Need to make .22uH RFC

Discussion in 'General Electronics Chat' started by doug08, Mar 12, 2011.

  1. doug08

    doug08 Thread Starter Member

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    I made a choke of .22uH from an open air coil, but that is not the same as an rf choke....correct? I was told to wind some 30-32 ga magnet wire around a 1meg ohm resistor....but how many turns? Just cover the entire resistor? My tester only reads down to 1uh or .001mh. Can't check the value.
  2. t_n_k

    t_n_k AAC Fanatic!

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    That's a small inductance.

    The likely reason you would use the 1MΩ resistor is to provide a convenient means of winding the coil and then terminating (soldering) the conductor ends on the resistor leads.

    With such a small inductance every length of wire contributes a small 'parasitic' inductance. The resistor body would also add other parasitic components.

    Another method of obtaining a small inductance is to loop a conductor one or more times through a ferrite bead.

    Also consider SMD chip inductors as an option.
    Last edited: Mar 13, 2011
  3. Papabravo

    Papabravo AAC Fanatic!

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    "parts is parts" and "inductance is inductance"
    My point is that it doesn't matter how you created if it still does the job.
    Last edited: Mar 13, 2011
  4. SgtWookie

    SgtWookie Expert

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    9 turns of AWG-30 magnet wire wrapped in 1 layer around a 1/8" drill shank with no spaces between the windings will give you about 244nH. The resulting coil should be just under 0.1" long. 1 turn = 1 complete wrap around the drill shank.

    5 turns of AWG-30 magnet wire wrapped in 1 layer around a 1/4" drill shank with no spaces between the windings will give you about 246.7nH for a .055" long coil.

    4 turns of AWG-30 magnet wire wrapped in 1 layer around a Bic pen shaft (it's 0.33" in diameter) with no spaces between the windings will give around 235.5nH for a .044" long coil.

    In all cases, you can spread a turn (maybe two for the small diameter inductors) VERY slightly to decrease the inductance for tuning. A razor blade or X-acto knife is a handy tool to use. The smaller diameter inductor may be easier to tune. Dimensions are fairly critical.

    A short piece of straight wire has inductance that will be noticeable at those small values.
  5. AlexR

    AlexR Well-Known Member

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    No! An rf choke could quite easily be in the form of an open air coil, it all depends how it is used.
    The term RF choke describes the function of an inductor rather than its construction or any other attributes.
    The funtion of an RF choke is to present a high impedance to RF signals while having a low impedance to DC.
    Its hard to give a definitive answer without seeing you circuit diagram but I would have thought that for an RF choke you would need to use thicker wire than 30-32 gauge. After all the the object of the exercise is to provide a low resistance/reactance path for DC while blocking the RF.
    Using a 1M resistor as a former for an RFC is fairly standard practice. 6 turns of 22 gauge wire around a 1/4W resistor or 6 turns of 18 gauge around a 1/2W resistor should give you about 0.2uH.

    If you want to play around with other wire sizes and form factors then this site http://www.crystalradio.net/cal/indcal2.shtml provides quite a good online inductance calculator.
  6. doug08

    doug08 Thread Starter Member

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    I made one, and the circuits seems fine. Always good to get advice though.
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