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why are cellphone chargers so small?

i was thinking what kind of transformer are in those cellphone chargers. their output are so high considering their very small size.

is it possible to use audio transformers to step down mains AC? if yes, is it safe?
my professor gave us a very simple project - to make a power supply. of course i will use a transformer. when i pass my project she told me that i used a audio transformer. i don't know that the transformer was an audio transformer because it was labeled 220v 50Hz to 9v 200mA
i searched my area for transformers i found several with identical ratings but they are significantly larger. my conclusion is my transformer was small due to the fact that it is an audio transformer. when i searched for audio transformer i found a lot of very small transformer. now, is it safe to use those very small transformers to step down 220VAC mains?

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gmoon5 years ago
Cell phone chargers and other modern "wall warts" no longer utilize step-down transformers to convert mains voltage. The current technology uses switching regulation, often called SMPS--switch mode power supplies.

SMPS don't necessarily fully isolate the device from the mains, although they generally have sophisticated current-sensing circuitry to prevent any mishaps.

No, in general it isn't wise to use a small audio transformer as a step-down transformer on any mains voltage. Those are mostly for impedance matching. That's not to say that a large audio output transformer might not work, but they were not designed for that application. And large audio output transformers are more expensive than the average step-down wallwart anyways...
codestroy7 (author)  gmoon5 years ago
but i don't see any switching regulator controller capable of handling 220VAC or 311V peak to peak.
codestroy7 (author)  steveastrouk5 years ago
thank you steveastrouk!
They not only make SMPS units for 220VAC, the same inexpensive wallwarts can be used for both 120 and 220--without modification, and without having to explicitly set the unit for the input voltage.

I'm holding one in my hand right now--the light weight alone tell me there's no step-down transformer inside.
There are many -- they just have to be rated for it - looking online, most ac>dc converters are smps these days.
codestroy7 (author)  frollard5 years ago
can you give me an example? a part number maybe?
Sometimes you have to accept your teachers are actually wrong. If it was labelled as you say, it was just a very small mains transformer, they exist - yours is rated at less than 2W, perfectly simple to buy and use.

Steve