A transformer is an electric device that uses mutual induction to change alternating current voltages. It consists of two coils - a primary coil and secondary coil - wound around an iron core. Transformers can either step up or step down voltages. A step up transformer has fewer turns in the primary coil than the secondary coil, while a step down transformer has more turns in the primary coil. The alternating current passing through the primary coil induces an alternating current in the secondary coil through electromagnetic induction. Transformers are used to render current earth-free and alter voltages for different applications.

1. Construction and their working

2. What is transformer?
 A transformer is an electric device which is used for
changing the a.c. voltages and to render the current
earth-free
 A transformer which increase voltages is called as
step up transformer
 A transformer which decrease voltages is called as
step down transformer
 Principle: based on the principle of mutual induction

3. Circuit Symbol for Transformer

4. Construction of transformer
 Consist of two coils (primary
and secondary) of insulated wire
wound onto a laminated soft iron
frame
 Two coils may be wound on top
of one another on central bar or
on opposite side of the frame
 Frame is usually rectangular in
shape
 One coil usually contain more
turns than other

5. Working of Transformer
 An alternating current is passed through primary coil
which sets up magnetic field which cut the secondary coil
 Emf is induced in secondary coil due to change in
magnetic field of primary coil
 For an emf to be induced, current should be a.c.
 No electrical connection b/w two coils both are insulated
from each other, current is induced via EMI
 Soft iron core serves to concentrate the mag. lines of
forces
 Soft iron is laminated to prevent eddy currents

6.  Step up transformer: No. of turns in primary coil is less
than that in secondary coil
 Converts the low voltages into high voltages
 Primary coil is made of thick insulated copper wire with
less no. of turns
 Secondary coil is made of thin insulated copper wire with
larger no. of turns
 Step down transformer: No. of turns in primary coil is
larger than that in secondary coil
 Converts the high voltages into low voltages
 Secondary coil is made of thick insulated copper wire with
less no. of turns
 Primary coil is made of thin insulated copper wire with
larger no. of turns

8. Function of Transformer
 To render the current earth free: The main current
passes through primary coil induces emf in secondary coil,
as there is no elec. connection b/w primary and secondary
coil, which has no connection to earth so render the
current earth free
 If connection b/w earth and sec. coil made, no current
passes to earth as circuit is not complete, so no earth
shock occur
 To alter the voltage of an alternating current: via step
up and step down transformer
 Emf induced in secondary coil is directly proportional to
no. of turns in this coil

9. Types of transformer
 Static transformer (step up, step down and even ratio
transformer)
 Variable transformer
 Autotransformer

10. Variable transformer
 That can put out different amount of voltage from the same
input voltage
 Tapings may be provided so that either primary or secondary
coil can be varied in length
 To use the same apparatus on supply of different voltages by
varying no. of turns
 Higher the voltage of supply the more turns of wire are
included in primary coil
 Care should be taken to see that control is in right position for
supply used
 Tapings may be taken from different points on secondary coil
to supply different voltages to separate circuits in same
apparatus

11.  Used to speed up and slow down electric motor
 Make lights dimmer and brighter

12. Autotransformer
 Consist of a single coil of insulated wire wound over
laminated soft iron core
 Tapped at two points, C and D
 When used as a step up transformer , coil b/w C and D act
as primary and A and B act as secondary and vice versa
 It has certain disadvantages
 It can be used only for a small step up and down in
voltage
 Do not make current earth free in secondary coil

14. Choke coil
 It is included in circuit in order to produce a self induced EMF
 Low frequency choke coil: consist of many turns of insulated
wire wound on soft iron usually on central bar of rectangular
frame
 High frequency choke coil: consist of usually less no. of turns
of insulated wire wound on bobbin of non conducting material
as current varies rapidly
Uses:
 Even out variations in the intensity of current
 To prevent the flow of high frequency current and allow the
passage of lower one

15. Dynamo
 It is a device which is used for the production of an EMF
via electromagnetic induction
Construction
 A coil of wire lie b/w poles of magnet and by some mean
rotation of the coil is provided via motor, steam engine or
water power
 When coil rotates, emf is induced in coil first in one
direction than other
 So current produced is alternating type
 Collecting device is used to convey current from coil to
external circuit

16.  Supply both types of currents to external circuit either
a.c or by converting a.c into d.c
 Used on large scale for production of electricity
including main supply