Transformers

What is a transformer?

A classical transformer is used to transform an electrical alternating voltage (primary voltage) into another alternating voltage (secondary voltage).

The primary and secondary voltages are not electrically connected . . The specialist speaks of a galvanic separation between the primary side and the secondary side . .

In the simplest case, a transformer consists of a primary coil, an iron core and a secondary coil . The two coils consist of a winding of copper wire, which has been coated with a layer of paint for insulation.

Transformer with clearly visible separate primary coil (P) and secondary coil (S).

Mains transformer with 9 V and 12 V output voltage.

Depending on the design and intended use, the secondary voltage may be lower, equal to or higher than the primary voltage.

The height of the output voltage is determined by the winding ratio (number of windings) between the primary and secondary coils. How much power the transformer can transmit depends on the cross-section of the copper wire used.

For voltage adjustment, the primary and secondary windings may have taps. This allows a mains transformer with a secondary coil to output different voltages.

The more current flows over a conductor, the higher the losses on the cable.

The reason for this is the resistance of the conductor. In order to reduce the cable losses , it is theoretically possible to increase the cable cross-section . This is not done, however, as the cables become very quickly too expensive due to the high material requirements.

It is better to increase the voltage using a transformer before transmission . This is exactly what is done in practice, and that is why interland lines work with extremely high voltages of several thousand volts.

The formula makes it easy to understand the situation:
P = U x I
When the voltage (U) in the electricity plant is transformed by a factor of 1,000, the current (I) decreases by a factor of 1,000 with the same power (P).

The power loss calculated according to the formula P = I² x R (Strom² x line resistance) is then only 1/1,000,000 of the value without transformation. Thus, electrical energy can be transmitted across the whole country.

On the receiver side, in substations and transformer stations, the high voltage of several thousand volts is transformed down to a voltage of 230 V that is common for the public power grid.

Overhead power lines are operated from 10,000 V to 380,000 V.

Transformers are also used in plug-in power supplies.

The supply voltage of 230 V is perfect for lamps, heaters, power tools or kitchen appliances.

Due to the high voltage, "thin" cables with 1.5 to 2.5 mm² are perfectly sufficient for domestic electrical installations.

For other devices such as radios, stereos, computers, printers and other small devices, however, the voltage of 230 V is far too high.

For this reason, power supplies are required for the voltage adjustment, in which transformers in the most varied designs are also used.

The power supplies can either be built into the device or integrated externally between the device and the mains plug in the power line.

If signals are to be transmitted from one circuit to another circuit, transformers or, better said, transformers are also used. A classical example of this is a light organ .

The sound signals picked up at the loudspeaker output must be activated by electronic switches (thyristors) which control the light in the rhythm of the music .

However, the lamps of the light organ are directly connected to the 230 V mains. For safety reasons, however, it is not permissible that the mains potential is transferred back to the stereo system. Galvanic isolation is required.

For this reason, the impulses of the loudspeaker output are switched to the primary coil of the transformer. The secondary coil then generates the pulses for controlling the lamps.

The black block on the left side of the board is the transformer.

The structure of this isolating transformer shows the functional principle.

If a voltage is applied to the coil on the primary side, the primary coil builds up a magnetic field when it is switched on . This magnetic field is bundled and amplified by the iron core .

In the short moment when the primary coil builds up the magnetic field , the magnetic field strength increases from zero to maximum . At this moment, a voltage in the form of a pulse peak is generated (induced) in the secondary coil.

If a DC voltage is connected to the primary coil, the current flowing through the primary coil generates a consistently strong magnetic field that does not change. This means that no further voltages can be measured at the secondary coil apart from the short switch-on pulse.

The reason for this is that the secondary coil induces a voltage only when the strength of the magnetic field changes . Therefore, transformers do not work with DC voltage . They must be operated with an alternating voltage.

On the primary side, a mains transformer is supplied with an alternating voltage (e.g. 230 V from the public supply network) and outputs an alternating voltage at a height of 12 V at the secondary coil. Depending on the design, transformers are intended for installation or have their own housing with the necessary connection options.

A power supply is also operated with an alternating voltage , but includes a rectifier (BGR) and a sieve capacitor (C) in addition to the mains transformer. For this reason, power supplies usually output a voltage at a similar constant level (DC voltage). To ensure that the output voltage is always the same at different loads, many power supplies are also equipped with voltage stabilization.

Power transformer (upper figure) and power supply (lower figure).

This transformer with 70 W is 11 x 5.5 x 3.5 cm small and 120 g light.

