I want to use a transformer to convert AC voltage. Input is 220V AC at 50 Hz and output is, for example, 5V 30mA AC.

Many people told me, this transformer will be huge because the mains frequency is low. But, I still have no idea, how big it will be. (BTW, 50Hz main Frequency is so low, I am not sure if the transformer could magnetize..., maybe need big Fe core...)

For my converting requirement, and take Piezo transformer or any other high-advanced transformer design (like pcb stack twisting transformer) for example (smaller, better). How large it will be? Suprise me.

(I have no idea about the transformer design, I mean, how thick the conventional coil wire, etc. I just know the basic equations Vin x Iin = Vout x I out... some text book stuff, no experimental experience.)

How do I determine which core size is appropriate and how many windings of what type of wire I am supposed to use for primary and secondary?

• Do you want to design this transformer yourself, or buy one? If you only need 5V @ 30mA it can be quite small, even at mains frequencies. Jan 11, 2013 at 7:07
• If you look deep enought you probably have one at home: some old battery charger... Jan 11, 2013 at 7:58
• The questions title could use some improvement. Its very general.
– Rev
Oct 16, 2014 at 7:17

For the power level you need, the transformer can be small, here is an example part (yours would be the smallest, datasheet here with dimensions):

You'll note it says 2 x 5V for the secondary, but you only have to use one of the secondary windings.

For a switching supply, here is a small module that takes 85V-265V and outputs 5VDC, the dimensions are 26.7mm by 31.7mm:

Here is an example schematic, based on a teardown of the iPhone charger:

The above came from this site.

• The current for your shown device is 600mA, and it is big, too big. but I just need the current is around 30mA is enough. BTW, I hope I can design by myself, better to use the pcb transformer to design it. But I am not sure how big it will be.
– alan
Jan 11, 2013 at 7:37
• Can you define "too big" ;-) What size do you want it to be? Jan 11, 2013 at 7:39
• in the phone charge, the transformer style, compared to that one, how big it will be for my requirement
– alan
Jan 11, 2013 at 7:40
• I mean, like the iphnone charger transformer
– alan
Jan 11, 2013 at 7:42
• Yes, it is Switch mode.... the frequency is high, and current is big, but my frequency is low, and the current is small. you mean I cant design my transformer like that...
– alan
Jan 11, 2013 at 7:43

The power, not just the frequency, influence how big the transformer would be: In your case you need a 0.15VA ( yu probably would not find SO small ) that is small even at 50 Hz. You probably save money and time by buying one instead of wiring yourself: it is anyway a critical component in term of safety so if you are not experienced in the field, buying one is also safer for you and your circuit.

• can you give me some details, if I cant find it, can I design it on a pcb board? Or it is too big to build it.
– alan
Jan 11, 2013 at 8:08
• google for "Transformer design", find some source or some books, I can't point to anything since I can't say one source is better than another due to inexperience. In any case you probably cheaper buy one. Jan 11, 2013 at 8:12