Electronics Transformer

8/14/2019 Electronics Transformer

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Preface

One thing that obviously confuses many people is the idea of flux density withinthe transformer core. While this is covered in more detail in Section 2, it isimportant that this section's information is remembered at every stage of your

reading through this article. For any power transformer, the maximum flux densityin the core is obtained when the transformer is idle. I will reiterate this, as it isvery important ...

For any power transformer, the maximum flux density is obtained when thetransformer is idle.

The idea is counter-intuitive, it even verges on not making sense. Be that as itmay, it's a fact, and missing it will ruin your understanding of transformers. Atidle, the transformer back-EMF almost exactly cancels out the applied voltage.The small current that flows maintains the flux density at the maximum allowed

value, and represents iron loss (see Section 2). As current is drawn from thesecondary, the flux falls slightly, and allows more primary current to flow toprovide the output current.

It is not important that you understand the reasons for this right from thebeginning, but it is important that you remember that for any power transformer,the maximum flux density is obtained when the transformer is idle. Please don'tforget this

Introduction

As you look through this article, you may be excused for exclaiming "This is forbeginners? - the man's mad. Mad, I tell you!" This is probably fair comment, buttransformers are not simple, and there is no simple way to provide all theinformation you need to understand them properly. There are sections here thatprobably go a little bit deeper than I originally intended, but were just toointeresting to leave out.

There are parts of this article you may want to skip over, but I suggest that youdo read all of it if you can. A full understanding to the extent where you candesign your own transformer is not the aim, but the majority of the information isat the very least interesting, and will further your general electronics knowledge.

For those who wish to delve deeper, Section 2 does just that. It is recommendedreading, even for beginners, as there is a great deal to be learned abouttransformers, despite their apparent simplicity.

The principles that allow us to make use of electro-magnetism were onlydiscovered in 1824, when Danish physicist Hans Oersted found that a currentflowing through a wire would deflect a compass needle. A few years after this, it


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was found that a moving magnetic field induced a current into a wire. From thisseemingly basic concept, the field of electromagnetism has grown to the pointthat society as we know it would not exist without the many machines that makeuse of these discoveries.

Transformers are essential for all modern electronics equipment, and there arevery few devices that do not use them. Each transformer type has a specific use,and it is uncommon that a transformer made for one application can be used foranother (quite different) purpose.

Before embarking on a description of the different types, the basic theory mustbe understood. All transformers use the same basic principle, and only the finerpoints ever change. A transformer works on the principle of magnetic coupling totransfer the energy from one side (winding) to the other.

Transformers are bi-directional, and will work regardless of where the input is

connected. They may not work as well as they otherwise might, but basicfunctionality is unchanged. An ideal transformer imposes no load on the supply(feeding the primary) unless there is a load across the secondary - real lifecomponents have losses, so this is not strictly true, but the assumption can beused as a basis of understanding.

Power transformers are rated in Volt-Amps (VA). Using Watts is of no use, sincea load that is completely reactive dissipates no power, but there are still Voltsand Amps. It is the product of "real" voltage and current that is important - awattmeter may indicate that there is little or no real power in the load, but thetransformer is still supplying a voltage and a current, and will get hot due to

internal losses regardless of the power.

Transformer cores have a quoted permeability, which is a measure of how wellthey "conduct" a magnetic field. Magnetism will keep to the path of leastresistance, and will remain in a high permeability core with little leakage. Thelower the permeability, the greater is the flux leakage from the core (this is ofcourse a gross simplification, but serves well enough to provide an initialexplanation of the term).

A transformer may be made with various materials as the core (the magneticpath). These include

Air - provides the least coupling, but is ideal for high frequencies(especially RF). Permeability is 1.

Iron - A misnomer, since all "iron" cored transformers are steel, withvarious additives to improve the magnetic properties. Permeability istypically about 500 and upwards.

Powdered Iron - Steel magnetic particles formed into a core and heldtogether with a bonding agent, and fired at high temperature to create a


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ceramic-like material with very good properties at medium to highfrequencies (over 1 MHz). Especially suited to applications where there isa significant DC component in the winding or for very high power.Permeability is typically 40-90.

Ferrite - A magnetic ceramic, usually using exotic magnetic materials to

obtain extremely high permeability and excellent high frequencyperformance (from 50kHz to over 1MHz). An astonishing range of differentformulations is available for different applications. Permeability is fromabout 500 up to 9,000 or more.

