Toroid Cores

Introduction

Some of the information on this page came directly from a variety of documents on the Amidon Web site. Currently, this page only covers cores from Amidon, but will be expanded to other suppliers in the future, when I get time.

The part number for a toroidal core identifies the Type (FT = Ferrite, T = Powdered Iron), Size, and Core Mixture used. Together, this information can be used to select your core type. The mixture information includes μ (permeability ) and A_L (constant of proportionality or inductance factor). That information can then be used to calculate the number of turns required to obtain a specific inductance.

The toroids are designated by a code: T-xxx-yy or FT-xxx-yy. T = Iron Powder materials and FT = Ferrite. The first 3 digits (xxx) indicates the core outer diameter in units of 0.01” (10 mils). The last number (yy) designates the code for material type. The iron cores are color coded. The ferrite materials have much higher permeability and so require fewer turns to obtain a given inductance.

Choosing which kind of toroid to use can be difficult. While Ferrites have very high A_L, it can be difficult, or impossible, to design a inductor for a specific inductance. Because you can only Difficult to hit an exact value of inductance.

Very High inductances can be achieved.

Very wide banded.

Tendancy to saturate when used with high power.

Difficult to hit an exact value of inductance.

Increments of 1 turn only.

A_L can vary from core to core.

Material	μ range	Applications
Carbonyl Iron Powder	3 - 75	High-Q Inductors. Broadband transformers.
Hydrogen-Reduced Iron Powder	35 - 90	Interference Suppression. Low Frequency Chokes. SMPS (Switch Mode Power Supply) Filters.
Nickel-Zinc (Ni-Zn) Ferrite	20 - 850	High-Q inductors. Broadband Transformers. VHF Ferrite-Rod Antennas.
Manganese-Zinc (Mn-Zn) Ferrite	800 - 15000	SMPS. Interference Suppression. Medium Frequency Ferrite-Rod Antennas. Audio Chokes (Loudspeaker Crossover Neworks).

For Radio-Frequency applications, ferromagnetic core materials can be divided into four classes, all of which can be varied in composition to establish a desired degree of permeability. In general, Powdered Iron cores have the lowest permeabilities, and Mn-Zn cores the highest. However, the available permeability ranges overlap. The High-Permeability materials have the highest losses at high frequencies, and are most susceptable to magnetic saturation. Permeability ranges and areas of application are summarized in the table.

Apart from the physical dimensions of a toroid (outside and inside diameter, thickness) there is a value given for each particular core size and material, which is usually called the A_L value, and is the manufacturer’s inductance index for the core. Manufacturer’s data for Iron Powder and Ferrite cores are in the data tables and show all the required information. The A_L figure for Iron Powder cores is given as uH/100 turns, but for Ferrite cores it is quoted as mH/1000 turns. Note that many other manufacturers quote A_L as nH/t2 for both types of material. The conversion from uH/100 turns to nH/t2 gives a result which is divided by 10. The relevant A_L value is used in many calculations involving cores.

Ferrite Toroidal Cores

Ferrite Toroid Calculator
L N = 1000 × A_L L = A_L × N² 1×10⁶ A_L = L × 10⁶ N² Where: N = Number of Turns L = Inductance (mH) A_L = mH/1000 Turns
Core:	Material:
Core = FT-50, Mat. = 43, μ = 1100, A_L = 43 Resonant Circuit Frequency = undefined Wideband Circuit Frequency = undefined Noise Attenuation = undefined

On the right is a simple calculator for determining the number of turns required for a particular core materal. At the top are the equations for calculating number of turns, based on the A_L value, and A_L based on the number of turns.

To use the calculator, just select the Core Type and Core Material from the User Input selectors. As the selectors are changed the calculator will display a list showing the Number of Turns (1T to 10T) and the Inductance for each of those turns. If that is not the range that your interested in, use the buttons above the display area. The button marked Turns +5 increment the starting Number of Turns by 5 and redisplay the inductances. If you go past the range you are interested in, use the button marked Turns -5 to decrement the range. If you want to go back to the beginning turn range (1T to 10T), just click on the Reset button. The Reset button will only initialize the turns range, not the Core Type or Core Material.

Ferrite Toroidal Cores are well suited for a variety of RF circuit applications and their relatively high permeability factors make them especially useful for high inductance values with a minimum number of turns, resulting in smaller component size.

