About step up transformer

Thread Starter


Joined Nov 28, 2014
Hi, I am working on inverter circuit....i am going use a circuit from google...But my doubt is about the transformer i am going to use...I have 220 v to 12 transformer....can i use it as step-up by connecting in reverse ..?and one of my friend said that,there may be problems ,when any load is connected although it gives 230v output......What is the reason for this...?I i have 100 watts load how to choose the transformer?


Joined Aug 30, 2007
If you are going to use the 220/12 volt transformer to step up you must put in 12 volts AC . And at 20:1 ratio you'll need several amps to drive a 100w load.
Your "inverter" may not supply a sine wave. Your measurement system may not employ TRMS (True RMS).

Simple inverters might use a center tapped transformer. But there are "modified sine" and "sine wave inverters". The simplest is a square wave inverter.

Many meters are designed to read RMS only for a sine wave in, They a "Average responding" RMS reading, so the multiply the average by a "fudge factor". e.g. they just precision rectify and multiply by a constant.

There used to be meters that operated on how the waveform heated.

RMS = Root Mean Squared or SQRT (Mean()^2)

For the definition of MEAN, see calculus.


Joined Aug 30, 2007
I can't believe I missed the word "inverter" in the original post !!:eek: One of the usual problems I've seen is the transformer selected doesn't have large enough (wire diameter) windings on the secondary (12v side). My students have built a few of these, and as KISS stated, used a center tap transformer. The higher current needed on the now primary (12v side) needs a larger diameter wire to work properly.

Thread Starter


Joined Nov 28, 2014
Can one help me??Is it ok if i use 12/220v -1 amp step down transformer as step up...if so how much current will it produce at the output??


Joined Apr 24, 2011
Usually you can do this, one cannot be sure specifying an unknown part for an unknown circuit.

If you know the output power for step down you can use that same rating for step up. The transformer really has no clue which way the energy is flowing.

Do respect the voltage ratings. For exame, don't drive the 12v side with 24 volts.


Joined Mar 14, 2008
Can one help me??Is it ok if i use 12/220v -1 amp step down transformer as step up...if so how much current will it produce at the output??
Depends upon the type of inverter waveform driving the 12V side as to whether it will work to give you the proper voltage out.
Is the 12V side center-tapped?
A 12V, 1A step-down transformer can deliver 12W maximum at the 220V output or 55mA (power in equals power out) not including any inefficiencies.
P1=P2 Power primary = Power secondary

V1 * I1 = V2 * I2 = V = voltage, I=current)

You also have V1/V2 = n or the turns ratio

So, the transformer, transforms voltage within it's power limitations. You can't exceed either winding limitation.
The specifications would have to spec both at 100 W in your case. There will be some losses.


Joined Nov 13, 2015
So, you need 9 Amp transformer so that you will get 12V x 9 Amp = 100 watts power output and output voltage would be 220V, 450mA.


Joined Aug 23, 2012
You could buy one or if you want to DIY then you just google : 12v to 220v converter, then you could get a lot of infos.
as 12V to 220V DC to AC Converter Circuit.

The simple current calculation:
Assuming that Eff = 60%, Eff = Efficiency:
Wo = Wi * Eff
= Wi * 60%

Wi = Wo/60%
= 100 W/60%
= 166.7 Watts

Wi = Vi * Ii
Ii = Wi/Vi
= 166.7 W/12 V
= 13.9 A