Description of Inductors Elmer Station

This station provides facilities (clamps, tinning materials, extra wire/cores, LC Meter, microscope) to facilitate the builders' coil and transformer winding tasks. The Elmer should be ready to consult on:

  • winding the inductors (L1 and T1)
  • mounting the inductors (especially T1) on the board.


This station should be located in a well-lit, bright area, preferably by a window or under a bright light.



spool of #30 thernmaleze magnet wire1
Spare T25-6 toroid cores25
15' hank of color coded thermaleze bifilar wire1
spool of 63/37 solder .032"1 (for tinning)
jar of paste flux1 (for tinning)
black and yellow wire insulation1 (cut into 1/8" bits to serve as identifying "sleeves" for leads)
UNCOATED cookie sheet1 (for tinning)


    wooden skewers 6
    Irwin Clamps, short clips4
    25W soldering iron 1
    Tweezers 1
    diagonal cutters 1
    PC with microscope software loaded1
    PC microscope 1(for verifying turn count)


    LC Meter 1(optionally measuring L1 inductance)
    DMM 1(identification of secondary leads)


    Elmers should read/provide a copy of the Common Construction Techniques articles on Toroidal Inductors for reference. A pdf file is here for downloading and printing.

    L1: 15" of #30 enameler wire wound 30 turns on T25-6 for 2.5 µH

    • Wire

      Builder measures out about 15" of #30 wire
    • Wind

      Builder winds L1 coil: 30 turns of #30 wire on a T25-6 (yellow) core using fingers, or skewer, or Irwin clamp. It is adviseable to leave the leads relatively long at this point in case more turns are required.
    • Tin Leads

      Builder tins coil leads to within 1/8" of core. Tinning involves using paste flux and then running the wire thru hot solder blobs on the iron
    • (Optionally) Verify Turn COunt

      Builder can use PC microscope to help verify turns count
    • (Optionally) Test Inductance

      Elmer tests the coil's inductance on LC Meter:
      • release all switches (on the right side of the meter)
      • Push in on the "PWR" switch on the right side of the meter.
      • Once calibration is completed, push in the "LX" switch on the right side of the meter.
      • When meter reads "not an inductor", press and hold the push button on the top of the test harness plugged into the meter. This shorts the leads
      • Push the "ZERO" button on the right side of the LC Meterand then release it
      • Rrelease the pushbutton on the top
      • Insert the coil's leads into the pin holes on the test harness atop the meter
      • Read the inductance. It should be close to 2.5 µH (+/- 2µH) on the 20m RX's coil. Recognize that the inductance will change somewhat once it is installed onto the board and the leads are soldered and trimmed.
      • release all switches (on the right side of the meter)

    T1: T25-6 with primary and 2 secondaries

    See special instructions document for winding T1
    • Wire for Primary and for Secondaries

      Builder measures out two 7" strands of #30 wire, folding one of the wires over double and twisting it together about 3 twists to the inch
    • Wind Primary

      Builder winds transformer on a T25-6 core, using the 7" wire to wind 8 turns of #30 for the primary
    • Wind (bifilar) Secondaries

      Builder winds 4 turns of the folded over, twisted wire on top of and in the same direction as the primary.
    • Tin and Tag the 2 Primary Leads

      Builder tins (see above)and then identifies primary wilres (using bits of insulation slipped over the leads. Recommended colors:
      • Black insulation for primaries
      • red insulation for one of the secondaries
      • no insulation for the remaining secondary
    • Separate and Tin the 4 Secondary Leads

      Builder separates and tins (see above) both pairs of secondary leads
    • ID and Tag Secondary Leads

      Builder uses DMM's continuity or ohmmeter tester to identify which leads go to which pairs and identifies them using a different color of insulation
    • Finished

      Builder returns to builder station to mount inductors per builders notes.

    Pay Special Attention To

    • Turn counting - any pass through the center of the core = 1 turn
    • Make sure both the primary and the bifilar secondary windings ot T1 are wound in the same direction
    • L1 should end up with approximately a 15 degree "wedge" of clear core at the bottom.

    Build Stages Supported

    Band Pass Filter Stage