Technically, powdered iron and ferrites are both classified as soft (see below)ferrites, but they have very different characteristics, even within the same"family". They are generally unsuitable for low frequency operation, except at lowlevels. Ferrites are often used as signal transformers (such as isolationtransformers for telecommunications or other small signal applications), wherethe high permeability makes them an ideal choice for small size and high

inductance.

Core materials are generally classified as "soft" - this has nothing to do with theirphysical properties (they are all hard to very hard), but is a reference to theirability to retain magnetism (remanence). Hard magnetic materials are used formagnets, and they have a very high remanence, which is to say they retain avery large proportion of the original magnetic field that was induced into themduring manufacture.

All switchmode power supplies use ferrite transformers, since conventionallaminations cannot be made thin enough to prevent huge losses in the core.

Many limitations exist in any core material. For low frequency power applications,grain-oriented silicon steel (about 4% silicon) is by far the most common, as ithas a very high flux density before saturation. Almost all other materials areinferior in this respect, one of the main reasons this material is still so common.


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Toroidal E-I

Split Bobbin E-I Plug-Pack Conventional E-I

A small sample of some transformers is shown above (not to scale). The toroidaland E-I transformers are the same power rating, and a small selection of littletransformers and a plug-pack (wall transformer, wall-wart, etc) are shown as well.

1. Magnetism and Inductors

The transformer is essentially just two (or more) inductors, sharing a commonmagnetic path. Any two inductors placed reasonably close to each other will workas a transformer, and the more closely they are coupled magnetically, the moreefficient they become.

When a changing magnetic field is in the vicinity of a coil of wire (an inductor), avoltage is induced into the coil which is in sympathy with the applied magneticfield. A static magnetic field has no effect, and generates no output. Many of the


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same principles apply to generators, alternators, electric motors andloudspeakers, although this would be a very long article indeed if I were to coverall the magnetic field devices that exist.

When an electric current is passed through a coil of wire, a magnetic field is

created - this works with AC or DC, but with DC, the magnetic field is obviouslystatic. For this reason, transformers cannot be used directly with DC, for althougha magnetic field exists, it must be changing to induce a voltage into the other coil.

Try this experiment. Take a coil of wire (a loudspeaker crossover coil will donicely for this), and a magnet. Connect a multimeter - preferably analogue) to thecoil, and set the range to the most sensitive current range on the meter. As youmove the magnet towards or away from the coil, you will see a current, shown bythe deflection of the meter pointer. As the magnet is swung one way, the currentwill be positive, the other way - negative. The higher the coil's inductance and thestronger the magnet (and/ or the closer it is to the coil), the greater will be the

induced current.

Move the magnet slowly, and the current will be less than if it is moved quickly.Leave it still, and there is no current at all, regardless of how close the magnetmay be. This is the principle of magnetic induction, and it applies to all coils(indeed to all pieces of wire, although the coil makes the effect much greater).

If you now take a handful of nails and place them through the centre of the coil,you will see that the current is increased many times - the magnetic field is nowmore concentrated because the steel nails make a better magnetic path than air.

The ability of a substance to carry a magnetic field is called permeability, anddifferent materials have differing permeabilities. Some are optimised in specificways for a particular requirement - for example the cores used for a switchingtransformer are very different from those used for normal 50/60Hz mainstransformers.

The permeability of transformer cores varies widely, depending on the materialand any treatment that may be used. The permeability of air is 1, and mosttraditional cores have a much higher (i.e. > 1) permeability. A couple of notableexceptions are aluminium and brass, which are sometimes used to reduce theinductance of air cored coils in radio frequency (RF) work. This is much less

common than a ferrite "slug" core, which increases the inductance and is used totune some RF transformers.

As well as permeability, magnetic cores (with the exception of air) have amaximum magnetic flux they can handle without saturation. In this context,saturation means the same as in most others - when a towel is saturated, it canhold no more water, and when a magnetic core is saturated, it can carry no more


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magnetic flux. At this point, the magnetic field is no longer changing, so current isnot induced into the winding.

You will be unable to saturate your nails with the magnet, as there is a very largeair gap between the two pole pieces. This means that the core will always be

able to support the magnetic flux, but the efficiency is also very much lowerbecause the magnetic circuit is open. Nearly all the transformers you will seehave a completely closed magnetic circuit, to ensure that as much of themagnetism induced into the core as possible will pass through the winding(s).

There are some cases where a tiny air gap will be left deliberately, and this isdone routinely when a transformer or coil must sustain a significant DCcomponent as well as the AC. This is covered briefly below, but there is more onthis subject in the second section of the article.

