Transformers are widely used in all branches of electronics. One of their most well known uses is in power applications where they are used to transform the operating voltage from one value to another. They also serve to isolate the circuit at the output from a direct connection to the primary circuit. In this way they transfer power from one circuit to another with no direct connection.
Very large transformers are used on the National Grid to change the line voltages between the different values required. However for the radio amateur or home enthusiast transformers are commonly seen in power supplies. Transformers are also widely used in other circuits from audio up to radio frequencies where their properties are widely used to couple different stages within the equipment.
What is a transformer?
A basic transformer consists of two windings. These are known as the primary and the secondary. In essence power enters on the primary, and leaves on the secondary. Some transformers have more windings but the basis of operation is still the same.
There are two main effects that are used in a transformer and both relate to current and magnetic fields. In the first it is found that a current flowing in a wire sets up a magnetic field around it. The magnitude of this field is proportional to the current flowing in the wire. It is also found that if the wire is wound into a coil then the magnetic field is increased. If this electrically generated magnetic field is placed in an existing field then a force will be exerted on the wire carrying the current in the same way that two fixed magnets placed close to one another will either attract or repel one another. It is this phenomenon that is used in electric motors, meters, and a number of other electric units.
The second effect is that it is found that if a magnetic field around a conductor changes then an electric current will be induced in the conductor. One example of this can occur if a magnet is moved close to a wire or a coil. Under these circumstances an electric current will be induced, but only when the magnet is moving.
The combination of the two effects occurs when two wires, or two coils are placed together. When a current changes its magnitude in the first, this will result in a change in the magnetic flux and this in turn will result in a current being induced in the second. This is the basic concept behind a transformer, and it can be seen that it will only operate when a changing or alternating current is passing through the input or primary circuit.
Transformer turns ratio
For a current to flow an EMF (electro-motive force) must be present. This potential difference or voltage at the output is dependent upon the ratio of turns in the transformer. It is found that if more turns are present in the primary than the secondary then the voltage at the input will be greater than the output and vice versa. In fact the voltage can easily be calculated from a knowledge of the turns ratio:
Es = ns
Ep is the primary EMF
Es is the secondary EMF
np is the number of turns on the primary
ns is the number of turns on the secondary
If the turns ratio ns/np is greater than one then the transformer will give out a higher voltage at the output than the input and it is said to be a step up transformer. Similarly one with a turns ratio less than one is a step down transformer.
Voltage and current ratios across the transformer
There are a number of other factors that can be easily calculated. The first is the ratio of input and output currents and voltages. As the input power is equal to the output power it is possible to calculate a voltage or current if the other three values using the simple formula shown below. This fact does not take account of any losses in the transformer that can fortunately be ignored for most calculations.
Vp x Ip = Vs x Is
For example take the case of a mains transformer that gives out 25 volts at one amp. With an input voltage of 250 volts this means that the input current is only a tenth of an amp.
For some transformers the number of turns on the primary will be the same as that on the secondary, and the current and voltage at the input will be the same as that at the output. However where the turns ratio is not 1:1, the voltage and current ratio will be different at the input and the output. From the simple relationship shown above it will be seen that the ratio of voltage to current changes between the input and the output. For example a transformer with a turns ratio of 2:1 may have a 20 volt input with a current of 1 amp, whereas at the output the voltage will be 10 volts at 2 amps. As the ratio of voltage and current determines the impedance, it can be seen that the transformer can be used to change the impedance between the input and the output. In fact the impedance varies as the square of the turns ratio as seen by:
Zp = np2
Transformers are widely used in many applications in radio and electronics. One of their main applications is within mains power supplies. Here the transformer is used to change the incoming mains voltage (around 240 V in many countries, and 110V in many others) to the required voltage to supply the equipment. With most of today's equipment using semiconductor technology, the voltages that are required are much lower than the incoming mains. In addition to this the transformer isolates the supply on the secondary from the mains, and thereby making the secondary supply much safer. If the supply were taken directly from the mains supply then there would be a much greater risk of electric shock.
A power transformer like that used in a power supply is generally wound on an iron core. This is used to concentrate the magnetic field and ensuring the coupling between the primary and secondary is very tight. In this way the efficiency is kept as high as possible. However it is very important to ensure that this core does not act as a one-turn winding. To prevent this happening the sections of the core are insulated from one another. In fact the core is made up from several plates, each interleaved but insulated from one another as shown.
The two windings of a power transformer are well insulated from one another. This prevents any likelihood of the secondary winding from becoming live.
Although one of the major uses for transformers that the hobbyist will encounter is for transforming supply or mains voltages to a new level, they also have a variety of other applications for which they can be used. When valves were used, they were widely used in audio applications to enable low impedance loudspeakers to be driven by valve circuits that had a relatively high output impedance. They are also used for radio frequency applications. The fact that they can isolate the direct current components of the signal, act as impedance transformers, and as tuned circuits all in one means that they are a vital element in many circuits. In many portable receivers these IF transformers provide the selectivity for the receiver. In the example shown it can be seen that the primary of the transformer is tuned using a capacitor to bring it to resonance. Adjustment of the resonant frequency is normally made using a core that can be screwed in and out to vary the amount of inductance of the coil. The transformer also matches the higher impedance of the collector stage of the previous stage to the lower impedance of the following stage. It also serves to isolate the different steady state voltages on the collector of the previous stage from the base of the following stage. If the two circuits were not isolated from one another, the DC bias conditions for both transistors would be disturbed and neither stage would operate correctly. By using a transformer the stages can be connected for AC signals whilst still maintaining the DC bias conditions.
The transformer is an invaluable component in today's electronics scene. Despite the fact that integrated circuits and other semiconductor devices seem to be used in ever increasing quantities, there is no substitute for the transformer. The fact that it is able to isolate and transfer power from one circuit to another whilst changing the impedance, ensure that it is uniquely placed as a tool for electronics designers.