There are two basic Ferrite material groups:

Nickel-Zinc (Ni-Zn) Ferrite, Range: μ = 20 - 850 - High-Q inductors, Broadband Transformers, VHF Ferrite-Rod Antennas, ...
Nickel Zinc ferrite cores exhibit high volume resistivity, moderate temperature stability and 'High Q' factors for the 500 KHz to 100 MHz Frequency Range. They are well suited for low power, high inductance resonant circuits. Their low permeability factors make them useful for wide band transformer applications as well.
Manganese-Zinc (Mn-Zn) Ferrite, Range: μ = 800 - 15000 - Switch Mode Power Supplies, Interference Suppression, Medium Frequency Ferrite-Rod Antennas, Audio Chokes (Loudspeaker Crossover Neworks), ...
The Manganese Zinc ferrites, having permeabilities above 800 μ_i, have fairly low volume resistivity and moderate saturation flux density. They can offer 'High Q' factors for the 1 KHz to 1 MHz Frequency Range. Cores from this group of materials are widely used or switched mode power conversion transformers operating in the 20 KHz to 100 KHz frequency range. These cores are also very useful for the attenuation of unwanted RF noise signals in the frequency range of 20 MHz to 400 MHz and above.

A_L Value Chart for Ferrite Toroidal Cores
	Material Number
Core Size	43 μ=850	61 μ=125	63 μ=40	67 μ=40	68 μ=20	72 μ=2000	75/J μ=5000	77 μ=2000
FT-23	188	24.8	7.9	7.9	4.0	396	995	396
FT-37	420	55.3	19.7	19.7	8.8	884	2220	884
FT-50	523	68.8	22.0	22.0	11.0	1100	2740	1100
FT-50A	570	75.0	24.0	24.0	12.0	1200	2990	1200
FT-50B	1140	150.0	48.0	48.0	-	2400	-	2400
FT-82	557	73.0	22.4	22.4	11.7	1172	2940	1172
FT-114	603	79.3	25.4	5.4	12.7	1270	2940	1270
FT-114A	-	146.0	-	-	-	-	-	2340
FT-140	952	140.0	-	45.0	-	2240	-	2240
FT-240	1239	171.0	-	53.0	-	3133	-	-

Material 43 (μ=800) High volume resistivity. For medium frequency inductors and wideband transformers to 50 MHz. Optimum frequency attenuation from 40 MHz to 400 MHz.

Material 61 (μ=125) Offers moderate temperature stability and high 'Q' for frequencies 0.2 MHz to 15 MHz. Useful for wideband transformers to 200 MHz and frequency attenuation above 200 MHz.

Material 63 (μ=40) For high 'Q' inductors in the 15 MHz to 25 MHz frequency range.

Material 67 (μ=40) Similar to the 63 material. Has greater saturation flux density and very good temperature stability. For high 'Q' inductors, (10 MHz to 80 MHz). Wideband transformers to 200 MHz.

Material 68 (μ=20) High volume resistivity and excellent temperature stability. For High 'Q' resonant circuits 80 MHz to 180 MHz. For high frequency inductors.

Material 73 (μ=2500) Primarily a ferrite bead material. Has good attenuation properties from 2 MHz through 50 MHz.

Material 72 (μ=?) Material 77 is an upgrade from the former 72 material. The 72 material is still available in some sizes, but the 77 material should be used in all new designs.

Material 'J'/75 (μ=5000) Low volume resistivity and low core loss from 1 KHz to 1 MHz. Used for pulse transformers and low level wodeband transformers. Excellent frequency attenuation from 0.5 MHz to 20 MHz.

Material 77 (μ=2000) Has high saturatior flux density at high temperature. Low core loss in the 1 KHz to 1 MHz range. For low leverl power conversion and wideband transforers. Extensively used for frequency attenuation from 0.5 MHz to 50 MHz.

Wire Turns Chart for Ferrite Toroidal Cores core size v.s. wire size: single layer wound
core size	10	12	14	16	18	20	22	24	26	28	30	32	34	36	38	40
FT-23 --	0	0	0	0	2	4	7	11	15	21	28	37	48	62	79	101
FT-37 --	0	0	2	4	7	11	15	21	28	36	48	61	79	100	127	161
FT-50 --	2	4	7	10	15	19	26	34	45	58	750	95	121	154	194	245
FT-82 --	3	5	8	10	16	22	29	39	51	65	84	106	135	171	216	273
FT-87 --	10	14	19	25	34	43	56	72	92	118	150	188	239	302	374	478
FT-114 --	16	22	29	38	49	63	80	103	131	166	211	263	334	420	527	665
FT-150 --	16	22	29	38	49	63	80	103	131	166	211	264	335	422	529	667
FT-193 --	31	41	53	68	86	109	139	176	223	282	357	445	562	707	886	1115
FT-240 --	36	46	60	77	98	123	156	198	250	317	400	499	631	793	993	1250
Note: Allowance has been made for winding error. A few more turns may be possible with very careful winding and close positioning of each turn.

