TRANSFORMERS AND THEIR USES

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This article is not intended to be a definitive treatise on the subject, but is intended to give a general over-view, which I hope will be particularly interesting and helpful to the newcomer to Amateur Radio.

Firstly, what is a transformer?  For the purposes of this article, we can regard a transformer as two or more magnetically coupled coils, which may or may not share an iron or ferrite core.  These windings may be single layer, or may be several layers separated by layers of insulating material or the windings may be "wave" or "pile" wound.  The insulation material used between layers is normally acid-free paper or plastic film, such as Mylar.  The entire component, or at least the winding assembly, is often encapsulated in wax or varnish to prevent the ingress of moisture or other possible contaminants, which could attack the insulation or even the wire itself.

There are many different types of transformers, so I will limit the scope of this article to a brief description of the various types and their uses.  Inductors are described in the previous section.  The different types of transformer have different circuit symbols, as shown below.  The alternative symbol is now often used on many circuit diagrams and each of the variants shown can be applied to it.  Note that pre-set inductance transformers are sometimes represented by the variable inductance transformer symbol.  To be pedantic, transformer windings where the inductance adjustment involves the use of a trimming tool, are pre-set, whereas a variable winding inductance transformer is one having an operator control function.  Strictly speaking, the goniometer and the variometer are both transformers but they are described in the inductor section because the variometer is always configured for use as a variable inductor and the goniometer is similar in many respects to the variometer.

An isolation transformer is two or more separate windings, magnetically coupled together.  The turns ratio of such a transformer is given by:

A transformer can be used as an impedance matching device, where the relevant turns ratio can be calculated using the following formula

Auto transformers employ one tapped winding, where, in step down configurations, the total winding can be regarded as the "primary" and the winding between the tap and one end terminal can be regarded as the "secondary".  When used in step up configurations, the roles of "primary" and "secondary" are reversed.  The above formulae still apply but, obviously, there is no isolation between primary and secondary windings.

I have not included any other formulae relating to transformers, as this is a very complex subject, particularly at high frequencies.  It is necessary to understand the concepts of self inductance, mutual inductance, leakage inductance, coupling factor, copper losses, core (iron) losses, inter-winding capacity and insulation, skin-effect and various other parameters and is far beyond the scope of this article.  However, a few points concerning skin effect may be helpful.

The series resistance of a transformer winding is a combination of the intrinsic resistance of the wire and any effective increase due to skin-effect.  Formulae relating to skin-effect are extremely complicated and are beyond the scope of this article.  At low frequencies, the series resistance is equal to the DC resistance of the wire, but as the frequency increases, the effective resistance also increases.  This is because, at high frequencies, the signal current only flows in the outer layer, or "skin", of the conductor.  The skin-effect only becomes important above about 50kHz, becoming progressively more important as the frequency increases.  At frequencies above 100MHz, the signal current is only flowing in the outermost few microns of the conductor and, for this reason, conductors intended for use at these frequencies are usually silver plated copper, although the conductivity of the base material is unimportant, unless there is also a high DC current flowing, as the signal current only flows in the plating layer.  The Q of a transformer winding is inversely proportional to the effective resistance and any current flowing in this resistance will generate heat, which could result in excessive temperature rise.  Note that the Q of a transformer winding is only important when considering the operation of RF transformers, where the Q of the tuned primary winding is affected by many factors, such as losses reflected from the other windings.

I recommend that anyone wishing to investigate the subject of transformer design further should consult the large number of relevant text-books and the information available on the Internet.

RF and IF transformers are sometimes referred to as "coupled circuits", a term reflecting their configuration, whereas LF and mains transformers do not have any alternative name.

As with inductors, there are several different types of wire used in transformers.  The commonest type of conductor used is copper wire, which may be a single solid wire or several strands but other materials, such as solid aluminium wire and resistance wire, are also to be found in specialised applications.  All wires are available in a large range of diameters and coverings.  Outside of the USA, the thickness of wire, or its gauge, is normally stated in millimetres.  However, Imperial Standard Wire Gauge (SWG), based on inch sizes, was commonly used in the UK until a few years ago.  The USA still uses the American Wire Gauge (AWG), which is also based on inch sizes.  There are other gauges in use, such as Birmingham Wire Gauge, Brown and Sharpe Wire Gauge and Piano Wire Gauge, but these are not used for copper or aluminium winding wires or resistance wires.  Winding wires are generally available with diameters between 0.025mm and 5.0mm, although smaller diameters down to 0.009mm and larger diameters up to 12.7mm are available.