Figure 1.1 - Essential Workings of a Transformer

Figure 1.1 shows the basics of all transformers. A coil (the primary) is connectedto an AC voltage source - typically the mains for power transformers. The fluxinduced into the core is coupled through to the secondary, a voltage is inducedinto the winding, and a current is produced through the load.

The diagram also shows the various parts of a transformer. This is a simpletransformer, with two windings. The primary (denoted as such during the design)will induce a magnetic field into the core in sympathy with the current produced

by the applied AC voltage. The magnetic field is concentrated by the core, andnearly all of it will pass through the windings of the secondary as well, where avoltage is induced. The core in this case is typical of the construction of a "C-Core" transformer, where the primary and secondary are separated. Morecommon is the "traditional" EI (ee-eye) type, which although somewhat out offavour these days is still used extensively. This is shown below.


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The magnitude of the voltage in the secondary is determined by a very simpleformula, which determines the "turns ratio" (N) of the component - this istraditionally calculated by dividing the secondary turns by the primary turns ...

1.1.1 N = Ts / Tp

Tp is simply the number of turns of wire that make up the primary winding, andTs is the number of turns of the secondary. A transformer with 500 turns on theprimary and 50 turns on the secondary has a turns ratio of 1:10 (i.e. 1/10 or 0.1)

1.1.2 Vs = Vp * NMostly, you will never know the number of turns, but of course we can simplyreverse the formula so that the turns ratio can be deduced from the primary andsecondary voltages ...

1.1.3 N = Vs / VpIf a voltage of 240V (AC, naturally) is applied to the primary, we would expect24V on the secondary, and this is indeed what will be measured. The transformerhas an additional useful function - not only is the voltage "transformed", but so is

the current.1.1.4 Is = Ip / NIf a current of 1A were drawn by the primary in the above example, then logicallya current of 10A would be available at the secondary - the voltage is reduced, butcurrent is increased. This would be the case if the transformer were 100%efficient, but even this - the most efficient "machine" we have - will sadly never beperfect. With large transformers used for the national supply grid, the efficiency ofthe transformers will generally exceed 95%, and some will be as high as 98% (oreven more).

Smaller transformers will always have a lower efficiency, but the units commonly

used in power amplifiers can have efficiencies of up to 90% for larger sizes. Thereasons for the lost power will become clear (I hope) as we progress. For thetime being, we shall consider the transformer to be "ideal" (i.e. having no losses)for simplicity.

Figure 1.2 - E-I Laminations


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The conventional E-I lamination set is still extensively used, and a few pertinentpoints are worth mentioning. The centre leg is always double the width of theouter legs to maintain the cross-sectional area. Likewise, the "I" lamination andthe "back" of the E are the same width as (or sometimes slightly larger than) theouter legs. The winding window is where the copper windings live, and in a well

designed transformer will be almost completely full. This maximises the amountof copper and reduces resistive losses because the windings are as thick as theypossibly can be.

2. Magnetic Core Terminology

This list is far from complete, but will be sufficient to either get you started orscare you away. I have included the symbols and units of only three of theentries below, since most are of no real interest.

Coercivity -is the field strength which must be applied to reduce (orcoerce) the

remanent flux to zero. Materials with high coercivity (e.g. those used forpermanent magnets) are called hard. Materials with low coercivity (those used fortransformers) are called soft.

Effective Area - of a core is the cross sectional area of the centre limb for E-Ilaminations, or the total area for a toroid. Usually this corresponds to the physicaldimensions of the core but because flux may not be distributed evenly themanufacturer may specify a value which reflects this.

Effective length - of a core is the distance which the magnetic flux travels inmaking a complete circuit. Usually this corresponds closely to the average of the

physical dimensions of the core, but because flux has a tendency to concentrateon the inside corners of the path the manufacturer may specify a value for theeffective length.

Flux Density - (symbol; B, unit; Teslas (T)) is simply the total flux divided by theeffective area of the magnetic circuit through which it flows.

Flux linkage - in an ideal inductor the flux generated by one turn would becontained within all the other turns. Real coils come close to this ideal when theother dimensions of the coil are small compared with its diameter, or if a suitablecore guides the flux through the windings.

Magnetomotive Force - MMF can be thought of as the magnetic equivalent ofelectromotive force. It is the product of the current flowing in a coil and thenumber of turns that make up the coil.

Magnetic Field Strength - (symbol: H, unit; ampere metres (A m -1)) when currentflows in a conductor, it is always accompanied by a magnetic field. The strength,


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or intensity, of this field is proportional to the amount of current and inverselyproportional to the distance from the conductor (hence the -1 superscript).

