Introduction
Perhaps it is a about time for us to banish for good the fear that people seem to have of
these circular critters. With that in mind this talk will cover the construction and use of
Toroidal Inductors, which are generally referred to as toroids. It is intended to cover from
understanding what a toroid is, to building and connecting the finished toroid into a
circuit.
There is no way we can cover everything, and in some cases things maybe simplified,
doubtless due to my limited understanding!
So if you have any questions; observations; or hints please feel free to raise them as we
go along.
Don’t worry if you can’t remember the links, articles and tools mentioned, as the contents
of this presentation can be found on my web site www.m1hog.com
Acknowledge sources
What is Toroid?
A toroid is a doughnut-shaped object whose surface is a torus. Its annular shape is
generated by revolving a circle around an axis external to the circle.
What is a Toroidal Inductor
A coil of insulated wire (usually around a core of iron or similar metal) in a doughnut
shape. These are used as inductors in transmitters and receivers because they possess
higher inductance and carry greater current than similarly constructed solenoids. They are
also used as transformers in main power supplies. Toroidal coils reduce resistance, due to
the larger diameter and smaller number of turns in the winding. The magnetic flux in a
toroid is confined to the core, preventing its energy from being absorbed by nearby
objects.
Why use Toroids
Mention other inductors (Kank et al) Advantages / Disadvantages
Toroidal inductors / transformers have high performance. They offer the smallest size (by
volume and weight) and lowest electromagnetic interference (EMI). Their geometry leads
to near complete magnetic field cancellation outside of its coil, hence the toroidal
inductor has less EMI when compared to other inductors of equal power rating.
1. They can be mounted closely together
3. Can be made in a wide range of values and characteristics
4. Simple to make
5. Q - High quality factor (or Q) - The ratio of its inductive reactance to its resistance at a
given frequency, and is a measure of its efficiency. The higher the Q factor of the
inductor, the closer it approaches the behavior of an ideal, lossless, inductor.
Some Example Uses
RF Choke – an inductor designed to have a high impedance to RF current and low
impedance to DC current.
Transformer / impedance matching
Variable-frequency oscillator (VFO) The frequency is set by a tuned circuit using
capacitors and inductors. The frequency can be changed by adjusting the components in
the tuned circuit.
Band pass filter - Small Capacitor used to tune
Mixers – Used in transformers for Diode Ring Mixers. Ready built and homebrew.
ATUs
Toroid T37-2 Characteristics
Specs for T37-2 RF Toroids
Physical Dimensions
Magnetic Dimensions
OD = 0.375 in. / 9.53
mm
ID = 0.205 in. / 5.21
mm
Ht = 0.128 in. / 3.25
mm
= 2.31 cm
A = 0.064 cm2
V = 0.147 cm3
Dimensional Tolerance
OD - +/- 0.015 in.
ID - +/- 0.015 in.
Ht - +/- 0.02 in.
AL
uH = (AL * Turns2) / 1000
4.0
Basic Iron Powder
Carbonyl E
length
in
# of Inductance inches
Turns
in uH
includes
1 inch
tails
6
.15
5.0"
8
.26
6.0"
10
.41
7.0"
12
.58
8.0"
14
.80
9.0"
16
1.1
10.0"
18
1.3
11.0"
20
1.6
12.0"
Material Permeability (µ°)
Temperature Stability (ppm
/°C)
10
95
* averaged from -55C° to +125C
Magnetic Tolerance
* tested with evenly-spaced
windings
+/- 5 %
Toroidal Color Code
Red/Clear
Resonant Circuit Frequency
Range
* range is given to optimize Q and
core loss
250KHz - 10MHz
22
2.0
13.0"
24
2.4
14.0"
26
2.8
15.0"
28
3.2
16.0"
30
3.6
17.0"
32
4.1
18.0"
34
4.7
19.0"
36
5.2
20.0"
38
5.8
21.0"
40
6.5
22.0"
Toroid T37-6 Characteristics
Specs for T37-6 RF Toroids
Physical Dimensions
Magnetic Dimensions
Dimensional Tolerance
OD = 0.375 in. / 9.53
mm
ID = 0.205 in. / 5.21
mm
Ht = 0.128 in. / 3.25
mm
= 2.31 cm
A = 0.064 cm2
V = 0.147 cm3
OD - +/- 0.015 in.
