Transformers (not the robots)

Discussion in 'General Electronics Chat' started by jaygatsby, Nov 23, 2011.

  1. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
    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.
  2. crutschow


    Mar 14, 2008
    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.
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    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.
  4. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
    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
  5. DerStrom8

    Well-Known Member

    Feb 20, 2011
    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.
    jaygatsby likes this.