transformer basics

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published: Jun-17-2015, last edit: Mar-16-2017

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Transformer basics

Operation of transformers is quite basic and easy to understand. They work based on an alternating magnetic field.  When an alternating or direct CURRENT passes through a wire a magnetic field is created. When a wire is wound into a coil the magnetic field inside that coil becomes higher. The higher the current and the more windings the higher the magnetic field.

It also works the other way around, but NOT for a continuous magnetic field. When an alternating magnetic field is present within a coil an alternating Voltage (not current) is generated. Transformers thus work by the grace of an ALTERNATING magnetic field only. The picture below shows the basics.

trafoAn AC voltage is applied to a coil which has a certain resistance. Because of this a current will flow through that coil. That current creates an alternating magnetic field. This coil is called the primary coil. All the electric energy is thus converted to magnetic energy. In a coil a magnetic field will be present in the center of it and this magnetic field bends outwards around that coil in all directions. This is where the core comes in. A magnetic field likes to take the route of least resistance and the core provides just that. It is made of a material that is easily magnetised and doesn’t ‘hold’ that field like a magnet does as. It is made of a different material than what magnets are made from. Depending on the frequency of the AC voltage different materials have to be used. For anything within the audible range (20Hz to 20kHz) a special type of ‘electric steel‘ has the best properties, for higher frequencies (data and SMPS) ferrite cores are more suited. Different types of ferrite are optimal for different frequencies. For mains frequencies (50Hz or 60Hz depending on where in the world one is) only metal cores can be used. The transformers discussed in this article ONLY covers mains transformers and is not about audio-transformers (which need slightly different mechanical constructions and materials) or ferrite core transformers.

The biggest portion of the generated magnetic field will stay within that metal core and only a small percentage will also venture outside the magnetic core. This is called the stray magnetic field and is a ‘loss’ factor. Also it will take some energy to magnetise and demagnetise the core when the polarity of the AC signal alters its direction. There is a slight amount of ‘memory’ of magnetic field which also causes some losses of magnetic field strength, this is called the hysteresis.

The generated magnetic field passes through a second coil which, not surprisingly, is called the secondary coil. When an alternating magnetic field passes through a coil a VOLTAGE is induced (generated) and it is present on the ends of the coil’s wire. It is not until something (a load) is connected to that coil a current will flow.

The amplitude of the output voltage is determined by the winding ratio. When for instance the winding ratio is 10:1 and a 230V voltage is applied to the primary coil a voltage 10x as low will be present on the secondary coil. In this case 23V. The diameter of the wires is determined by the current it must be able to handle. As little power is lost (efficiency depends on a few factors but is quite high for transformers) there is a clear relation between voltage and current and thus also between voltage and wire diameter. The lower the voltage, the higher the current, the thicker the wire. The higher the voltage, the lower the current and the smaller the diameter needs to be.

Transformers are not limited to a single input voltage nor a single output voltage. Transformers can have multiple primary (input) and secondary (output) windings. Primary windings may be consist of 2 or more separated windings (so for instance 2 x 115V) or could be connected so they are in series. This is called a Centre Tap in this case. Two of those separate 115V windings in series will give such a transformer an input voltage of 230V. 2 of these 115V windings in parallel will give the transformer a 115V input voltage. Such a transformer can thus be used on 2 different input voltages. Transformers exist with much more than 2 ‘taps’ in small increments for instance.

The same goes for secondary windings. A transformer can have several secondary windings with similar or different output voltages and some windings may or may not be connected to each other. With multiple windings it is important not to exceed the total maximum power rating a transformer can handle.

A fun property of a transformer is it also works the other way around. Meaning when we apply a current to the secondary winding we get an output voltage on the primary winding.

The coils are usually not wound directly on the metal core but are found on a former, also known as a bobbin. In the early days they were made of Pertinax but was soon replaced by plastic.

Transformers exist in many physical shapes and sizes. The overall size differences are related to the power rating (power = voltage x current). It is easy to understand why. The used metal core has quite linear properties, but when too much magnetic field strength is present they start to become saturated and do not work as they should any more. That surplus in power is converted into heat. So for this reason a small transformer can only handle a small amount of magnetic ‘force’ and if more power is needed more metal is needed that can ‘conduct’ the magnetic field without being saturated. The bigger the transformer in size the more power it can handle.

Cores in transformers are not made of solid metal but consist of laminated stacks of metal pieces. The reason for this is to avoid magnetic ‘whirlwinds’ in solid cores that lower the efficiency and would cause the transformer to overheat and draw a lots of not-needed power. Cores (of all the above mentioned types) are thus always made of a laminated special type of steel’. Those laminated steel sheets are usually < 1mm thick and often have an extra isolating coating on it.