Winding wires may be bare, enamelled or enamelled with a cotton, silk or rayon covering, depending on application.  Glass covering is also used in very special applications where the insulation must withstand very high temperatures or chemical attack.  There are several types of enamel used, the oldest being shellac based.  Modern synthetic enamels are often "self-fluxing", where the enamel melts at normal soldering temperatures, acting as a soldering flux in the process.  For higher temperature applications, non self-fluxing enamels such as Lewmex, Bicalex or one of their equivalents, are often used.  Important parameters of wire coverings are mechanical thickness, abrasion resistance, colour and flexibility, temperature rating and voltage breakdown.

In litz wire, several solid, insulated, wires are woven or twisted together and then several of these bundles are woven or twisted together.  Sometimes, several of these composite bundles are then woven or twisted together, the process being repeated until the desired overall thickness of wire has been achieved.  It is essential that the individual wires are insulated from each other, except at the end connections, where it is essential that they are all connected.  The 60kHz transmitter at the time-code broadcasting station WWVB uses 6075/36 (6075 strands of 36 AWG wire).  This type of wire results in a very much higher Q for a given coil than if an equivalent solid conductor had been used, by reducing the effective skin resistance of the conductor.  The weaving or twisting pattern of litz wire is designed so that individual wires will reside for short intervals on the outside of the cable and for short intervals on the inside of the cable.  This allows the interior of the litz wire to contribute to the cable's conductivity. Litz wire is extremely expensive.

Bunched wire, or "poor man's litz wire", also uses a number of insulated solid conductors, but these are twisted together as one bundle.  This also results in higher Q coils but is not as effective as litz wire.  However, bunched wire is considerably cheaper than litz wire.

Any DC current flowing in a winding having a ferrous material core will magnetise the core material.  If this current is high enough, the core material will "saturate", causing the inductance to decrease considerably.  The same will occur if the maximum permitted flux density in the core is exceeded due to large signal current peaks.  If saturation occurs in an audio frequency transformer, considerable distortion to the audio waveforms will occur.

Iron dust cores are, as their name implies, made of powdered iron, compressed together with a binding medium, to form a "slug" of material.  A thread may be moulded into the outside surface to enable them to be used in variable inductance transformers, where they are threaded into non-magnetic formers upon which the actual coils are wound.  A variation of this is where a threaded brass rod is bonded into one end of the slug.  The purpose of using iron dust is to reduce the eddy-current losses.  A specialised type of iron dust core is the pot-core, which is described in the appropriate sections below.

Ferrite cores can be made of a variety of ferrite materials depending upon the frequency of intended use and the required relative permeability.  Otherwise, the comments about iron-dust cores apply.  Specialised types of ferrite core exist, namely, pot-cores, ferrite rods, as used in ferrite rod aerials, single hole ferrite beads, multiple hole ferrite beads and ferrite rings or toroids.  The uses of these specialised types are described in the appropriate sections below.

Iron cores usually take the form of a stack of thin, silicon-iron, laminations, arranged to form the core of low frequency transformers.  Individual laminations are insulated from one another, usually by a very thin oxide layer, to reduce eddy current losses.  Laminations are made in "C" and "T", "E" and "I" or modified "E" and "T" combinations, with other styles being available for specific purposes.

In a mains transformer, or any other transformer where the windings carry no DC current, each successive set of laminations is rotated 180 degrees as the stack is built up. This has the effect of virtually elininating any air gap in the magnetic path as the butt-joint in one set of laminations is covered by the solid part of the sets on either side.  If appreciable DC current flows through any of the windings of a transformer, "E" and "I" laminations are normally used and successive sets of laminations are not rotated through 180 degrees.  In this application, the core consists of a stack of "E" laminations and a stack of "I" laminations butted together leaving a small air gap.  It is normal to insert a layer of insulating material in this gap, prior to clamping up the complete assembly, to ensure an air gap of consistent width.

