A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer’s coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer’s core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or “voltage”, in the secondary winding. This effect is called mutual induction. If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp), and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows,
By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be “stepped up” by making Ns greater than Np, or “stepped down” by making Ns less than Np.
In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception. All operate with the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household (“mains”) voltage. Transformers are essential for high-voltage electric power transmission, which makes long-distance transmission economically practical.
The ideal transformer as a circuit element
Ideal power equation is as follows,
If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power: giving the ideal transformer equation
Transformers normally have high efficiency, so this formula is a reasonable approximation.If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by the square of the turns ratio. A transformer makes use of Faraday’s Law and the ferromagnetic properties of an iron core to efficiently raise or lower AC voltages. It of course cannot increase power so that if the voltage is raised, the current is proportionally lowered and vice versa.
Types of transformers:
In general, transformers are used for two purposes: matching and power supplies.
Power transformers are used to convert from one voltage to another, at significant power levels.
A “step-up transformer” allows a device that requires a high voltage power supply to operate from a lower voltage source. The transformer takes in the low voltage at a high current and puts out the high voltage at a low current.
A “step-down transformer” allows a device that requires a low voltage power supply to operate from a higher voltage. The transformer takes in the high voltage at a low current and puts out a low voltage at a high current.
An “isolation transformer” does not raise or lower a voltage; whatever voltage comes in is what goes out. An isolation transformer prevents current from flowing directly from one side to the other. This usually serves as a safety device to prevent electrocution.