Make toroid winding easier #t1 #filter #qcx #lpf #bpf


I've noticed a number of posts regarding winding toroidal inductors and the infamous T1 transformer.  I'd like to share my technique.
I've now finished my QCX and Ultimate Relay-switched LPF kits with 20+ toroid inductors and a transformer.  I started by winding several toroids the common way with a long length of enameled wire, then counting and recounting turns.  How tedious and frustrating!  
I remembered some techs 40+ years ago talking about winding toroids using a bobbin.  I could not find such a gadget in this century via an Internet search.  I tried making a bobbin using some heavy duty business card paper, but it bent and frayed quickly.  I cut a scrap of thin plastic material, but the points broke and the thin wire jammed in the V-notch.  So I 3D-printed my own design.  I takes about 6 minutes and 20 cm of filament to print one.  It has worked very well for both T37- and T50-sized toroids.  Bobbins for larger toroids can be scaled up from the T37-size using the slicer software for the printer.  See the bobbin at .
The toroid bobbin makes winding toroidal inductors and transformers relatively easy (OK- less tedious and frustrating) by holding at least 1m (T37-size bobbin) of #28 AWG (0.35 mm) enameled copper wire on its 4 cm length.  If longer lengths of wire are needed, simply put the wire on the bobbin until its full, then start your windings at that mid-point.  Once you use all the wire on the bobbin, then wind the other part of the wire on the bobbin and continue in the other direction.  Or use 2 or 3 bobbins in series, or make the bobbin 5-6 cm long.
The bobbin provides a grip for tightening the wire around the toroidal form.  It can also be used as a lead bending form for the vertical-mounting of resistors.
The way I make sure the wire ends of the inductor match how QRP Labs lays out the PCB holes is to hold the toroid between my left thumb and forefinger with the hole facing me.  I pass the wire end through the hole towards me (that's turn 1) and hold about 2-3 cm under my thumb.  Then I start winding turns counter-clockwise (anti-clockwise to some of us) around the toroidal form.  After the first 3-4 turns I'll snug the wire to the form.  Any fewer turns might result in pulling the wire completely off the toroid.
My revised method for counting turns is to wind five turns, then mark the fifth turn wire with a contrasting-ink very fine point marker (while relaxing my arthritic hands).  (I guess what the spacing between turns should be (based on experience now), and will adjust when finished.)  I wind another 5 turns, and another mark.  Repeat until all turns are on.  I don't mark the last few turns.  (Remember that each time the wire passes through the hole is one turn.)  I cut the bobbin loose leaving 2-3 cm of wire at the end of the inductor.
I adjust the turn spacing after all the turns are on so they are even, while leaving about a 30 degree gap between the starting turn and the final turn.  It's very easy to count that the toroid has 33 turns on it and my eyes don't cross.
I burn the enamel off the wire for about 6-7 mm very close to the finished inductor using a soldering iron and a blob of solder.  I clean the tinned wire, then feed the 2-3 cm of wire ends through the PCB holes, check that tinned wire shows both above and below the hole after snugging the inductor onto the PCB, and solder into place.


Thank you! What a great idea, even if it is a blast from the past.

Downloaded and printing now. I only have PLA so I will let you know how it works out. PLA pretty tough after it cures a few days.



Yes, please let us know how PLA works out.  I don't remember the material of the old ones, maybe handmade of bakelite or plexiglas, and for larger sizes than T37.

73, Greg  WA7IRW