A variation of the laminated transformer core is the C-core, where a continuous strip of core material is wound into a stack which is then divided into two parts.  The cut faces are then polished such that the two halves form a continuous magnetic path, with virtually no air gap, when tightly clamped together.  The photograph below shows the clamping straps on a typical C-core transformer.

Solid metal cores or winding bobbins are never used as they would be coupled to the coil and would cause large eddy current and "shorted-turn" losses.  If such a core or former were to be used in a power transformer, the result would a spectacular rise in temperature, leading to rapid catastrophic failure.

 

FIXED RATIO TRANSFORMERS

Fixed ratio transformers are used at both high and low frequencies.  High frequency transformers are used in receivers and transmitters at low and high powers.  Low frequency transformers are used at frequencies below about 100kHz and include switch mode power supply transformers, AF coupling transformers, line and loudspeaker matching transformers and mains transformers.

RF and IF Transformers

Variometers and goniometers are types of air cored RF transformers.  Variometers are always used as variable inductors and are therefore described under "air cored variable inductors" in the "Inductor" section.  The radio goniometer is a device in which a rotatable search coil is coupled to two individual fixed coils arranged at 90 degrees to each other.  Goniometers resemble variometers and are described in the "Inductor" section, under "air cored variable inductors".

RF transformers usually consist of a main winding which is tuned by a capacitor and an un-tuned coupling winding which is magnetically coupled to the main winding.  Adjustable tuning is either provided by using a fixed inductor and a variable trimmer capacitor, or by using a fixed capacitor and a variable inductor adjusted by a movable ferrite or iron dust core.  These components are often housed in screening cans to eliminate interaction with other components.  Screening cans are usually manufactured from aluminium and therefore provide predominantly electrostatic screening.  If magnetic screening is needed, mu-metal cans are normally used but they are not often required.

IF transformers usually consist of two parallel tuned circuits, magnetically coupled together.  The bandwidth of the combination depends on the coupling between the two tuned circuits.  Transformers have been made that allow mechanical adjustment of the spacing, and therefore the coupling, between the two tuned circuits.  Less than critical coupling results in the sharpest response, critical coupling results in a medium bandwidth, whereas greater than critical coupling results in "double-humping" and therefore a wide bandwidth.  Adjustable tuning is either provided by using fixed inductors and variable trimmer capacitors, or by using fixed capacitors and variable inductors adjusted by movable ferrite or iron dust cores.  These components are nearly always housed in screening cans to eliminate interaction with other components.  Screening cans are usually manufactured from aluminium and therefore provide predominantly electrostatic screening.  If magnetic screening is needed, mu-metal cans are normally used but they are not often required.

Wide-band, untuned, transformers are often wound on two-hole ferrite beads or pot- cores.

The current transformer is another specialised type of wide band transformer, where a single wire "primary" is passed through the centre of a secondary coil wound on a ferrite ring.  AC current flowing in the primary wire induces a current in the secondary winding, which can then be measured.  The advantage of the current transformer is that negligible impedance is introduced into the wire being monitored and the measuring circuit is completely isolated from the wire.  Current transformers are often used to measure the current flowing in high current, high voltage, power lines and in RF current measuring applications, such as VSWR bridges.  Current transformers intended for low frequency, low current applications often employ several turns on the primary and the example shown below produces a secondary current of 10mA for a primary current of 5A.  The other photograph shows a current transformer as used in a typical VSWR bridge.

      

LF Transformers

AF transformers normally use silicon-iron laminations or ferrite cores, with a primary winding and one or more secondary windings.  These types of transformer are used for general purpose matching and are used to couple loudspeakers or 600ohm lines to audio output stages or for using high impedance microphones with low input impedance amplifiers or vice-versa.  Microphone transformers are often fitted into mu-metal screening cans to prevent hum pick-up.  This type of component is nearly always impregnated to prevent the ingress of moisture and may be of "open" construction, or may be enclosed in a sealed metal container.

Mains Transformers

Unlike other types of LF transformer, mains transformers often incorporate a screen between the primary winding and the secondary windings.  This normally takes the form of an earthed metal sheet, inter-wound with the other windings but with the ends of the sheet insulated from each other, so that a shorted turn does not exist.  The purpose of this screen is two-fold.  Firstly, it prevents the primary becoming connected to the secondary should an insulation failure occur.  Such a fault would result in the primary being earthed, which should blow a fuse.  Secondly, the screen provides an effective way of eliminating mains-borne interference that can transmitted via inter-winding capacity between primary and secondary.  Obviously, in auto transformers, screens would be ineffective and are therefore not used.