Magnetic Flux - (symbol: ; unit: Webers (Wb)) we refer to magnetism in termsof lines of force or flux, which is a measure of the total amount of magnetism.

Permeability - (symbol; , units: henrys per metre (Hm-1) is defined as the ratio offlux density to field strength, and is determined by the type of material within themagnetic field - i.e. the core material itself. Most references to permeability areactually to "relative permeability", as the permeability of nearly all materialschanges depending upon field strength (and in most cases with temperature aswell).

Remanence - (or remnance) is the flux density which remains in a magneticmaterial when the externally applied field is removed. Transformers require thelowest possible remanence, while permanent magnets need a high value of

remanence.

I mention these here for the sake of completeness, but their real importance isnot discussed further in Section 1. Section 2 of this article will revisit the terms,and their importance is somewhat enhanced in context.

3. How a Transformer Works

At no load, an ideal transformer draws virtually no current from the mains, since itis simply a large inductance. The whole principle of operation is based oninduced magnetic flux, which not only creates a voltage (and current) in the

secondary, but the primary as well! It is this characteristic that allows anyinductor to function as expected, and the voltage generated in the primary iscalled a "back EMF" (electromotive force). The magnitude of this voltage is suchthat it almost equals (and is effectively in the same phase as) the applied EMF.

Although a simple calculation can be made to determine the internally generatedvoltage, doing so is pointless since it can't be changed. As described in Part 1 ofthis series, for a sinusoidal waveform, the current through an inductor lags thevoltage by 90 degrees. Since the induced current is lagging by 90 degrees, theinternally generated voltage is shifted backagain by 90 so is in phase with theinput voltage. For the sake of simplicity, imagine an inductor or transformer (no

load) with an applied voltage of 230V. For the effective back EMF to resist the fullapplied AC voltage (as it must), the actual magnitude of the induced voltage(back EMF) is just under 230V. The output voltage of a transformer is always inphase with the applied voltage (within a few thousandths of a degree).

For example ... a transformer primary operating at 230V input draws 150mA fromthe mains at idle and has a DC resistance of 2 ohms. The back EMF must besufficient to limit the current through the 2 ohm resistance to 150mA, so will be


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close enough to 229.7V (0.3V at 2 ohms is 150mA). In real transformers thereare additional complications (iron loss in particular), but the principle isn'tchanged much.

If this is all to confusing, don't worry about it. Unless you intend to devote your

career to transformer design, the information is actually of little use to you, sinceyou are restrained by the "real world" characteristics of the components you buy -the internals are of little consequence. Even if you do devote your life to thedesign of transformers, this info is still merely a curiosity for the most part, sincethere is little you can do about it.

When you apply a load to the output (secondary) winding, a current is drawn bythe load, and this is reflected through the transformer to the primary. As a result,the primary must now draw more current from the mains. Somewhat intriguinglyperhaps, the more current that is drawn from the secondary, the original 90degree phase shift becomes less and less as the transformer approaches full

power. The power factor of an unloaded transformer is very low, meaning thatalthough there are volts and amps, there is relatively little power. The powerfactor improves as loading increases, and at full load will be close to unity (theideal).

Now, another interesting fact about transformers can now be examined.

We will use the same example as above. A 240V primary draws 1A, and the 24Vsecondary supplies 10A to the load. Using Ohm's law, the load resistance(impedance) is therefore 24/10 = 2.4 Ohms. The primary impedance must be240/1 = 240 Ohms. This is a ratio of 100:1, yet the turns ratio is only 10:1 - what

is going on?

The impedance ratio of a transformer is equal to the square of the turns ratio ...

3.1.1 Z = NTransformers are usually designed based on the power required, and thisdetermines the core size for a given core material. From this, the required "turnsper volt" figure can be determined, based on the maximum flux density that thecore material can support. Again, this varies widely with core materials.

A rule of thumb can be applied, that states that the core area for "standard" (if

indeed there is such a thing) steel laminations (in square centimetres) is equal tothe square root of the power. Thus a 625VA transformer would need a core of (atleast) 25 sq cm, assuming that the permeability of the core were about 500,which is fairly typical of standard transformer laminations. This also assumes thatthe core material will not saturate with the flux density required to obtain thispower.


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The next step is to calculate the number of turns per volt for the primary winding.This varies with frequency, but for a 50Hz transformer, the turns per volt is(approximately) 45 divided by the core area (in square centimetres). Fewer turnsare needed for a 60Hz transformer, and the turns per volt will be about 38 / corearea. Higher performance core materials may permit higher flux densities, so

fewer turns per volt might be possible, thus increasing the overall efficiency andregulation. These calculations must be made with care, or the transformer willoverheat at no load.

