TRANSFORMER

•  a devise that transfers electrical energy from one circuit to another circuit using inductively coupled conductors.
• is an electromagnetic devise used to increase or decrease an alternating current input voltage.
• it also used to step up voltages if higher voltages are required or to step down voltages if lower voltages are required.
• used in power supplies to isolate the electrical circuits from the AC voltage source.
• if transformers are used in power supply, the AC power source is connected only to the primary of the transformer. This isolates the electrical circuits from the power source.
• Otto Blathy the father of transformer.

Parts of transformer

• a transformer consists of primary winding, secondary winding and core.
• primary winding-coil forming the part of an electrical circuit such that changing current in it induces a current in a neighboring circuit.
• secondary winding- coil such that current is induced in it by passing a current through the primary coil. 
• core is made of layers of iron.
• when the current is passed trough the primary coil, it produces a magnetic field. The core is to provide a path for the lines of magnetic force so that they almost all pass through the secondary coil. 
• by putting two coils of wire close together while not touching, the magnetic field from the first coil effects the other coil. The effect called inductance.

Transformer is based on two principles:

• an electric current can produce a magnetic field (electromagnetism).
• a changing magnetic field within a coil of coil induces a voltage across the ends of the coil (electromagnetic induction).

Two types of transformer

1.Step-up Transformer 

• a transformer in which voltage across secondary coil is greater than primary voltage.
• the number of turns in secondary coil is greater than in primary coil.
• converts low voltage AC into high voltage AC.
• more current flows in primary coil. Therefore one must use thicker wire in primary coil as to compared to the secondary coil.
• it delivers higher voltage than the source.

Symbol of Step Up Transformer

2. Step-down Transformer

• a transformer in which voltage across secondary coil is lesser than primary voltage.
• the number of turns in secondary coil is less than in primary coil.
• converts high voltage AC into low voltage AC.
• more current flows in secondary coil. 
• the larger gauge wire used in the secondary winding is necessary due to the increase in current.
• it delivers lower voltage than the source.

Symbol of Step Down Transformer

* The step up and step down effect coil turn ratios in a transformer is analogous to gear tooth ratios in mechanical gear systems, transforming values of speed and torque in much the same way.

* Transformer is not an energy conversion devise, but it is devise that changes electrical power at one voltage level into electrical power at another voltage level through the action of magnetic field but with a proportional increase or decrease in the current ratings without change in frequency.

* TURNS AND VOLTAGE RATIOS

• • Where:
  •   VP  -  Primary voltage
  •   VS  -  Secondary voltage
  •   NP  -  Primary turns
  •   NS  -  Secondary turns
  • 
    
  • Then:
  • Vs = Ns  or  Secondary Voltage = secondary turns
  • Vp    Np       Primary Voltage        primary turns
  Notice the equation shows that the ratio of secondary voltage to primary voltage is equal to the ratio of secondary turns to primary turns.

  The equation can be written as:

  
  

                                      VpNs =  VsNp
  
  

  
  

  
  

  Example:

  
  

  
  

  A transformer has 50 turns in the primary coil and 200 turns in the secondary coil. The amplitude of the primary AC is 9V. What is the amplitude of the secondary AC?

  
  

         
• Vs = VpNs    = 9V(200)    = 36V
             Np              50

         Answer: Vs = 36V

The National Grid

• the voltage is altered in the national grid with the use of step-up and step-down transformers.
• the voltage is stepped-up when it leaves the power station to reduce the current.
• the voltage is then stepped down before it reaches our homes.
• transmission lines are sets of wires called conductors that carry electric power from generating plants to the substations that deliver power to customers.

Uses of transformers

           - a transformer is used in almost all ac operations.

1. in voltage regulator for TV, refrigerator, computer, air-conditioners etc.
2. a step down transformer is used for welding purposes.
3. a step down transformer is used for obtaining large current.
4. a step up transformer is used for production of X-rays and NEON-advertisement.
5. transformers are used in voltage regulators and stabilized power supplies.
6. transformers are used in the transmission of AC voltage over long distances.
7. small transformers are used in radio sets, telephones, loud speakers and electric bells etc.

Rectifier Circuits

The rectifier circuits is the heart of power supply. Its function is to convert the incoming AC voltage to DC voltage. There are three basic types of rectifier circuits used with power supply.

1. Half wave Rectifier

• it uses one diode only.
• a rectifier that changes only one half of a cycle of alternating current into a pulsating, direct current.
• advantage of a half wave rectifier is only that its cheap, simple and easy to construct.
•  It is cheap because of the low number of components involved.
•  Simple because of the straight forwardness in circuit design.
• current in the circuit flows in one direction only.
• not very efficient at producing DC voltage.

2. Full Wave Rectifier

• one kind of full-wave rectifier, called the center-tap design, uses a transformer with a center-tapped secondary winding and two diodes.
• operates on both alternations of the sine wave.
• rectifies both the positive and negative cycles in the waveform.

3. Full Wave Bridge Rectifier

• a bridge rectifier is an arrangement of four or more diodes in a bridge circuit configuration which provides the same output polarity for either input polarity.
• easier to filter
• center-tapped transformer not required
• bridge rectifiers are widely used in power supplies that provide necessary DC voltage for the electronic components or devices. 
• it has the advantage that it converts both the half cycles of AC input into DC output.