Different shapes of cores all have their own properties. Some properties may be more desirable in certain cases where they are undesirable in other cases. The shape used in the example above is very similar to U-core and R-core transformers. Most widely used, however, are E-core (also called E-I-core) and toroidal transformers. Each of these types has properties that may be desirable over the others and this is the reason why all these types exist.

E coresE-Core (also called E-I core) transformers are the most common types. They are called E core  because the laminated steel has the shape of an E and an I. They are stacked alternately. Another version of this design is the E-E core. The coils are situated around the middle ‘leg’ of the E. The primary and secondary coil(s) may be wound on top of each other (with a isolating foil layer between them) or can be wound in separate ‘chambers’ next to each other. The latter has a lower capacitive coupling between the primary and secondary winding than the version where the coils are wound on top of each other. For audio equipment a low capacitance between primary and secondary wires is desirable as less high frequency ‘garbage’ on the mains will be present and also less 50Hz (60Hz) ‘leakage‘ will be present. Efficiency is quite good and these type of transformers are easy to produce. It needs a bit more steel (and is thus heavier in weight) than toroidal transformers. E-cores are square and thus the coils are also wound around a square bobbin. Sometimes these types of transformers are not very silent and can emit a humming noise.

U cores

U-core (U-I core) transformers are very similar to an E-core with stacked laminates in the shape of a U and an I. It lacks the middle ‘leg’ the E has. It is similar to the core used in the example above that illustrates the basic function.

Because it misses the middle leg the coil(s) are thus moved to one or both of the outer legs. Variations of this type of transformers are C-core and L-core types. In most cases the primary and secondary sides are mounted on the opposite ‘legs’ (shown in the bottom right illustration on the left) but sometimes, for certain reasons, the two coils are directly above or on top of each other on one ‘leg’.

The U-core is slightly less efficient than the E-core but when made with the two coils on the opposite ‘legs’ the capacitance between the primary and secondary windings is very low.

U-cores with all the windings on 1 side are very common in TV’s used as high voltage line transformers, except these aren’t made of electric steel but have ferrite cores as they operate on a higher frequency (15625 Hz).

R cores

R-Core transformers are similar to C-core except for the fact they have a rounded core diameter and do not consist of two halves connected together, but are more similar in construction to toroidal transformers. The bobbin(s) and coil(s) are round instead of square. This makes them somewhat more efficient and have less ‘stray‘ magnetic fields outside of the transformer.

They do not have a square appearance like that of the E- and C-core types but have rounded corners as well as a circular core (the diameter of the core is circular). The bobbins are also circular and thus the copper wires do not have to make tight bends around the edges.

These transformers also have a very low capacitance between the primary and secondary windings and have a higher efficiency. They are more expensive to make and thus not very common. I would say these would be first choice as a transformer for audio applications.

O cores

Toroidal transformer cores consist of a very long and thin metal strip that is wound up (like a spring) in a large circle rather than exist of lots of individual metal parts. A cross-section of the core thus looks like a laminated square and this gives them their distinctive shape that is different from an O-core.

The wires around a ‘square’ core run straight and bend sharply around each corner where as in the O-core the core diameter is ‘round‘. Square toroids are cheaper to make than an O-core.

Power/size ratio is best of all the mentioned transformer types, so when a small size and relatively high power ratings are needed AND price is also an issue a toroid is the way to go.

Just like the O-core it has very little stray magnetic fields outside of the transformer and is part of the reason of the higher efficiency. For these reasons (price-weight-size/power ratio) these transformers are used quite often in compact equipment.

Of course toroids also have a downside… just like the O-core it has a rather high capacitance between the primary and secondary winding(s) because the primary and secondary windings are very close together over a very large surface which makes them less suited for audio when it comes to this specific property. A toroid also does not use bobbins but like the O-core the windings are made on top of a thin isolating layer wound around the metal core. In most cases though the price-weight-size-efficiency/height/power properties may be more important. Very few (and expensive) toroids, however, are equipped with a special conductive layer between the primary and secondary winding (which lowers the efficiency  slightly) and this acts as screen ensuring HF garbage on the mains cannot couple capacitively to the secondary winding(s) but is diverted to ground. Most of the existing toroids do NOT have this special shield though.

Generally toroids do not emit as much sound (hum) as the E-U cores described above. The primary and secondary winding(s) are wound all around the core and are layered and on top of each other. Toroids have a higher capacitance .

Some things that are good to know.

50Hz and 60Hz seem very close together and one might expect that 60Hz transformers can also be used for 50Hz systems and vice versa. This, however, is not always the case. Transformers designed for 50Hz can always be used on 60Hz BUT transformers specifically designed for 60Hz can NOT be used for 50Hz systems. If the transformers have 50/60Hz written on it they can be used on both systems.

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