WindingToroids - It isn't really that tough!
By Michael S. Fisher  WT9W

Hopefully, if you're reading this, you are sold on the idea of kit building.   I find kit building to be one of the most enjoyable experiences in Amateur Radio.  However, there are certain tasks that some feel, don't add to the enjoyment.  One of these tasks is winding toroids.   If you are into kit building you will need to face this task at some point since toroids are used in many circuits such as oscillators, filters and are also used as transformers.  This guide will give you some tips to help the process go smoothly and may actually make it a bit more enjoyable.

SUGGESTED TOOLS
First, you will need some special tools to perform this task as painlessly as possible.   They include:

Magnifying Glass or Magnifying Visor (preferred)
Butane Lighter
Small Jewelers Screwdriver
Fine point black Magic Marker (sharpie)
Fine Sandpaper
Soldering Iron
Solder
Solder Pot (optional)
Long nose pliers
Ohmmeter

WINDING THE TOROID
The first step is to follow the kit manufacturer's instructions  very carefully.  The instructions will specify which core (ferrite or iron powder), the length of wire to use, and how many turns are required.  Many of the cores that you will use are color coded.  If not you may find the cores bagged separately to minimize the possibility of using the wrong one.  You must make sure you select and use the proper core and wire.  Some toroids may specify a smaller gauge wire when many turns must be wound on the core.  The instructions will indicate how many turns are required for the toroid and how to wind it (start point and which direction to wind in).  The method of winding is very important for two reasons.   First, if the toroid is not wound as designated, it may not fit on the board as intended.  Second, some windings on the toroid may need to maintain a particular polarity.   These toroids are said to be wound with a certain "sense" (reference of one winding to another) that must be maintained for the circuit to perform properly.  YOU MUST FOLLOW THE DIRECTIONS CAREFULLY!   Refer to any diagrams or pictures that are provided.

After winding a toroid I always use a magnifying glass and a small jewelers screwdriver to check the number of turns.  I first separate the turns as much as possible on the core.  Then I view the toroid under a magnifying glass or magnifying visor and use the small jewelers screwdriver as a pointer to help count the number of turns.  I always count twice.   EACH TIME THE WIRE PASSES THROUGH THE CENTER OF THE TOROID COUNTS AS ONE TURN.

Some toroids will require a seconding winding.  You must follow the instructions carefully to make sure that it is wound with the correct "sense".  There are also some toroids (bi-filar and tri-filar) that require that two or three pieces of magnet wire be twisted together before winding them on the toroid.  Kit manufacturers are usually very good at explaining how the toroids should be wound and how they should look when complete.  An excellent example is located on the Elecraft web site.  Click here to view a picture of the toroids that were wound by AC6KW for the Elecraft K2. In this picture L18 is a toroid using a single wire (it has 2 leads), T5 is a transformer wound with two separate pieces of wire (it has 4 leads) and T3 is a bi-filar (two wires twisted together)  transformer (it has 4 leads).  

PREPARING THE LEADS
Now that you have wound the toroid it is time to prepare the leads.  First you must determine how the toroid is to be mounted on the board (laying flat or standing vertical).  You will then need to mark the wire leads using the fine tip black Magic Marker (Sharpie).  I mark the leads at the edge of the toroid just before the lead would hit the pc board when the toroid is mounted flush.  I mark it this close to the toroid because you will need to unwind one turn from each lead in order to remove the enamel.  When you rewind each turn, the leads will usually be slightly longer due to the windings being a bit tighter on the toroid.  This is caused by the enamel removal and rewinding process.  For this technique, the mark on the wire will wind up being at the perfect location for removing the enamel so that you end up with a good solder connection and minimum lead length.

REMOVING THE ENAMEL
Now you will remove the enamel from each lead.  YOU MUST EXERCISE CAUTION SINCE YOU WILL BE USING AN OPEN FLAME IN THIS PROCEDURE.  First, unwind one turn from each end of the toroid.  If this is a bi-filar or tri-filar toroid you will need to untwist the leads and work with one at a time.  You must make sure you have enough clearance between the untwisted leads so that heating one won't cause a short with any of the other leads.  Now while holding the toroid by the core, place the end of one of the leads over the flame from the butane lighter.  You will find that the enamel burns at a very predictable rate.  When you remove the wire from the flame, a bit more of the enamel will burn.  With some practice you will be able to burn off the enamel to the precise point that you marked the wire with the black Magic Marker (Sharpie).  Until you become proficient at this, I suggest using slightly longer pieces of wire when winding the toroids.  The leads should be 1 - 2 inches long after winding all of the turns on the toroid.  This will give you a more comfortable amount of wire to work with when burning off the enamel.  Once you have burned the enamel you can remove the ash by using the fine sandpaper.  Take a small piece approximately 1/2 X 2 inches and fold it in half  lengthwise.  Place the wire between the folded pieces of the sandpaper and apply a small amount of pressure. Then gently draw the wire away from the sandpaper.  Do this several times slightly rotating the wire after each time.   This should remove all of the ash and leave a shinny wire.  Do not apply too much pressure since this may cause the wire to break. 

Now using the soldering iron and solder, tin the bare leads.  Usually any enamel that was not removed by the sandpaper will come off in the tinning process.  When you have completed all of the leads rewind the one turn for each lead that you removed.  If the toroid was a bi-filar or tri-filar, don't forget to re-twist the leads before winding the one turn back on the toroid.  After re-twisting the wires, make sure that the bare tinned leads are not shorted.  Next you must count the turns of wire on the toroid to confirm that you have the correct number.

