Step down transformers

A transformer consists of two coils (often called 'windings') linked by an iron core, as shown in figure below. There is no electrical connection between the coils, instead they are linked by a magnetic field created in the core.

Step-down transformers

Step down transformers is to reduce a voltage. Most power supplies use a step-down transformer to reduce the dangerously high mains voltage to a safer low voltage.   The input coil is called the primary and the output coil is called the secondary. 

For this project, it will use a step down transformers that give output 9V or 12V as picture below.

The ratio of the number of turns on each coil, called the turn’s ratio, determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage.

Below are transformers formula,

Turns ratio = Vp/ V_S = Np/N_S

Power Out= Power In

V_S X I_S=V_P X I_P

Vp = primary (input) voltage
Np = number of turns on primary coil

Ip  = primary (input) current     

Transformers have two great advantages over other methods of changing voltage:

1. They provide total electrical isolation between the input and output, so they can be safely used to reduce the high voltage of the mains supply.

1. Almost no power is wasted in a transformer. They have a high efficiency (power out / power in) of 95% or more.