First post, though I've enjoyed the site as an anonymous surfer in the past.

I am looking for a transformer to turn US wall voltage into something smaller, like 24, 12, or 9V. Searching Jameco I see that these transformers do exist, but they're huge! And expensive. My question is: how do cell phone chargers and similar devices do it in such a small space? I'd like a smaller transformer but they don't seem to be available. Do they limit the AC input some other way and put that into a smaller transformer?

Thanks, and it's good to be on the forum.
J

 

The newer small wall-worts use electronic switching converters which operate at a high frequency so they require much smaller transformers. For a given power output, the size of the required transformer magnetics is inversely related to the frequency.

 

It all depends on how much power is required. Perfectly ordinary transformers capable of dealing with a few watts can be made small enough to fit into a biggish wall-wart. Only the higher power versions, from tens to hundreds of watts (VA, really) upwards can by any stretch of the imagination be called huge.

Nowadays however, a lot of power supplies use switch-mode techniques which effectively turn the low-frequency incoming AC (usually 50Hz or 60Hz) into much higher frequencies, perhaps tens of kHz or more. These higher frequencies allow very much smaller transformers to be used.

 

I see - thanks. The ones I was looking at have as little as 250mA output, and are aprox 50mm cubed. I tore apart a cell phone charger and the transformer inside of it was half that size. Perhaps the alternative method of transforming power you've described is what's going on there.

Thanks again

 

I see - thanks. The ones I was looking at have as little as 250mA output, and are aprox 50mm cubed. I tore apart a cell phone charger and the transformer inside of it was half that size. Perhaps the alternative method of transforming power you've described is what's going on there.

Thanks again

The one you tore apart was probably a switch-mode supply, so it is solid-state. I imagine it had a few capacitors and things in with the small transformer? However, these do not very often supply a large amount of current when put in a wall-wart style adapter. They're common for cell phones, but it would take a bit for one to be able to supply, say, 5 amps. In many cases, they would have to be much larger than your average cell phone charger.