For a 625VA transformer, it follows that you will need about 432 turns for a 240Vprimary, although in practice it may be less than this. The grain-oriented siliconsteels used in better quality transformers will often tolerate much higher total fluxper unit area, and fewer turns will be needed.

You can determine the turns per volt of any transformer (for reasons that willbecome clearer as we progress) by adding exactly 10 turns of thin "bell wire" or

similar insulated wire to an existing transformer, wound over the existingwindings. When powered from the correct nominal supply voltage, measure thevoltage on the extra winding you created, and divide by 10 to obtain the turns pervolt rating for that transformer.

Now, what earthly use is this to you? Well, you might be surprised at what youcan do with this knowledge. Assume for a moment that you have a transformerfor a fair sized power amplifier. The secondary voltage is 35-0-35V which is muchtoo high to power the preamp circuit or even its power supply - but you will beable to do that with a single 16V winding. Another transformer would normally beused, but you can also add the extra winding yourself. This is almost too easy

with toroidal transformers, but with others it may not be possible at all. If thetransformer uses (say) 2 turns per volt, a mere 32 extra turns of bell wire (orsimilar) will provide 16V at the typical 100mA or so you will need. Add a 10%margin, and you still have only 36 turns to add, and this can be done in a fewminutes. Make sure that the extra winding is securely taped down with a goodquality tape (Kapton is highly recommended if you can get it). Do notuseordinary electricians' tape - it is not designed for the temperature thattransformers may operate at under consistent load.

NOTE: Ensure that there is no possibility whatsoever of the added winding shortingbetween turns - this will cause the smoke to escape from the insulation in a spectacularfashion, and you may ruin the transformer itself.

4. Interesting Things About Transformers

As discussed above, the impedance ratio is the square of the turns ratio, but thisis only one of many interesting things about transformers (well, Ithink they areinteresting, anyway :-)


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For example, one would think that increasing the number of turns would increasethe flux density, since there are more turns contributing to the magnetic field. Infact, the opposite is true, and for the same input voltage, an increase in thenumber of turns will decrease the flux density and vice versa. This is counter-intuitive until you realise that an increase in the number of turns increases the

inductance, and therefore reduces the current through each coil.

I have already mentioned that the power factor (and phase shift) varies accordingto load, and this (although mildly interesting) is not of any real consequence tomost of us.

A very interesting phenomenon exists when we draw current from the secondary.Since the primary current increases to supply the load, we would expect that themagnetic flux in the core would also increase (more amps, same number ofturns, more flux). In fact, the flux density decreases! In a perfect transformer withno copper loss, the flux would remain the same - the extra current supplies the

secondary only. In a real transformer, as the current is increased, the lossesincrease proportionally, and there is slightly less flux at full power than at no load.

5. Examples of Transformer Uses

This is only a brief discussion of the many uses of transformers. I have avoidedswitchmode supplies in this section, and will only present the most commonlinear applications. Power supply applications are covered more fully in Section2, and also in the article on Linear Power Supply Design.

It would be impossible to cover all aspects of transformers and their uses, since

they are so diverse and are used in so many different things. Computer networkinterface cards, modems, through to power amplifiers and microwave ovens, carand marine ignition systems, Tesla coils and moving coil phono preamps are avery small sampling of the diversity of the humble transformer (well, maybe it isnot so humble after all :-)

5.1 - Push-Pull Valve Output StageApart from the obvious uses in power supplies, transformers are used in otherareas as well. Valve power amplifiers nearly all use a transformer for the outputstage, and this converts the high impedance of the anodes to the loudspeakerimpedance, as well as providing the voltage feed to the output valves. No biasing

or other support components have been shown here - for more information onthis, have a look at How Amplifiers Work.


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Figure 5.1 - Push-Pull Valve Output Stage

Figure 5.1 shows how this works. The primary winding acts in a manner that maysurprise you at first, but it is quite in accordance with all the theory. The supplyvoltage shown is 500V, and we will assume that the valve can turn on hardenough to reduce this to zero alternately at each end of the winding. This isnever the case, because valves do not have a low enough internal impedance,but it makes the explanation simpler :-)

Neither valve will draw appreciable current with no signal, and the amount drawndoes not magnetise the core. The reason is simple - an equal amount of currentis drawn through each section of the primary winding, but effectively in opposite

directions. The magnetic field created by one half of the winding is cancelled bythat from the second half, leaving a nett steady state magnetic field of zero.

