Have you ever thought of what is inside that charger, charging your cell phone or electric razor? What is powering a multitude of electronic equipment that we can't do without. The answer is, a transformer as the central piece inside any power supply.
Power supplies are very important in electronics and in our everyday lives, supplying voltages and currents necessary for circuits to operate. Power supplies are constructed from four common elements:
- Transformer
- Rectifier
- Filter
- Regulator
Of these, the transformer is the central element as it provides the power needed. The other components merely make this power available in a suitable, reliable form. In general, the user needs power to be supplied and at the right voltage, different from mains voltage. This requirement is precisely the function of the transformer!
Ideal Transformer
When Faraday discovered electromagnetic induction, he had virtually invented the transformer. Any transformer simply consists of two coils positioned so that the magnetic flux from one passes through to the other. A transformer changes a given electrical current into another current of different voltage. There are two kinds of transformers:
- Step-up transformer to increase the voltage
- Step-down transformer to decrease the voltage.
Figure 1 is a schematic diagram of a transformer. It consists of insulated wires wound around a conductor, such as an iron loop. A changing current in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a changing voltage in the second circuit (the secondary). By adding a load to the secondary circuit, one can make current flow in the transformer, thus transferring energy from one circuit to the other.
This arrangement allows for step-up or step-down of voltages depending on the number of turns of wire present, and according to the simple equation:
Vp / Vs = Np / Ns
where Vp is the primary voltage, Vs is secondary voltage, Np is the number of primary turns, and Ns is the number of secondary turns. By appropriate selection of the numbers of turns, a transformer thus allows an alternating voltage to be stepped up — by making Ns more than Np — or stepped down, by making it less.
Worked Examples
For a step-down tranformer, if the secondary has half the primary turns, then by proportion, the secondary voltage must be half the primary voltage. Using numbers, if the primary has 5000 turns and the secondary has 500 turns, then the turns ratio is 10:1. So, if the primary voltage is 240 volts then the secondary voltage will be smaller by this same ratio, to 24 volts. For a step-up transformer, if the secondary has twice as many turns, then by proportion, the seconday voltage must be twice the primary voltage.
Power Losses
When power provided by the primary equals the power taken by a load on the secondary, then we have an an ideal transformer. Consider a 24 watt lamp connected across a secondary coil: the primary coil supplies exactly 24 watts to match the power consumed. In a non-ideal transformer, however, there are power losses due to resistive heating in the windings. In the example, the power supplied would be somewhat smaller, say 22.6 watt for 5% losses.
Understanding Transformers
Michael Faraday showed that current in one coil can be induced in another, thus discovering the underlying working principle of a transformer. The modern transformer is normally located inside the power supply. Its working concept is simple: two coils wound around a common iron core transform voltage, as needed, according to the number of turns present in each coil.
The reader may be interested in more details on this topic or to learn about power transmission and usage.
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