How to Wind a Toroid Inductor: Turns, AL Value and Tuning
If you build low-pass filters, matching networks or small receivers, sooner or later you will need to wind a toroid inductor. The ring-shaped core looks mysterious to a beginner, yet the arithmetic is simple and the winding takes a few minutes with a little patience. A toroid is popular with builders because it packs a lot of inductance into a small space, keeps most of its magnetic field inside the core and does not couple much with nearby parts. This guide shows how to read the core’s data, calculate the number of turns for the inductance you want, wind and count the turns correctly, and measure the result so that your filter works as designed.
Why Toroids Are Used
The Wikipedia article on toroidal inductors and transformers lists the main advantages. Because the toroid is a closed-loop core, it has a higher inductance and Q factor than an inductor of the same mass with a straight core, since most of the magnetic field is contained within the core. Due to the symmetry of the ring, little magnetic flux escapes, so a toroidal inductor couples less interference to adjacent circuits. For a homebuilder this means that you can put two coils close together on a small board without much interaction, which is not true of air-wound coils, and you get a compact filter with low loss.
Two main types of core are used in amateur work. Powdered-iron cores, with a code starting with T, such as T50-2, are used in resonant circuits and low-pass filters where a good Q and a stable inductance are important. Ferrite cores, such as FT50-43, have a much higher permeability and are used for chokes, baluns and broadband transformers, where a high impedance over a wide range is the aim. This guide is about powdered-iron inductors, while the winding techniques for ferrite are similar.
Reading the Core Code
The code on a powdered-iron core has two parts: the first number is the outside diameter in hundredths of an inch, and the second is the material, which decides the permeability and the frequency range. A T50-2 is a core of 0.50 inch outer diameter in the material called 2, and it has a red colour code. According to the data published at toroids.info, the T50-2 has an outside diameter of 12.7 mm, an inside diameter of 7.70 mm and a height of 4.83 mm, and its AL value is 4.9 with a tolerance of 5 per cent. The same page gives the optimum range for the highest Q and lowest loss as 250 kHz to 10 MHz. A T50-6, in the yellow material, has AL of 4.0 and is optimised for 3 MHz to 40 MHz.
| Core | Outside diameter | AL (nH per turn squared) | Best range for high Q |
|---|---|---|---|
| T37-2 (red) | 9.53 mm | 4.0 | 250 kHz to 10 MHz |
| T50-2 (red) | 12.7 mm | 4.9 | 250 kHz to 10 MHz |
| T68-2 (red) | 17.5 mm | 5.7 | 250 kHz to 10 MHz |
| T50-6 (yellow) | 12.7 mm | 4.0 | 3 MHz to 40 MHz |
The values in the table come from the same data source, and manufacturers’ own datasheets are the final reference. When you buy cores from a seller, check the printed data for the exact type, because material and size codes differ between brands.
The Formula for Turns
The inductance of a toroid wound with N turns is proportional to the square of the number of turns. With AL in nanohenries per turn squared, the formula is:
L (µH) = AL × N² / 1000
and, turned around to find the turns for a wanted inductance:
N = √(L (µH) × 1000 / AL)
As an example, you need 1 µH on a T50-2 for a filter. N = √(1000 / 4.9) = √204 = 14.3. You cannot wind a fraction of a turn in a useful way, so choose 14 turns, which gives 4.9 × 196 / 1000 = 0.96 µH, or 15 turns, which gives about 1.10 µH. A difference of 4 per cent is within the tolerance of the core itself, and you can adjust the value afterwards by spreading or squeezing the winding. A second example: 2.2 µH on the same core gives N = √(2200 / 4.9) = 21.2, so use 21 turns for 2.16 µH. And a 1 µH coil on a T50-6 with AL of 4.0 needs about 15.8 turns, so 16 turns gives 1.02 µH.
These calculations ignore the effect of the tails, the end leads that connect the coil to the circuit, and they ignore the slight change caused by how the turns are spread over the core. For ordinary filter work this is acceptable, and the measurement at the end will tell you the true value.
Choosing the Wire
Use enamelled copper wire (magnet wire) for the winding. The wire has to carry the current of your circuit and must fit through the hole of the core for all the turns in one layer. For a T50-2 with 14 to 22 turns, wire of about 0.5 to 0.8 mm diameter, which is 24 to 20 AWG, is a convenient size. In a low-pass filter for 100 watts, the current in the coils is moderate and the wire does not have to be thick, but the heating in the core and the wire increases with the power, so use the thicker size that fits. As a rule, the first and last turns should not overlap, and the winding should not fill the hole completely, because you need room to pass the wire through each time without damaging the enamel.
Winding and Counting Step by Step
- Cut the wire with some excess. A rough allowance is the circumference of the core cross-section times the number of turns, plus 5 to 8 cm for each tail. For 14 turns on a T50-2, about 40 cm is enough, but a longer piece is easier to handle.
- Start the winding by passing the wire through the hole from the top, leaving a tail of a few centimetres.
- Count one turn each time the wire passes through the hole. This is the rule that causes most mistakes. The number of turns is the number of times the wire goes through the centre, not the number of visible loops on the outside of the ring. A wire that goes through the hole once and comes out is one turn.