With the usual mains voltage of 230 V/50 Hz in Germany, 50 sine waves are generated in the electricity plant per second. Subjectively, this is quite fast, if the voltage and thus also the current direction are reversed 100 times per second.

In electronics, however, the frequency of 50 Hz is infinitely slow . Therefore, if you want to transform high-power voltages, you need large, thick and heavy transformers.

In order to save material, weight, space and money, ingenious developers have come up with the idea of controlling transformers much faster. To do this, you only need to make a direct voltage from the mains voltage of 230 V.

This DC voltage is switched to the primary coil of a transformer with an electronic switch at the desired frequency of several thousand Hertz. 

Since this transformer then works with a much higher frequency, it is much smaller, lighter and more cost-effective at the same power than its slow partner from the 50 Hz fraction.

Practical implementation:
The diagram opposite shows the technical implementation of an electronic transformer in a simplified form. The bridge rectifier (BGR) converts 230 V AC voltage into a direct voltage of approx. 320 V. This is smoothed using the two capacitors (C1 and C2) . Each of the two capacitors charges to approx. 160 V (half direct voltage).

The two electronic switches (S1) and (S2) are controlled in such a way that only one of the two switches can be closed at a time. If both switches close in the event of a fault, the fuse (SI) would immediately trigger .

When the switch (S1) is closed , (C2) charges itself to the full operating voltage via the upper coil (primary coil) of the mains transformer (TR1). At the same time, the capacitor (C1) discharges via the switch (S1) and the mains transformer. In this phase, current flows through the primary coil from left to right .

Simplified circuit diagram of an electronic transformer.

Classic mains transformer with base bracket for mounting.

Mains transformers or mains transformers are used to convert the mains voltage of 230 V into a voltage suitable for the respective device to be supplied.

The transformers are either placed directly in the respective device or in an external power supply or also in a control cabinet.

For industrial applications, there are also mains transformers that can be operated with input voltages of 400 V or more.

Select the suitable transformer, in addition to the primary and secondary voltage, also observe the power that the transformer must have.

For a 12 V power supply with max. 3 A, the transformer can be roughly selected according to the following formula:

12 V x 3 A = 36 VA x factor 1.4 = 50 VA

Due to the factor 1.4, the mains transformer has a max. Load an utilization of approx. 70% . This means that it will not be overloaded even if the maximum load is sustained for a long time .

In connection with mains transformers, the term safety transformer or safety transformer still applies.

These transformers fulfill special safety features and are subject to stricter test conditions .

Due to internal safety measures, the transformers are short-circuit proof or partially short-circuit proof and have reinforced insulation .

This ensures that no electrical connection of the primary and secondary circuits can occur even in the event of a fault.

In addition, safety transformers are often equipped with safety devices or fuses.

Safety transformer with protective housing and fuses.

Control transformers are used in industrial applications.

Control transformers are used in energy and automation technology as well as in control cabinet construction.

Control transformers ensure a reliable supply of control and auxiliary circuits and, thanks to additional taps, enable a perfect adaptation to the local power grid .

In the event of a fault, the short-circuit current in the control circuit is limited and faults caused by switching inductive loads are reduced .

Compact power supply transformers are more than just conventional mains transformers .

In addition to the transformer, the rectifier and charge electrolyte capacitor are also integrated in the housing.

This means that compact power supply transformers do not output AC voltage but a DC voltage.

In the case of compact power supply transformers for experimentation (see figure), different AC voltage connections are routed outwards.

The user can then decide for himself whether to use the internal rectifier and the charge electrolyte capacitor.

Compact power supply transformers are also ideal for experimenting.

PCB transformer with mounting brackets on the side for mounting.

Print or platinum transformers are preferably used as mains transformers for small devices that do not require too much energy.

The transformers are molded into plastic housings and have connections that are led out downwards . This means they are perfectly designed for PCB mounting.

For smaller PCB transformers, the mechanical strength of the solder joints is sufficient to give the transformer sufficient hold . For larger transformers, there are usually still tabs on the housing, with which the transformer can be screwed to the circuit board.

When selecting the correct PCB transformer, the distances between the connection legs must be checked in addition to the electrical values such as primary and secondary voltage, as well as current and power.

Halogen transformers have been used in the past preferably for the power supply of low-voltage halogen lamps.

The transformers were previously designed as physical transformers with primary and secondary winding, but this made them very clumpy, heavy and expensive.

Halogen transformers are now available as electronic transformers, which can also be installed in a much easier way.  lassen.

