Transformer working principle: step down and step up  



1) there are two types of transformer


a) Step-down transformer:

this transformer is used to decrease the incoming voltage. So output voltage is lower than the input voltage.



b) Step-up transformer:

this transformer is used to increase the output voltage. So output voltage is higher than the input voltage here.

 

 

 1) A transformer works on mutual induction principle.



2) In a transformer, there are two coils wounded in a soft iron-core, as shown in the diagram.


3) First coil called the input coil, suppose the input coil has Ni no of turns.


4) And the second coil is called the output coil, suppose the output coil, has N0 no of turns.


5) Before going details we assume that there is no heat lose and the transformer is ideal so that there is no flux leakage.

6) When an alternating current  ii      (ii  - input) 

is  applied across the input, this will generate an alternating magnetic flux 


and this   is also related to the output coil.

 



7) Then alternating magnetic flux through the output coil will generate an emf on the output coil.

From Faraday’s Law of induction. We get that




Whereis the emf generated on the output coil




8) Now an emf also generated on the input coil. This emf is





Now  on     

  








 



From the above equations we see that

 

9) We see that is if no of turns on the output coil is increases


N output   >   N input   


Then will also increase. This is what happen in case of a step-up transformer.



10) if no of turns on the output is less than the input


Noutput   <   Ninput  


Then will also decreases. This is what happen in case of a step-down transformer.



 

For current:


1) We know that power


P = VI



2) Now as the transformer is ideal, So power lose is zero,

Pinput = Poutput

 

 



 



 

For output current



 

 

From the above equations we see that

 


3) If the no of turns in the output increases

Noutput  >  Ninput

Then output current decreases [this is opposite to the voltage, as discussed previously in step-up transformer]




4) And if no of input turn increases

Noutput   <   Ninput 

Then output current will increase. [this is opposite to the voltage, case as discussed previously in step-down transformer]

 

 


So finally we can conclude that

 

1) In step-up transformer, output voltage increases but output current decreases.

 

2) In step-down transformer, output voltage decreases but output current increases.