When valve V1 turns on completely, the voltage at its end of the winding isreduced to zero, and the voltage at the anode of V2 is 1,000 volts. This must bethe case, or the transformer theory is in tatters. The primary is operating as an"auto-transformer". Likewise, when V1 turns off and V2 turns on, the situation isreversed. You may well ask why 2 valves are needed at all? The voltage fromone valve is quite capable of swinging the voltage from one extreme to the other,it would seem.

This is not the case. Since the valve can only turn on, it will only be able tosupply current for 1/2 of the waveform. A Class-A push-pull design will normallyhave each valve carrying 1/2 of the maximum peak current required. In the caseof a push-pull design, there is no core saturation because of the DC current(which cancels out as before), so although two valves are needed, thetransformer will be smaller and will have very much better performance. Single-ended Class-A amps require a very large core with an air-gap to preventsaturation. This reduces the performance of the transformer dramatically, and


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increases distortion and gives a poorer low frequency response because of thelower inductance. High frequencies can also be adversely affected, because theair-gap causes some of the magnetic flux to "leak" out of the core. This is thecause of leakage inductance (covered in more detail in Section 2).

It is worth noting that the effective peak to peak swing across the entiretransformer primary is 2,000V. When V1 is turned on completely, it has zero volts(for our example only) at the plate, and V2 turns on it has a plate voltage of1,000V. V2 has exactly the same voltage peaks, but they are 180 degrees out ofphase. The total voltage across the transformer is therefore the sum of the twovoltages. From an AC perspective, the B+ supply line can be considered thesame as zero volts (remember it will be bypassed with a large capacitance).

The RMS voltage is easily calculated from the standard formula ...

5.1.1 Vp = Vp-p / 2

To obtain the peak value from peak to peak, then ...5.1.2 Vrms = Vp / 2To find the RMS value.

In this case, the peak to peak voltage is 2,000V, so peak is 1000V. The RMSvalue is 707V.

5.2 Single Ended Triode (SET) OutputFigure 5.2 shows the basic arrangement of a SET amplifier output stage. The fullDC current must flow through the transformer primary, and as discussed above,an air-gap must be introduced into the core to prevent saturation. Because an air

gap reduces the efficacy of the magnetic path, the core needs to be considerablylarger than would otherwise be the case.


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Figure 5.2 - Single Ended Triode Output Stage

The core operates with only one polarity of flux, which varies with the signal. Onemight think that this alone would reduce distortion, since the flux never crossesthe zero point, but this is not the case. It is still necessary for the flux to change,and the characteristics of magnetic materials indicate that the resistance tochange (rather than the absolute polarity of the magnetic field) is the dominantfactor. The valve (and transformer primary) must now carry a current equal to thepeak AC current demanded by the load - subject to the transformation ratio, ofcourse. Maximum negative swing (valve turned on) will double this current, and itwill be reduced to nearly zero as the valve turns off (positive swing). As thecurrent is reduced below the average standing (quiescent) current, the voltageacross the transformer increases in the opposite polarity - hence the fact that theplate voltage exceeds the supply voltage.

For the same power output, the valve in a single ended circuit must beconsiderably larger than that required for a push-pull circuit, because of thehigher dissipation needed for the extra current. There are also many other issueswith this arrangement, and they will be covered in more detail in Section 2.

Not the least of these is that the (probable) advantage of the additional voltageswing when using a centre tapped transformer is now gone, so the maximumRMS voltage that can be developed is 353V - a significant drop in primary ACvoltage.

5.3 Line Level ApplicationsTransformers are also used for "line-level" low power applications, typicallybalanced microphone inputs and line output stages. A transformer isunsurpassed for real-world balanced circuits, as the input or output is trulyfloating, and requires no connection to earth. This means that common mode


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signals (i.e. any signal that is common to both signal leads) are almostcompletely rejected.

Figure 5.3 shows a transformer balanced input, converting to unbalanced. Thesignal is amplified, and sent to the output transformer for distribution as a

balanced signal again. The "amplifier" will typically be a mixing console, and willtake mic or line level signals as the input (having run from the stage to the mixingarea), and the final mixed output is sent back to the stage for the main (Front ofHouse) public address amplifiers and speakers. There may be in excess of 100metres of cable from the microphone to the mixer and back to the main amps,and barely any noise will be picked up in the process.