Mains transformers are seldom found with neither the primary winding nor the secondary winding equipped with taps.  These transformers are either standard multi-winding transformers or are auto-transformers.  Both types employ fully interleaved soft iron laminations, which will saturate if appreciable DC currents flow in any of the windings.  Fixed ratio types are often used in stabilised power supplies, where it is unimportant whether the mains input voltage is 200V or 250V, as the final output voltage is electronically stabilised.  Isolation transformers used to supply power to 110V portable appliances on building sites and other potentially hazardous environments are nearly always impregnated to prevent the ingress of moisture and are always enclosed in sealed plastic containers.  Mains transformers intended for incorporation into equipment are not always enclosed.

     

 

MULTIPLE RATIO TRANSFORMERS

Multiple ratio transformers are normally only used at low frequencies.  These transformers are used at frequencies below about 100kHz and include switch mode power supply transformers, AF coupling transformers, line and loudspeaker matching transformers and mains transformers.

Multiple Ratio RF and IF Transformers

There is no reason to manufacture RF and IF transformers with multiple ratios.  Indeed, this would be extremely difficult to achieve whilst maintaining good Q.

Multiple Ratio LF Transformers

Multiple ratio LF transformers normally use silicon-iron laminations or ferrite cores and are often manufactured with taps on either primary, secondary or both windings.  These types of transformer are used for general purpose matching and are used to couple loudspeakers or 600ohm lines to audio output stages or for using high impedance microphones with low input impedance amplifiers or vice-versa.  This type of component is nearly always impregnated to prevent the ingress of moisture and may be of "open" construction, or may be enclosed in a sealed metal container.  Microphone transformers are often fitted into mu-metal screening cans to prevent hum pick-up.

EHT transformers, as found in CRT monitors and TV receivers, are a special type of multiple ratio transformer and always use ferrite cores.  They usually include the EHT rectifier and a focus control.

 

Multiple Ratio Mains Transformers

Multiple ratio mains transformers are either standard multi-winding transformers where one or more windings has taps, or are tapped auto-transformers.  Both types employ fully interleaved soft iron laminations, or are toroidally wound, which will saturate if appreciable DC currents flow in any of the windings.  Even if the secondary windings are untapped, the primary winding is nearly always provided with tapping points to cater for various mains input voltages.  This type of component is nearly always impregnated to prevent the ingress of moisture and may be of "open" construction, or may be enclosed in a sealed metal container.  Small mains transformers sometimes incorporate over-temperature protection devices that are interwound with the primary winding and connected in series with it.  These devices are seldom self-resetting and are usually inaccessible.  This means that once the protection device has operated, the whole transformer has to be replaced.

A Variac (which is a trade name, although these transformers are manufactured by several companies) will provided any voltage between 0V and about 10% more than the input voltage, at the turn of a knob.  This is achieved by using a specially wound toroidal transformer with a carbon brush slider connecting to a region of the winding where the enamel has been removed.  This brush provides the output tap.  Variacs are auto-transformers and are subject to waveform distortion when supplying heavy loads, particularly when DC current is also present.  Variac windings are always impregnated to prevent the ingress of moisture but complete transformers are seldom enclosed in metal containers, unless the transformer is a free-standing bench unit.

For the sake of completeness, it is worth mentioning the high power transformers found in power stations, electricity sub-stations and mounted on poles in rural areas.  These transformers are sometimes equipped with taps for minor adjustments to the output voltage and are used to transform the three-phase transmission voltages of up to 400kV down to the domestic 440V, three-phase system from which the single phase domestic 230V supplies are derived.  These transformers use oil immersed windings, where the oil is cooled in radiators or cooling fins.  When one considers the powers involved, even an efficiency of 99% would result in the generation of considerable amounts of heat.  A 400kV line can carry around 700MW of power, with a 132kV line carrying 70MW, an 11kV line carrying 3MW and the final 440V distribution circuits carrying 150kW.

     


This article is intended to give an overview of the subject and none of the items has been dealt with in depth.  Further information is available from transformer manufacturer's literature or from the Internet.

 

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