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I am trying to design an arduino board with an integrated power transformer that powers the board after off-course stepping down the voltage and then rectifying it to generate a 5V stable output. I have found this website where I can get cheap transformers:- https://www.aliexpress.com/store/1459305

If you open the link, you will find that on the left side the type of the transformer is mentioned like EE, EP, ETD, RM..etc etc. My problem is that I cannot decide which type is suitable for my application. I just have simple theoretical knowledge about transformers. I just want to use the transformer to step down 220V AC (single phase 50Hz) to 9V or 12V AC which I will then rectify and feed to a 7805 ic, that finally powers the atmega328p microcontroller.

I did my own research and have only come to know that EF means Earth Fault type Transformer and have no idea what it means. So please provide me with some information regarding the types of these transformers and please suggest which one is best for my application.Here you can see the types being highlighted by the red box:-

enter image description here

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You need to get rid of switch-mode-power-supply as a filter. You will be looking for a 50/60Hz (or just 50Hz) transformer with E-I lams and made from silicon steel or steel.

The pictured ones appear to have ferrite cores and will only work with a complex switching power supply circuit. Here is what a typical one looks like:

We, in North America, anyway, normally call these 'power transformers' for historical reasons.

enter image description here

As you say, you'll want about a 9-10VAC output and rated for at least 1.6x the current you expect out of the 7805, assuming a full wave rectifier.

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It appears that AliExpress doesn't even sell traditional mains-frequency power transformers anymore. At least none of their categories showed anything but: Usage: High Frequency.

Is there some compelling reason you want to INTEGRATE the mains power supply on the same board as the micrtocontroller? There are many disadvantages to this scheme and I can't think of any significant advantages.

Is there some compelling reason you want to design your OWN power supply? There are so many SMPS wall-wart and board-modular supplies out there already designed, assembled and tested which can be acquired for less than the cost of the parts. It hardly seems practical these days unless you are required to do this for some formal education requirement.

Is there some compelling reason to use a LINEAR, mains-frequency supply when your research on AliExpress shows that the whole world has moved to switch-mode power supplies (SMPS). Old-school mains-frequency designs and components (particularly transformers) are no longer economical for at least a decade or two.

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    \$\begingroup\$ Actually there are plenty there, but ali would be about my last choice for a safety-critical device. Most lack even UL markings (and probably fewer again are legitimate). Prices are pretty high too, they're old tech as you point out, and it's expensive to ship iron by air. Digikey has plenty of appropriate product in stock and probably there are similar stockists in India. \$\endgroup\$ Nov 4 '16 at 23:00

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