What Is Step-Down Transformer? How It Works?

By Ashwini Kumar Sinha and Vinay Minj


A transformer is a passive device that converts the voltage level either from high to low or low to high. The transformer that converts the high voltage power to low voltage power is called a step-down transformer and the one that converts low to high voltage is called a step-up transformer.

A transformer works on the mutual induction principle, also known as Faraday’s Law of Electromagnetic Induction, which states that the magnitude of voltage is directly proportional to the rate of change of magnetic flux.

To get a deeper view into the performance of a transformer, let’s understand its basics.

Mutual Induction

It means that a current gets induced in a coil when it comes in proximity of a current-carrying coil having varying magnetic flux. This induced current is directly proportional to the rate of change in current.

Faraday’s Law

According to the Faraday’s Law, any change in the magnetic field near the coil or conductor leads to the production of an electromotive force (EMF) that is induced inside the coil due to a change in magnetic flux.


A transformer consists of the following main parts:


The coils that are winded up on a certain material collectively form a transformer core. These cores are made of very high permeable material that is able to carry the flux. The core of a transformer acts as a path or channel for the easy flow of magnetic flux. These cores are constructed using ferromagnetic materials that have high permeability such as iron. In transformers, we use thin metallic iron sheets instead of a single solid core because the single solid core causes a greater generation of eddy currents and this decreases the efficiency of a transformer.


The transformers are winded with wires called coils. Here, we use wires with less resistance and good conductivity, which is required to get good efficiency from the transformer. Generally, copper is used in transformer winding as it has good electrical conductivity and very low resistance in comparison to others. It is also not expensive like gold, silver and platinum.


A transformer works on the principle of mutual induction. So, when there is a change in the current of one coil, an electric current is also induced in the other coil in its proximity.

Every transformer consists of two coils or winding: primary and secondary. The primary winding is connected to the AC power source and the secondary to the load. When AC current is provided to the primary winding of the coil, a magnetic flux gets generated. Through the transformer core, the magnetic field completes its path. As the secondary winding comes in contact with this magnetic flux, an EMF is induced on it. The strength of the generated EMF depends on the number of turns in winding of the secondary coil.


where, N1 = Number of turns in the 1st coil

N2 = Number of turns in the 2nd coil

The relation between voltage and number of turns in a coil is:

Vp/ Vs = Np/Ns

where, Vp = Voltage in primary coil

Vs = Voltage in secondary coil

Np = Number of turns in primary coil

Ns = Number of turns in secondary coil

Step Down Transformer
Step Down Transformer

What is a Step-Down Transformer?

The transformer that has a larger number of turns in the primary winding and a smaller number for secondary winding is called a step down transformer. So as we can see from the previous equation for the relation between the number of turns in winding and voltage, if the number of turns in primary is greater than the number of turns in secondary then the EMF generated in the secondary is less than the primary input. Hence, we get a lower voltage in the secondary coil of a step down transformer. As the name indicates, the step down transformer is used for converting higher voltage power into  lower voltage power.

Now let’s understand the above process with an example. Suppose we have an AC source that can produce 210 V. If we use a transformer having Np (number of turns in the primary coil) = 20000 and Ns (number of turns in secondary) = 100, then Vs (voltage at the secondary coil) is given by

Vs = (Vp * Ns)/Np

Put the above values in this equation to calculate the voltage at the secondary coil. I have left this work for you. Mention the right value in the comments.

Application of Step-Down Transformer

Step down transformers are used in power adaptors and rectifiers to efficiently decrease the voltage. They are also used in electronic SMPS.

Other applications include:

  • Power transmission lines
  • Welding machines
  • Voltage stabilizers and inverters

Advantages of Step-Down Transformers

  • Step-down transformers are very efficient and can give the desired output with an efficiency of up to 99 per cent.
  • We can get the desired output voltage easily without loss of much power.
  • They are less expensive and more reliable.
  • They can be used to provide high currents and low voltages.




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