Chapter 15 - Toroids

When it comes to any transformer, what ever be the core used or the frequency applied all connected theories and rules are relevant. Today, ferro magnetic cores are widely used to make transformers of varying kinds. Slugs, ferrite rods, beads and toroids .... all come under this category. Not only that more inductance is possible at low number of turns with ferro magnetic materials but also cores that suit to a variety of frequencies ranging from power frequency to giga hertz are possible. The self shielding habit of toroids even lets us keep RF transformers closer, without mutual interferences. Many times, to get a definite prescribed 'Q', similar types are perhaps the only solution.

Toroids that vary in diameter from the measurement of a pin head to many inches are available in the market. In sensitive points like 'inside a Crystal filter, well shielded pot core is used. Cup core mechanisms also are made of  ferro magnetic materials. Toroids are available in powdered iron and ferrite categories.  Toroid is reference to the shape of the core; thats it. Powdered iron cores are found in categories of carbonyl iron and hydrogen reduced iron. At the same time ferrite cores come from Nickel Zinc category and Manganese Zink. Quality aspects like the 'Q', stability and the frequency range varies, that sometimes heavily depending on the basic material of the core used. This difference is marked in the core data under 'mix no.' The cross sectional area of a any core is decided based on the load saturation there. Table C-15/1 shows the colour code for various Mixes and C-15/2 gives its specialities. 

C-15/1			Mix No 6	Yellow
Mix No 1	Blue		Mix No. 10	Black
Mix No 2	Red		Mix No. 12	Green and White
Mix No 3	Grey		Mix No. 15	Red and White

The code name used on a toroid reveals the facts like material of make, mix no. and size of the core.  'T' powder iron and 'FT' or 'TF' means ferrite category. The figure next to that shows the size of the toroid. According to the Universal standard we follow, no. 50 means  50/100 inches of outer diameter. Third figure is the mix no. For example, a T-50-2 toroid is very commonly used in Radio Frequency circuits. 

C-15/2                           Powdered Iron
Mix No	Material	Relative Permea-bility μr	Handling Frequency	Remarks
0		1	Up to 200 MHz	Inductance varies with the method of winding
1	Carbonyl C	20	50 KHz – 500 KHz	Higher volume resistivity
2	Carbonyl E	10	1 MHz – 30 MHz	Good Q and  higher volume resistivity
3	Carbonyl HP	35	500 KHz -5000 KHz	Good Q and stability
6	Carbonyl SF	8	20Mhz -30Mhz	Higher Q
10	Type W Power Iron	6	40 MHz -100 MHz	High stability and good Q
12	Synthetic  Oxide	3	50 MHz – 100 MHz	Good Q, moderate stability
15	Carbonyl Gs 6	25	1 MHz – 2 MHz	Excellent Q and stability – used in VLF
Ferrite
33	MZ	850	I KHz – 1 MHz	Low volume resistivity used for loop stick  antenna rods
43	NZ	850	To 50 MHz	High attenuation for 30 MHz to 400 MHz. used for MW inductors
61	NZ	125	2 MHz -15 MHz	Moderate temp. stability; used in wide band transformers
63	NZ	40	15 MHz – 25 MHz	Low permeability and high volume resistivity
67	NZ	40	10 MHz – 80 MHz	High flux – used for wide band transformers up to 200 MHz
68	NZ	20	80 MHz-180 MHz	Excellent temp. stability, High Q, High vol. resistivity

This does not mean that all manufacturers are following the same coding pattern.  In Chart C-15/3 we see the characteristics units appropriately explained. 

To make a coil of a definite inductance using a ferrite toroid first we have to find out its' AL value. AL is the 'Inductance Index' of a coil with relationship reference to its permeability and size. These details also are given in its elaborate data. 

A few examples are given in Chart C-15/4 This is in an environment showing the inductance in μh or mh for every 1000  or 100 turns. Here the inductance at 100 turns is given in μh.This is data given by a particular manufacturer only. Further to take the required no of turns at a specific situation follow the formula T=1000√Lmh+AL or T100 =√Lμh+AL. If the no. of turns is found, decide the gauge of the wire on that particular toroid.

Charts come to your rescue here also.  A sample chart is given in C-15/5

When using toroid for transformers, see that separate mixes are used for both narrow and broadband circuits. When assembling broadband RF chokes and transformers the inductive reactance should be four times the expected impedance of the circuit.

C-15/4
Material	Mix  No 2	Mix  No 6	Mix  No 10
T-80	55	45	32
T-68	57	47	32
T-50	49	40	31
T-44	52	42	33
T-30	43	36	25
T-20	27	22	16

C-15/5  Toroid Size (Outer Diameter)
Wire Gauge	68”	5”	.37”	.25”
20	29	23	14	6
22	38	30	19	9
24	49	39	25	13
26	63	50	33	17
28	80	64	42	23
30	101	81	54	29
32	127	103	68	38

Even at simple circumstances there are winding configurations like spaced winding and distributed winding. Disciplines like keeping a specific distance between turns, maintaining a 30  degree difference between the starting point and the ending point all help to reduce the stray capacitance between turns. AL value is calculated on an assumption that the parasitic capacitance (stray capacitance) is under strict check. In broadband transformers keeping the winding using all the portions of the toroid close to the ground is a general practise. In case the edges of a toroid is sharp, an insulator coverage is recommended before winding the coil. 

Today, bifilar and trifilar transformers using ferro magnetic cores also are found very generously. Similar transformers come under 'transmission line transformers'. When assembling bifilar transformers two wires are wound at a time and in trifilar there will be three wires instead of two. In some typical situations, more than one core is used to assemble a single coil. This practise seen with slugs and beads also. 

Whatever, it is always advised to go for the best quality toroid for purchase. A measurement or data change even in micro figures sometimes count heavily, resulting in serious circuit related problems. There are quality toroid manufacturers in India. Toroids are available in many kinds and sizes and some with specific winding instructions. It also is to be mentioned that making own ferro magnetic cores is also not at all a Herculean task for a committed home brewer.  

Chapter 14                                         Chapter 16