ID - +/- 0.015 in.
Ht - +/- 0.02 in.
length
in
# of Inductance inches
Turns
in uH
includes
1 inch
tails
6
.11
5.0"
8
.20
6.0"
10
.31
7.0"
12
.44
8.0"
14
.60
9.0"
16
.78
10.0"
AL
uH = (AL * Turns2) / 1000
3.0
18
1.0
11.0"
Basic Iron Powder
Carbonyl SF
20
1.2
12.0"
Material Permeability (µ°)
8.5
22
1.5
13.0"
24
1.8
14.0"
26
2.1
15.0"
28
2.4
16.0"
30
2.7
17.0"
Temperature Stability (ppm
/°C)
35
* averaged from -55C° to +125C
Magnetic Tolerance
* tested with evenly-spaced
+/- 5 %
windings
Toroidal Color Code
Resonant Circuit Frequency
Range
* range is given to optimize Q and
core loss
Yellow/Clear
3MHz - 40MHz
32
3.1
18.0"
34
3.5
19.0"
36
3.9
20.0"
38
4.4
21.0"
40
4.9
22.0"
Toroid T200-6 Characteristics
Specs for T200-6 RF Toroids
Physical Dimensions
Magnetic Dimensions
Dimensional Tolerance
OD = 2.00 in. / 50.8
mm
ID = 1.250 in. / 31.8
mm
Ht = 0,550 in. / 14.0
mm
= 13.0 cm
A = 01.27 cm2
V = 16.4 cm3
OD - +/- 0.025 in.
ID - +/- 0.025 in.
Ht - +/- 0.030 in.
length
in
# of Inductance inches
Turns
in uH
includes
1 inch
tails
6
.38
13.4"
8
.67
17.2"
10
1.1
21.0"
12
1.5
24.8"
14
2.1
28.6"
16
2.7
32.4"
18
3.4
36.2"
AL
uH = (AL * Turns2) / 1000
10.4
Basic Iron Powder
Carbonyl SF
20
4.2
40.0"
Material Permeability (µ°)
8.5
22
5.1
43.8"
Temperature Stability (ppm
/°C)
35
24
6.0
47.60"
26
7.1
51.4"
28
8.2
55.2"
30
9.4
59.0"
32
10.7
62.8"
34
12.1
66.6"
36
13.5
70.4"
38
15.1
74.2"
* averaged from -55C° to +125C
Magnetic Tolerance
* tested with evenly-spaced
windings
+/- 5 %
Toroidal Color Code
Yellow/Clear
Resonant Circuit Frequency
Range
* range is given to optimize Q and
core loss
3MHz - 40MHz
40
16.7
78.0"
Terms
Permeablity
http://en.wikipedia.org/wiki/Permeability_%28electromagnetism%29
AL
Rough rule of thumb
Type -2 and -6 materials are great for narrowband tuned circuit inductors
Type 43 and type 61 ferrite materials are more broadband
Toroids In Schematics
Single winding
Transformers / taps
Bifilliar and beyond
Show example schematics / values required (Spreadsheet calc)
Calculating values
In a circuit
Phasing Dots
An increase in voltage at one dot produces and increasing voltage at the other. Current
entering one dot causes current leave the other dot. if two windings are shown with a dot
at the same end it means that both windings should be wound on the core the same way.
http://en.wikipedia.org/wiki/Dot_convention
Construction - Before we start
People seem to get very concern over the precise construction details for a toroid, even it
seems down to the temperature at which it was wound! (Yes). But fortunately for us the
construction is generally not “that” critical, as the toroid is generally used with a variable
component, perhaps a capacitor to allow fine adjustment if required.
Counting turns, each pass through the core counts as one turn
Coverage 85%
Evenness of windings
Overlapping!
Wind in the same direction (sometimes direction matters) if only to get the coloured sides
all facing the same way!
Neatness is not generally critical. But get a tidy result and it makes any subsequent
checking easier!