Figure 5.3 - Balanced Microphone and Line Outputs

The telephone system used to be completely dependent on transformers to feedthe signal from the exchange (or Central Office in the US) to the customerpremises equipment (CPE). The phone switch used in offices, (PABX - PrivateAutomatic Branch Exchange, or PBX for the US) equipment still uses

transformers for nearly all incoming circuits whether analogue or digital. Theprinciple is exactly the same as for the audio application shown above, exceptthat for telephone circuits there is usually a DC voltage present to power the CPE(in the case of a home telephone) and to provide some basic signalling. Allmodern PABX circuits use ferrite cored transformers, with DC isolation circuitry toensure that no DC flows in the transformer windings, as this degrades theperformance in the same way as with the output transformer for a SET poweramplifier.

Audio applications for transformers in balanced circuits came from thetelecommunications industry where the concepts were first thought of. A

telephone line may be 4km or more in length, and is not shielded, so a method ofeliminating hum and noise was essential.

6. Safety

Safety is a major consideration for any power transformer (and in the case oftelecommunications, the isolating transformers), and electrical contact betweenprimary and secondary must not be allowed underanyrealistic fault condition. All


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countries have safety standards that apply to transformers where electricalisolation is important, and if in any doubt about the safety of a transformer for aparticular purpose, make sure that you verify that the transformer complies withthe relevant standard(s). It is well beyond the scope of this article to cover all thepossibilities of standards and compliance issues, so I shall leave that to you.

Many power transformers are fitted with an internal "once only" thermal fuse thatwill become open circuit in the event that a preset temperature is exceeded. Thistemperature is chosen to be the maximum safe temperature of the windingsbefore the insulation melts or breaks down, so in the event of a fault, the thermalfuse will open before the insulation is damaged and the component becomespotentially dangerous. It also helps to prevent the risk of fire (and no, this is notintended to be humorous - a friend of mine had his house burned to the groundbecause of a faulty power transformer in a video recorder - as determined by thefire investigators. True story!). See Figure 6.1 (below) as an example of how badthings can get if the transformer is not protected.

Once the thermal fuse opens, the transformer must be discarded, as it is usuallynot possible to gain access to the fuse for replacement. This is not as silly as itmay sound, since the thermal effects on the insulation cannot be predicted, andthe transformer may be unsafe if it were still able to be used.

There are transformers that are designed to be "intrinsically safe", and theseusually have the windings on separate sections of the core, not in physicalcontact with each other. If the core is connected to the electrical safety earth(which is usually a requirement), no method of failure (including a completemeltdown) in the primary will allow mains voltage to appear at the secondary.

Side by side windings are the next safest, and although primary and secondaryare on the same bobbin, the material used is selected to withstand hightemperatures and will maintain separation of the windings. Toroidal cores andother concentrically wound transformers are the least safe, since the onlyseparation between primary and secondary is a rather thin layer of insulation.This is one of the reasons that thermal fuses are often used with toroids.


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Figure 6.1 - Transformer Meltdown

Figure 6.1 shows a transformer I removed from a repair job. It is a complete

meltdown, and the remains of the plastic bobbin can be seen quite clearly. In anycircuit, it is extremely important to protect the user from coming into contact withthe mains should this happen. In this case, the bobbin had melted away from thewindings, dribbled on the base of the equipment, and generally made a big mess.Despite all this, there was no electrical connection between primary andsecondary or the laminations. This was a well made transformer (it failed due togross continuous overload, not any failure in the transformer itself).

Proper safety earthing is the only real way to ensure that a transformer that failscatastrophically (such as that shown) does not cause the chassis to become live- not all transformers are created equal when safety is concerned. Correct fusing

will ensure that the fuse blows - hopefully before the electrical safety iscompromised. A thermal fuse would have prevented the situation from becomingas bad as shown, but the transformer would have been just as dead.

7. Noise

Transformers make noise. This is not only the electrical noise that is created bythe nasty current waveform through the windings, diodes and into the filtercapacitors, but actual audible noise. One source is winding vibration, due to thewire moving because of the magnetic field and the current flowing through theconductors. This is to be avoided at all costs, since constant vibration will

eventually wear away the insulation, the windings will short circuit, and thetransformer is ruined. Fortunately, this is rather unusual, but it can (and does)happen on occasion.

