selection of transformer

hiiiiiiiii
i need transformer for 12v dc 2a
can i use this formula for transformer secondary voltage

Vsec(rms) = (Regulated DC + Regulator Drop + Ripple voltage + Rectifier Drop)/√2

where, regulated dc=12vdc
regulator drop= 3v
ripple voltage=1v
rectifier drop=1.4v

Vsec(rms)=12.30ac

if any mistakes please correct me

Vsec should be Vpeak not rms ( because after rectification, filter capacitor holds peak voltage) and according to Vmin input (Minimum input mains voltage, transformer should work and output to be regulated).

Vsec(rms) >= 12.30ac

Click to expand...

will also do as √2 is already included in formula.

It's not quite as simple as that in practice because the formula does not take into account the effects of transformer losses, the filter capacitance and load current. The effects of these factors are interrelated and cannot be added to the previously calculated value by simple arithmetic.

For example, the transformer losses act approximately like a series resistance. This slows down filter charging on the rectified peak. It lowers ripple but also lowers the average output voltage. It also makes the voltage dependent on load current. With low-quality transformers that have a high equivalent series resistance, the average DC voltage at the input to the regulator may change quite a bit from low load to full load. In some practical cases using a cheap 12.3V transformer and a bridge rectifier, the filtered DC may vary from about 16V at very light load to less than 13V at full load. Ripple will cause that to swing below 13V. Mains fluctuations will make the situation worse.

The actual calculations are quite complex and I usually don't bother with it. I generally use graphs from books which I scanned and saved for convenience. Long practical design experience has made it unnecessary for me to refer to the graphs every time. Unfortunately, it will be illegal to upload the graphs, but there must be free online resources of a similar nature.

I agree with PJDD. Some safe working thumb rules also help most of the times unless it is quantitative assessment issue.