Powdered Iron Toroidal Cores

Material	μ range	Applications
Carbonyl Iron Powder	3 - 75	High-Q inductors. Broadband transformers.
Hydrogen-Reduced Iron Powder	35 - 90	Interference suppression. Low Frequency chokes. SMPS (Switch Mode Power Supply) filters.

The Carbonyl Iron cores are especially noted for their stability over a wide range of temperatures and flux levels. Their permeability range is from less than 3 μ_i to 35 μ_i and can offer excellent 'Q' factors from 50 KHz to 200 MHz. They are ideally suited for a variety of RF applications where good stability.and good 'Q' are essential. Also, they are very much in demand for broadband inductors, especially where high power is concerned.

The Hydrogen Reduced lron cores have higher permeabilities ranging from 35 μ_i to 90 μ_i Somewhat lower 'Q' can be expected from this group of cores. They are mainly used for EMI filters and low frequency chokes. They are also very much in demand for input and output filters for switched mode power supplies.

The next several pages are devoted to iron powder materials and the toroidal core configuration in particular. You will find physical dimensions of available items, their As values and other magnetic properties, as well as how to select the proper core for your application.

In general, toroidal cores are the most efficient of any core configuration. They are highly selfshielding since most of the flux lines are contained within the core. The flux lines are essentially uniform over the entire length of the magnetic path and consequently stray magnetic fields will have very little effect on a toroidal inductor. It is seldom necessary to shield a toroidal inductor.

The A_L value of each iron powder core can be found in the charts on the next several pages. Use this As value and the formula below to calculate the number of turns for a specific inductance.

A_L Value Chart for Powdered Iron Toroidal Cores
Core Size	41-Mix Green μ = 75	3-Mix Grey μ = 35 0.05-5 MHz	15-Mix Red & Wht μ = 25 0.2-2 MHz	1-Mix Blue μ = 20 0.05-5 MHz	2-Mix Red μ = 10 2-30 MHz	6-Mix Yellow μ = 8 10-50 MHz	10-Mix Black μ = 6 30-100 MHz	12-Mix Grn & Wht μ = 3 50-200 MHz	0-Mix Tan μ = 1 100-300 MHz
T-12	112	60	50	48	20	17	12	7.5	3.0
T-16	130	61	55	44	22	19	13	8	3.0
T-20	175	76	65	52	25	22	16	10	3.5
T-25	225	100	85	70	34	27	19	12	4.5
T-30	375	140	93	85	43	36	25	16	6.0
T-37	308	120	90	80	40	30	25	15	4.9
T-44	229	180	160	105	52	42	33	18.5	6.5
T-50	320	175	135	100	49	46	31	18	6.4
T-68	420	195	180	115	57	47	32	21	7.5
T-80	450	180	170	115	55	45	32	22	8.5
T-94	590	248	200	160	84	70	58	32	10.6
T-106	900	450	345	325	135	116	NA	NA	19.0
T-130	785	350	250	200	110	96	NA	NA	15.0
T-157	970	420	360	320	140	115	NA	NA	NA
T-184	1640	720	NA	500	240	195	NA	NA	NA
T-200	755	425	NA	250	120	100	NA	NA	NA

Number of Turns v.s. Wire Size and Core Size Approximate maximum of turns - single-layer-wound enameled wire
Wire Size	T-200	T-130	T-106	T-94	T80	T-68	T-50	T-37	T-25	T-12
10	31	17	10	10	8	7	5	1	1	0
12	41	23	14	14	12	9	6	3	1	0
14	53	30	20	20	17	12	8	5	5	0
16	68	40	27	27	23	15	11	7	3	1
18	86	51	35	35	30	21	16	9	4	1
20	109	66	45	45	39	28	21	12	5	1
22	139	83	58	58	51	36	28	17	7	2
24	176	107	75	75	66	47	37	23	11	4
26	223	137	96	96	84	61	49	31	15	5
28	282	173	123	123	108	79	63	41	21	8
30	357	220	156	156	137	101	81	53	28	11
32	445	275	195	195	172	127	103	67	37	15
34	562	348	248	248	219	162	131	87	48	21
36	707	439	313	313	276	205	166	110	62	29