Electrical Transformers are devices that transfer electrical current from one circuit to another through electrical induction. They consist of two coils of wire, wrapped around an electrically conductive core, which is usually made from laminated iron.

A basic 'step down' transformer

The coil on the left of the diagram, called the "Primary winding", carries the input current. The coil on the left is known as the "Secondary winding", and carries the output current. Both the input and output are Alternating Currents.

'Step Up' and 'Step Down' transformers[edit | edit source]

Transformers are generally used to change the voltage of an electrical circuit. A Step up transformer is one that increases the voltage, and a Step down transformer is one that decreases the voltage. For example, a step up transformer could have an input voltage of 10V AC and an output voltage of 15V AC.

The increase or reduction in voltage depends on the ratio of primary winding coils : secondary winding coils. The formula used to express this is , where is the number of coils, is voltage; refers to the primary winding and refers to the secondary winding.

In other words, the ratio between the number of coils on the windings is the same as the ratio of voltages across each winding. Coils on primary winding:Coils on secondary winding = Input voltage:Output Voltage.

For example, if there is one coil on the primary winding, and 8 coils on the secondary winding, then the ratio between windings will be 1:8. This means that the ratio between input voltage and output voltage will be 1:8, and therefore the output voltage will be 8 times the size of the input voltage.

If there are twice as many coils on the secondary winding than there are on the primary winding, then the voltage will be doubled. If there are half as many coils on the secondary winding as there are on the primary winding, then the voltage will be halved, and so on.

Typically, a step up transformer will be used to decrease the current in the electrical wires in the National Grid. This reduces the amount of heat energy lost by resistance in the wires. Multiple step down transformers will be then used to reduce the voltage down to practical, household levels. 

Power Inputs and Outputs in Transformers[edit | edit source]

Transformers are close to 100% efficient. For the purposes of this explanation, we will assume that transformers are exactly 100% efficient, meaning power in=power out.

In order to fully understand the following, it is necessary to understand that Power= Current x Voltage, or .

In a transformer, if the power input= power output, then . Basically, if the voltage in the primary coil () is bigger than voltage in the secondary coil (), then the current in the secondary coil ()must decrease so that I x V remains the same for both sides of the transformer. If voltage increases, then current must decrease in order to keep the total power the same.

How it works[edit | edit source]

An alternating current from the primary coil will create a changing magnetic field in the iron core. This, in turn, will induce a alternaiting potential difference and therefore current in the secondary coil. The voltage of this will depend upon the number of turns on the secondary coil. A input voltage as direct current will not work as no changing magnetic field will be created.

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