- Pull each turn snug against the core, so that the wire lies flat and the winding does not loosen. Keep the turns close together and in a single layer.
- Space the turns evenly over the core, leaving a gap between the start and the finish. Many designers leave a gap of about 30 degrees, which keeps the ends apart and avoids capacitance between them. In a coil that has to be adjusted, a gap of a few millimetres is easier to work with.
- Count again. Hold the core up to the light and count the wires through the hole. Do it twice, and do it again if the count differs.
- Remove the enamel from the tails with a sharp knife or fine sandpaper, or by melting it with a hot soldering iron, then tin the ends. Check with a meter that there is contact through the solder.
- Fix the winding if it needs to stay stable: a little drop of nail varnish, polyurethane or wax holds the turns without changing the value much, and prevents the winding from moving when the coil is handled.
Measuring the Result
The only way to know that the coil has the right value is to measure it. An LC meter gives a reading in a second, and a NanoVNA can measure the inductance at the frequency of interest if you build a small test fixture, as described in our guide to the NanoVNA and antenna analyzers. Measurements on a very small inductor are sensitive to the lead length, so keep the leads short and use the same fixture each time. If the value is a little high, you can lower it by spreading the turns over a larger part of the ring, or, in extreme cases, by taking one turn off. If it is a little low, squeeze the turns closer together on the core, which increases the inductance by a few per cent, and fix them in place. Adjust the value in this way for the first coil, then wind the others in the same way and with the same spacing.
Where Toroid Inductors Appear in the Shack
The most common use is in low-pass and band-pass filters. In a harmonic filter after a transmitter, the coils are usually wound on T50-2 or T68-2 cores for the lower bands, and on T50-6 for the higher bands, and the capacitors are silver mica or NP0 ceramic types. Our guide to the low-pass filter for harmonics shows what the filter does for your signal. Another use is in a matching network or an antenna tuner, as discussed in the article on L network matching. A third is the balun or unun, where the core is typically ferrite, and the winding style, a pair or a trifilar, determines the turns ratio; see our introduction to baluns and ununs.
Taps, Trimming and Keeping Records
Some circuits need a tap on the coil, for example to match a low impedance to a tuned circuit. Wind the first part of the coil, twist a small loop of wire to make the tap, and continue winding. Scrape and tin the loop later. The turns before the tap count in the same way as the others, so the inductance at the tap follows the same formula with the smaller number of turns. When you build several filters, write down the core type, the turns, the measured inductance and the spacing for each coil, and keep the notes with the project. A coil wound for a 40 m filter will not look different from one for 20 m, and a short label saves you from a confusing hour with the meter later.
Common Mistakes
- Counting both the inside and the outside wire. Each turn has one segment through the hole and one on the outside of the ring, so counting both doubles the number.
- Using the wrong material. A red T50-2 and a yellow T50-6 have different AL values and different frequency ranges. A mix-up changes the inductance by 20 per cent and also the loss at the operating frequency.
- Overheating the core. A powdered-iron core can be damaged by too much power at too high a flux level, which may lower the Q or change the inductance. For high power, use a larger core or a material suited to it.
- Leaving enamel on the tails. A joint that looks soldered but has no contact gives an unpredictable result. Always check continuity.
- Touching the winding with a metal object. A screwdriver or a nearby shield changes the value; measure the coil in its final place when you can.
The Bottom Line
To wind a toroid inductor, find the AL value of the core in the manufacturer’s data, use N = √(L × 1000 / AL) to find the number of turns, and wind the wire through the hole in a single layer while counting each pass through the centre as one turn. For example, a T50-2 with AL of 4.9 needs 14 turns for about 0.96 µH and 21 turns for about 2.16 µH. Space the turns evenly, strip and tin the tails, fix the winding and then measure it with an LC meter or a NanoVNA. Fine adjustment is done by spreading or squeezing the turns. It is a quick job that opens up a whole world of home-built filters and matching circuits.
Frequently Asked Questions
How do I count the turns on a toroid?
Count each time the wire passes through the centre hole of the ring. The number of turns is the number of wires you see through the hole, not the number of loops on the outside.
What is the AL value?
It is the inductance factor of the core, in nanohenries per turn squared. A T50-2 has an AL of 4.9, so the inductance is 4.9 times the turns squared, divided by 1000, in microhenries.
How many turns do I need for 1 µH on a T50-2?
About 14 to 15 turns. Fourteen turns give about 0.96 µH and fifteen turns about 1.10 µH, using the formula with AL of 4.9.
Can I change the inductance after winding?
Yes, a little. Spreading the turns over more of the core lowers the value, and squeezing them together raises it by a few per cent. Fix the turns with varnish when you are satisfied.
What is the difference between T and FT cores?
T cores are powdered iron, used for resonant circuits and filters with good Q. FT cores are ferrite with higher permeability, used for chokes, baluns and broadband transformers.
What wire should I use?
Enamelled copper wire that fits through the hole for all turns in one layer. For a T50-2 with about 14 to 22 turns, wire of 0.5 to 0.8 mm is a common choice.