If the halogen lamps are replaced by power-saving LED lamps , some electronic transformers may have problems.

Due to the significantly low operating current of the LED light bulbs, the minimum load that some electronic transformers require may not be reached.

The slim design enables a concealed installation.

The round design brings advantages but also more cost.

Due to the circular design, toroidal transformers have a better efficiency than block transformers. The magnetic scattering field is also significantly less pronounced.

This is also the reason why toroidal transformers like to be installed in hi-fi systems.

Other advantages are the low space requirement , a reduced weight and virtually no humming noise .

However, the production costs are much higher, which also makes the transformers more expensive.

Powerful toroidal transformers require a inrush current limiter in the form of a PTCs.

Series transformers are used for voltage adjustment .

This can happen, for example, if an American device designed for 110 V/60 Hz is to be operated for a limited time in the European power grid at 230 V/50 Hz.

However, ballast transformers only change the voltage and not the line frequency .

In most cases, however, this is rather non-critical.

In order for series transformers to be used individually, the input voltage as well as the output voltage can be set individually for some devices.

Ballast transformer with housing and suitable mains plug sockets.

A variable transformer provides individual output voltages.

Servo or control transformers are primarily used in measurement and control technology , in electronics laboratories and in the service sector.

With these transformers different output voltages can be generated at an input voltage of 230 V/50 Hz. For this purpose, the output voltage is set with a rotary grinding contact on a transformer coil mounted in a ring pattern.

Thus, it is very easy to determine how, for example, the connected TV set can regulate voltage fluctuations at the input.

The transformers can be designed as an autotransformer with only one winding without galvanic isolation (see sketch in the figure) or as a control isolating transformer with two windings.

An autotransformer usually consists of a coil with one or more taps for the output voltage.

Autotransformers are preferably used when the input and output voltage are very close together.

For an autotransformer, only the voltage difference between the input and output is transformed .

Galvanic isolation between the input and output voltage is not available for an autotransformer .

Low-power transformers can be used with just one winding.

Laboratory transformer with control and galvanic isolation.

Laboratory transformers are used for development, service or repair work on electronic circuits.

To avoid short circuits with grounded measuring devices and life-threatening electric shocks, the circuit or the device on which work is being carried out must be disconnected from the mains potential. This galvanic separation is achieved by a isolating transformer to which the device to be repaired must be connected.

The isolating transformers have either a fixed output voltage or the output voltage can be set individually. In this case it is called a control isolating transformer .

Due to the galvanic isolation from the mains supply, isolating transformers at the output do not have a protective conductor contact or the protective conductor is not connected at the output for cold device sockets .

The operation of several loudspeakers in an amplifier system is critical, because loudspeakers cannot be wired together so easily. However, in public buildings, restaurants or hotels, the simultaneous operation of several loudspeakers is often necessary.

Here ELA transformers or PA transformers help. An ELA transformer is connected to the output of the amplifier, which has the same connection values as a loudspeaker box on the primary side (input side).

The signals of the amplifier (music or speech) are now transformed on the secondary side of the ELA transformer (100 V).

The loudspeakers (LS1 - LS4) also use an ELA transformer that transforms the high pulses on the 100 V line back to a voltage compatible with the loudspeaker.

Several ELA transformers with the corresponding loudspeakers can be connected in parallel without any problem on the 100 V cable. Even long lines between the loudspeaker and amplifier no longer matter.

Several loudspeakers can be operated with ELA transformers.

Transformers are more about signal quality than power.

Transformers work primarily like transformers .

But unlike transformers, transformers do not need to effectively transform large power at a given frequency.

On the contrary!
Transformers must be able to transmit a wide frequency spectrum in the highest possible quality, e.g. as audio transmitters for microphones. The microphone circuit and the amplifier circuit are not electrically connected (galvanic isolation).

Transformers are also used in power supplies or in HF and digital technology. Here, they are called pulse transformers and do not have to be designed as broadband in terms of the transmission frequency.

Transformers convert current into current in magnetic fields and magnetic fields. They do so well, however, that they have proven themselves in the most varied areas of technology for over 100 years.

Thanks to the relatively simple construction, the use of robust materials and the absence of mechanically moving components, transformers are extremely low-maintenance and long-lasting. Provided that they are operated within the limits specified in the technical data.

Today's developers and engineers appreciate these simple and easy-to-use features. For this reason, the latest equipment, machines and systems still contain old-proven transformers.

Modern power supply with transformers and transformers (with yellow border).