First of all, I am not an Electronics guy originally. I love to spend time with Electronics. May be this is a basic question. But I am struggling to find the answer on internet.

I am from India and I often see that locally manufactured step-down Transformers are not sold with Volt-Ampere ratings. Instead they specifically mention following parameters:

  1. Input Voltage
  2. Output Voltage
  3. Max Output Current

Well, I know how to calculate VA from those params :) But my question is how do they control/determine what is the max allowed output current? I am assuming that following is the principle

Assume, V1=220VAC V2=12VDC

Since, V1/V2 = I2/I1 = N1/N2

N1/N2 = 18.333

N1=3666.6, N2=200 [Less turns on Primary/Secondary = Higher Current Output]

N1=9166.5, N2=500 [More turns on Primary/Secondary = Lower Current Output]

Is this understanding correct? If yes, then my last question is, how does this theory influences the gauge of the primary and secondary wires?


For the development of a transformer a few steps are required.

  1. Power rating

2.Voltage levels (primary and secondary)

3.Currents on both sides

4.Primary and secondary coils wire diameter/size

5.Iron Core area

6.Numbers of turns (primary and secondary)

First the size of the magnetic core has to be determined. From there the number of turns per volt.Next step is to calculate the windings for the required voltages. Then the wire gauge is to be calculated based upon the alowable current density.

The way you calculate the N1/N2 is ok but you can not reduce the number of turns just like that since that depends on the properties of the magnetic core.

More in detail you can find a complete calculation in the mentioned link. There they explain all the calculations involved in designing a small transformer.


  • \$\begingroup\$ Well, that is it. The link explains it all. Thank you so much Decapod. I will write comments if I get stuck with further doubts! \$\endgroup\$ – sribasu Oct 15 '16 at 5:08

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