.
  • Remove the insulation from the coil ends.

Example

Suppose you want to make an inductor which produces an inductance of 10 μH. The diameter of the coil is 1 inch and the coil length is given by 1.25 inches. You will have to find the number of turns of the coil.

Thus substituting the values in the above equation

L = 10 inches

d = 1inch

l = 1.25 inches

n = √{L [18d * 40l]} / d = 26

Thus, the number of turns of the coil will be 26.

Number of turns/inch = 20.8

Author

66 Comments

  1. Adedoyin Oyebanji

    Please how many turns for a coil wire of 0.2mm/32awg with 25mm diameter

  2. BAMIDELE DAVID

    Pls how can I wind a 220uH inductor?

  3. i would like to ask how can I make a 0.1 uH coil?

    • L = [d2 n2] / [18d + 40l]

      n = √{L [18d * 40l]} / d

      which is the correct one please ??

  4. Sir,can you please tell me how this equation is derived L = [d2 n2] / [18d + 40l] ,for designing an air core inductor. Sir please help me asap …….thank you

  5. how can i make a 680picohenry bobbin inductor

  6. I’m not understanding why the length of the coil should be “equal to or 0.4 times the diameter of the coil.”

    In the example the diameter is 1 inch and length is 1.25 inches.

  7. shawn wallace

    very helpfull,now i can make my own filters.i hate those nasty kickbacks from a tesla coil

  8. sir how to make 100mH inductor using 22swg copper wire?

  9. Sir,
    what is the gauge number of wire to use in equition

  10. Sir , can u please tell me how many turns will be required for making 1nH and what length and diameter we have to assume?

  11. M.Ali ismaeel

    how to build a 115mh inductor.plz inform me.

  12. zahid rasheed

    How to make an inductor of 0.098 Micro Henries [µH]

  13. lavesh parab

    Sir I want to make 3.3uH air core and I have wire of 22 gauge so how many turns I need to make and what will be the diameter of air core

  14. Hi.
    Doesn’t it better to apply the epoxy over the coil surface before making the winding?
    Since when the winding are already tight,it seems that the epoxy won’t get between the winding turns and the winding will short each other.Doesn’t the winding turns should be isulated from each other?while the epoxy purpose is to make this insulation?

  15. respected sir..
    can you tell how can i make 1nH & 2.2nH inductor. I am confused with the formula.plz help me sir.

  16. Sir, what kind of wire and the diameter do you suggest in making 200uH inductor for AM transmitter. thanks.

  17. what gauge is the best for 220uh with 6+ A peak and a 1000uh with 100ma? how many turns and the dia?

  18. chetan gupta

    how many turns and which gauge of copper wire be used to make the 22nH inductor?

  19. how to make a inductor of 10 mH which is being used
    in metal detector circuit…

    • Amin Avish

      You have to wind coil on ferrite core,it means you need a ferrite or iron core inductor..

  20. sir, i would want you to tell me the number of turns for making 0.01uH. thanks

    • seetharaman

      Use 6 mm dia plastic former wind with 22 swg wire wind 65 turns of 0.5″ length multilayer.

  21. will you please tell me how to calculate gauge of wire in this?

    • Seetharaman

      The gauge depends on the current flow required to flow through it. for RF application it should be at least 18 SWG or thicker as the signal current flows through the surface of the wire. hence to increase the conducting area use a thicker wire.

  22. Shailendra

    Sir I want to make a coil of 150uH. So what should be the values according to this.Please tell me.

  23. sir i make 6Mh inductor
    with 272 turns,
    22 Gauge copper wire
    2inch(50.8mm) diameter OD
    with PVc material as forme.
    now i want to calculate the value of l,N, d
    can you please tell me how to find these values

    • Seetharaman

      use the formula
      L = [d2 n2] / [18d + 40l]
      Where’ L ‘ is the inductance in Micro Henries [µH]
      ‘d’ is the diameter of the coil from one wire centre to another wire centre. It should be specifies in inches.
      ‘l’ is the length of the coil specified in inches.
      ‘n’ is the number of turns.