Just a quick word about those windings that are only 1 - 4 turns.  You will need to handle these a little differently.  Wind the toroid as described previously and mark the leads for removal of the enamel.  Remove all of the windings from the toroid.   This is necessary since attempting to sand the wire with the windings still installed on the toroid will cause you to pull the wire right off the core.  There just aren't enough turns to hold it in place.  Hold the wire with the long nose pliers when burning off the enamel and removing the ash with the fine sandpaper.  I usually allow a little extra wire for the ends.  If you hold the wire at the very end while removing the enamel, the spot where you held it will wind up being cut off after the soldering process.  That way you won't have to worry about any damage to the wire where it was held by the long nose pliers.  Once the enamel has been removed you can rewind the toroid and tin the leads as described previously.

ALTERNATE METHOD OF REMOVING THE ENAMEL      
There is an alternate method of removing the enamel that is favored by many kit builders.  This method uses a solder pot or soldering iron to remove the enamel and tin the wire in one step.  If you have a solder pot, turn it on and wait until it comes up to temperature.  When it is at the proper temperature, the solder will be in a molten state.  To remove the enamel, insert the wire into the molten solder up to the mark you made to indicate how much enamel needed to be removed.  The enamel will melt off  and the solder will tin the wire in one step.  The ash from the enamel will remain in the solder in the pot. 

A soldering iron can also be used to remove the enamel.  The method works best with a temperature controlled iron or workstation.  If you can adjust the temperature on your iron, you will want to set it between 750 and 800 degrees F.  If your iron has removable tips, put on a larger tip then you would normally use for pc board soldering.  The larger tip will hold more solder and will maintain a more constant temperature during the enamel removal process.  To start removing the enamel, melt a large blob of solder on to the tip.  The solder should melt almost instantly and should remain molten on the soldering iron tip.  Next, run the wire slowly through the solder on the tip until you reach the mark you had put on the wire.   You will see the enamel being removed and the wire being tinned at the same time.  The solder will retain the ash from the enamel.  This will require you to clean the contaminated solder off of the soldering iron tip and melt fresh solder on to the tip frequently.

SPACING THE LEADS AROUND THE TOROID
At this point you will want to make sure the windings are spaced as evenly as possible on the core.  In most cases the windings should cover about 85% of the core.  The instructions will usually tell you how the finished toroid should look.  The spacing of the windings does affect the inductance of the toroid.  If you have a meter that measures inductance, check the inductance of the toroid and compare it to  the value listed on the circuit schematic or table of parts.  If you need to adjust the inductance, try spreading the turns to increase the inductance or compressing the turns to decrease the inductance.  The spacing of the windings can also be adjusted for best performance during the final circuit alignment.  There will usually be references in the alignment instructions on how to do this.

MOUNTING AND SOLDERING
The next step is to mount the toroid by pulling the leads through the proper holes in the board.  YOU MUST BE VERY CAREFUL, ESPECIALLY WHEN THE TOROID HAS MULTIPLE LEADS.  It is very easy to put the leads in the wrong holes.  Pull the leads so that the toroid is snug against the board.  Bend all of the leads to hold the toroid in place.  Now solder one lead.  When soldering the leads pay careful attention to how the solder flows.  One of the biggest causes of problems with toroids is not getting a good solder connection between the toroid leads and the solder pads.   The solder joint should look shinny, smooth and rounded without any voids.  If the lead appears to be an "island" in a small pool of solder, chances are that it is not making good contact.  Now check to make sure the toroid is still snug against the board.  If not, reheat the solder joint you just made while applying gentle pressure to the toroid so that it will be snug against the board.  Be careful not to burn yourself since the toroid will get hot.  I usually place a folded piece of paper over the toroid so my finger doesn't get burned.   When you are satisfied with the position of the toroid, solder the remaining leads. 

After completing the soldering it is always best to double check the solder joints using an ohmmeter.  Place the leads from the ohmmeter on the solder pads (not the bare leads) of two of the leads from the same winding of the toroid or that are electrically connected in the circuit.  You should measure almost no resistance.  If  the resistance is high or infinite, re-check the solder joints.   If you still have a problem, you may want to try repositioning the wire by heating the solder joint and pulling a small amount of it from the toroid side of the board. Check the solder joint and add additional solder if necessary.  Once you are sure that you have good solder joints you can cut off the excess lead above the solder joints.

CALCULATING REQUIRED WIRE LENGTH FOR TOROIDS
The instructions in your kit will usually tell you how long the wires should be for winding the toroids.  I usually add a small amount (an extra inch) just to make sure there is sufficient lead length to help make the process of removing the enamel easier.  The kit manufacturers usually provide extra wire so this won't be a problem.  If you are unsure of how long the wire should be for the number of turns specified on a certain size core,  you can use the table below to figure the length.

Length in inches = (Desired Number of turns) X (IPT) + 3 INCHES

IPT = Inches per Turn
Table data and calculation by Robert Olson, WD4OHD published in QST Technical Correspondence - January 1999.


I hope this guide will be useful and help you get through winding your first toroids.   With practice this process will become fairly easy to perform and you should experience excellent results.  The more you do, the easier the process will become.  You may actually begin to enjoy it.  If you have any questions or comments, please e-mail me by clicking on the link below.