Insulation
Construction – Tools
Standard workshop tools
Pliers - fine
Soldering iron
Cutters
Vice / blutack etc to hold toroid while tinning leads etc
Magnifer - Ideally illuminated
Storage - If you are working on a few toroids. To avoid having to recount turns – was
that one 32 or 38 and which core was it wound on!
Construction – A word about wire
US vs UK wire sizes
Ideally use wire with a heat strippable insulator
Converting UK to US – equivalents chart
Construction - Winding
Remember every pass through the core counts as a turn
Overwind by 3 turns and remove these when complete
Pull each turn against the core to ensure it is tight.
Thread using a Blunt Darning needle. Not tried this techniques myself but some swear by
it.
[Multipig Band pass]
Rest. If there are many toroids to wind take a break it does not have to be a marathon!
The toroids for this band pass filter board was completed over several lunchtimes.
Skim through K2 Winding notes
Work through the process of building it
Hints and tips demoed throughout.
Tinning and scrapping techniques
Measuring / testing
Bifilar
Twisting wire
Colour
How to twist
6 to 8 turns per inch
Construction – Preparing the leads
Ideally use wire with a heat strippable insulator, this can be stripped using a hot pools of
solder or a butane gun. I have only used the solder pool technique.
Other wire can be stripped with sand paper or a craft knife, but this requires great care.
Some transformer toroids for ATU’s etc may use ordinary plastic coated wire which is
easily removed.
Strip them back close to the toroid. They will move away from it once tightened when
pulled through the pcb holes.
Construction – Testing and Measuring
[Bridge and Atlas Meter]
Test the leads with an ohm meter to ensure the insulation has been removed
In the case of transformers or bifilar etc windings check the windings are going in the
correct direction, and the correct ends are linked.
Check the Inductance with a bridge or component tester if you “really” want to. But
actual values do not always appear on circuit diagrams!
Inductance measurement is really for interest only, or if you are building a circuit from
scratch and want to check you calculations!
Construction - Stability
[Toroid in Wax – Racal rig]
I have never used any of these techniques, they were not suggested for construction of the
K2 or the Multipig+ But these techniques have been used by home brewers and in
commercial rigs, mainly to improve oscillator or mechanical stability.
Boiling for stability
If you need to stop toroid windings being rattled around in service, just place a droplet of
2 ton Epoxy on top and let it 'percolate' through the windings and spread out a little onto
the PCB. It's quite a thin epoxy resin and goes off in about 30 minutes - GM1SXX
Dipping the toroid in Wax
Construction - Installation
Pliers to pull the leads through the PCB holes tightly, solder then reheat the joint pulling
the wire through to get it tight.
Triple check the wiring before cutting of surplus leads. Can be difficult to reinstall with
trimmed leads!
Calculating Values
Calculators
Don's answer is right on.
In a transformer, the magnetizing impedance (or inductance) shunts source, shortcircuiting some of its capacity. So you want it to be large with respect to the load's
impedance. As Don said, ferrite gives you a lot higher inductance per turn squared. It
would be difficult to get enough inductance in lower HF frequencies to do the job with
iron powder. If you did choose to use iron powder, you'd probably want to resonate the
magnetizing inductance with a capacitor. But there goes the broadband thing.
What's the magnetizing inductance? Simply the value of the self-inductance of the
winding. So make its reactance 5 or 10 times the circuit impedance at that point in the
circuit (source side or load side) at the lowest frequency of interest. Then apply your
turns ratio to get the right impedance transformation ratio. Adjust up or down a bit to get
good integral values of turns for the desired ratio, but if you go down, make sure you're
not shaving too much margin off that 5 to 10 times ratio.
Suppliers
G-QRP
Jab
Sycom
Toroid Kits http://qrpkits.com/ favourable rate exchange rate
Links / Tools / References
The Mini Ring Core Calculator
Trimming the Inductance of Toroidal Inductors
Diz calculator
Toroid Winding Tips
Sprat article on building Kank equivalents
To take
Multipig Band pass board
Diode ring mixer SBL
Diode ring mixer toroid
ATU Toroid
Atlas