  24. sir i want to make a 0.1microhenry indutor coil so what will be the values of d n and l

  25. Seetharaman, is there any difference between 10uH and 10mH or are they thesame?

    • Seetharaman

      10mH / 1000 = 10uH ie 1000uH is equal to 1mH

  26. sir i have to filtering the 50vdc please tell me how much inductance are required to filter 50v dc and what is the length ,no of turns and insulator width and wire gauge

  27. Chinthaka

    Hai Dear Sir,

    1 need 1mH 1amp inductor and 4mH 1Amp inductor please kindly tell me how can i do this.

  28. I AM MAKING MOBIEL JAMMER NOW A DAY,s I REQUIER 22 MILE HENRY INDUCTER COIL
    WICH 500 MHZ USE IN CIRCUIT I DONT KNOW ABOUT IT I WANT TO MAKE IT PLEASE HELP OR SUGESTION ME.

  29. sir,
    if i want to make a 53uH coil by 24Awg,the arc is 15cm.then what will be the number of turn?

  30. I want a 0.156uH(Micro Henries) inductor and i use 18swg wire.Can you please tell me what would be the values of d,l and n.

  31. nice post thanks!!
    But L=MicroHenries

    So in your example is must be a typo “L=10 inches” it should be 10 uH

  32. Seetharaman

    Hi Shinoj AWG is american standard for wires. SWG is standard wire gauge for British and Europien standards. There will be a difference between both the standards. there are conversion tables available to compare both standards. a sample is shown below
    http://www.qsl.net/lx2sm/conversion1.htm

    • Hello Sir, am very greatful on your contribution to educate the electronics curious people which am also the beneficiary of ur programme, please kindly publish a wireless transmission signal circuit for speakers to sound transmitting system.

  33. seetharaman

    Hi Anish you have contact a motor/transformer winder or a good electric supplier who will be in a position to supply.

  34. Thankyou sir,
    But where do I get these wires. Like tell me some local shops where I can buy these wires. And how to ask them. Once I asked for a 24AWG copper wire in a radio house where i buy electronic components.. They are blinking.

  35. Thankyou sir, but where do I get these wires. Like tell me some local shops where I can buy these wires. And how to ask them. Once I asked for a 24AWG enamelled copper wire in a radio house where i buy electronic components.. They are blinking and saying they never heared such thing.

  36. seetharaman

    Hi Anish it depends on the application. For higher current a thicker (lower gauge number say 18 or 16SWG) of wire is required. For lower current thinner (higher gauge number say 32 or 38SWG) wire will do. For low frequency coils say Medium Wave a litz wire is preferred. For higher frequencies say at 200MHz or greater thicker wire is preferred, not that it has to flow higher current but higher frequency current flows through the skin of the wire. You refer to gauge table you can find the details.

  37. Hi sir,
    What type of coil wire to use? 22SWG, 24SWG etc,,, they recommend different type for different applications. How to find that?

  38. Abhishek

    How to use it as Centre tapped Inductor , for connections to Antenna?? in A radio receiver.

  39. Jady Burg

    great joob really help fulll but insulater is must for winding or any other material can be used
    ??????????????????????

  40. Someone asked how to make a 200 to 500 uH coil. These were the parts values given for the L3 in the 2 km FM transmitter. This choke is not needed for this project. I explain why in my blog here.

    watsonseblog.blogspot.com/2011/01/2011-jan-25-circuitstoday-fm-tx.html

  41. Seetharaman

    Hi Jatin 1mH(milli henry) is 1000micro henries substute in the above formula.

  42. Seetharaman

    Hi Fady you mean 200 to 500uH (micro henrys) use tha above formula. it cannot be variable but either 200 or 500uH can be made.

    L = [d2 n2] / [18d + 40l]

    •Where’ L ‘ is the inductance in Micro Henries [µH]
    •‘d’ is the diameter of the coil from one wire centre to another wire centre. It should be specifies in inches.
    •‘l’ is the length of the coil specified in inches.
    •‘n’ is the number of turns.

    • abhijith sivan

      sir. i am new to this feild but i am very interested in this. would u plz tell me how many turns required for 22nh at which size wire??

  43. how to conect the headphone socket in the circuit as it has 3 plugs
    plz ans me as soon as possible