Most of the noise is from the laminations or other core material, which contractwhen subjected to an intense magnetic field. This is called magnetostriction, andhappens to a greater or lesser degree with all magnetic materials. A stethoscopewill verify the source of the noise, and there is little or nothing that will stop it. A


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resilient mounting will stop most of the noise from being acoustically amplified bythe chassis, and generally the noise will be worse at no load. In some cases, atransformer may have been designed for 60Hz, but is used at 50Hz. In this case,the flux density will probably exceed the maximum allowable for the core, and thetransformer will get much hotter than it should, and will almost certainly be a lot

noisier as well. Toroidal transformers will generally be much quieter than EIlaminated (i.e. conventional) types.

Most (all?) transformers designed specifically for 60Hz will eventually fail with50Hz mains, due to overheating. The reverse is not true, and 50Hz transformerscan be operated quite safely on 60Hz.

Another problem with E-I laminations is that they may not have been fastenedtogether well enough, and this allows the outer laminations in particular tovibrate. Better quality conventional transformers will commonly be impregnatedwith varnish (sometimes under vacuum) and baked in a moderate oven until

tender .... oops, I mean until the varnish is completely dry. This binds thelaminations and windings together, preventing noise, and also making thetransformer more resistant to damage by water or other contaminants, and/ orunder conditions of high humidity (such as in the tropics).

Section 2

Click here to view the second part of the article. As I am sure you have noticed,transformers are not so simple after all.

References

These references are common to both sections of the article, although most onlyapply to Section 2. Countless different Web pages were researched during thecompilation of these articles, and although some were interesting, the majoritywere of minimal use. Of those who I actually remember (a daunting task in itself,considering the sheer amount of searching I had to do), I must "thank" thefollowing Web pages (in alphabetical order) ...

Amidon ATDL School (US Army) Jensen Transformers

Mitchell Electronics Corporation Tomi Engdahl - (ePanorama.net)

In addition, I have used various other references, but notably (in order ofusefulness) ...

Radiotron Designer's Handbook - F Langford-Smith (4th Edition)
http://sound.westhost.com/xfmr2.htmhttp://sound.westhost.com/xfmr2.htm


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Magnetism: quantities, units and relationships (E-mail:[email protected])

Basic Electronics - Grob, Third Edition (McGraw-Hill) Motorola Rectifier Applications Handbook (3rd Edition)

Download Silvio Klaic's neat littletransformer calculatorfrom his websiteThe following (slightly edited) definitions are from Units of Measurement

Units of Measurement site copyright by Russ Rowlett and University of North Carolina at ChapelHill.

Definitions used by permission.

Tesla (T) - flux density (or field intensity) for magnetic fields (also called themagnetic induction). The intensity of a magnetic field can be measured byplacing a current-carrying conductor in the field. The magnetic field exerts a forceon the conductor which depends on the amount of the current and the length ofthe conductor. One Tesla is defined as the field intensity generating one newton

of force per ampere of current per meter of conductor. Equivalently, one Teslarepresents a magnetic flux density of one Weber per square meter of area. Afield of one Tesla is quite strong: the strongest fields available in laboratories areabout 20 Teslas, and the Earth's magnetic flux density at its surface, is about 50microteslas (T). One Tesla equals 10,000 gauss. The Tesla, defined in 1958, isnamed after Nikola Tesla (1856-1943), whose work in electromagnetic inductionled to the first practical generators and motors using alternating current (much tothe annoyance of Edison, who claimed DC was "safer").

Weber (Wb) - magnetic flux. "Flux" is the rate (per unit of time) in whichsomething crosses a surface perpendicular to the flow. In the case of a magnetic

field, then the magnetic flux across a perpendicular surface is the product of themagnetic flux density, in Teslas, and the surface area, in square metres. If avarying magnetic field passes perpendicularly through a circular loop ofconducting material (one turn), the variation in the field induces a electricpotential in the loop. If the flux is changing at a uniform rate of one Weber persecond, the induced potential is one volt. This means that numerically the flux inwebers is equal to the potential, in volts, that would be created by collapsing thefield uniformly to zero in one second. One Weber is the flux induced in this wayby a current varying at the uniform rate of one ampere per second. The unithonours the German physicist Wilhelm Eduard Weber (1804-1891), one of theearly researchers of magnetism.
http://www.ee.surrey.ac.uk/Workshop/advice/coils/terms.htmlmailto:[email protected]://student.math.hr/resources/transformer01.exehttp://student.math.hr/resources/transformer01.exehttp://student.math.hr/~sklaichttp://www.unc.edu/~rowlett/units/index.htmlhttp://www.ee.surrey.ac.uk/Workshop/advice/coils/terms.htmlmailto:[email protected]://student.math.hr/resources/transformer01.exehttp://student.math.hr/~sklaichttp://www.unc.edu/~rowlett/units/index.html

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