Increasing mA in a wall wart

Discussion in 'General Electronics Chat' started by Gdrumm, Jul 20, 2014.

  1. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    I'm trying to help a friend who has a radio / record player that's missing the adapter. It calls for 9v 800mA.

    I have a bunch of adapters, but have yet to find one at 800 mA.
    I tried one 9v 200mA, and it fits the plug, turns on the panel lights, etc. but doesn't have enough power to run the record player, radio, ec.

    I've repaired wall warts before, so I know a little bit, but is is possible to add / change a couple of components to up the amps?

    I have others, at higher voltages, with more amps.
    If I find one of those with 800mA, could I lower the voltage to 9v?

    Thanks,
    Gary
     
  2. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    You can not up the Amps of a wall wart and keep the same voltage. With a buck converter, you can get twice the output current with half the voltage, three times the current with one third the voltage. four times the current with one quarter the voltage. From this we see that to get 800ma at 9v from 200ma we'd need a wall wart with output of at least 36 volts at 200ma, and this is very approximate.

    If the wall wart is already 800ma however, then all we need is a voltage regulator. If the voltage is not too much higher than a linear will work, or else a switcher is required to keep heat down to a minimum.
    A 12v 800ma wall wart would regulate down to 9v 800ma nicely, maybe with an LM317 with small heat sink.
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    Have you Googled 9V 800ma power supplies? There are many hits. Here for example. You can always use one that delivers more than 800mA of course.
     
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  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the current rating for the wall wart is mostly due to the size of the transformer, also the most expencive thing in there.
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    You can pick up a new 9V 1A adapter for $5 shipped.. or so

    You always replace an adapter with one rated the same or higher than the original..
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Yes, but as noted the only relevant component that needs to be beefed up is the transformer. Unless you already have the right transformer on hand, buying one will be more expensive than just getting a new wall wart.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Wall warts now are rarely linear supplies, most are SMPS switching supplies, many of the original linear types did not have any regulation and relied on the rated current to bring the voltage down to the rated level, switching supplies now however are inherently regulated.
    Max.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Hmmm, to me "wall wart" is reserved for only the old fashioned type. I guess it now applies to all.

    Either way, the point is the same, that it will be easier to buy than to make.
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    IMO .. "wall wart" is just a gross black blob sticking out of the outlet on your wall.
    So both linear and switching fall into that.
    The term does not segregate :)
     
  10. wayneh

    Expert

    Sep 9, 2010
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    As long as we're at it, then any PS with a cord to the wall is NOT a wall-wart? The only defining characteristic is the black blob plugged directly into the wall? I can live with that.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Me too. :)
     
  12. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Yeah.. I'm gonna starting a new trend.. The laptop type AC/DC power supplies where the black box is incorporated along the cord somewhere but not attached to the outlet prongs are now called "Floor Warts"..
     
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  13. RichardO

    Well-Known Member

    May 4, 2013
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    A fellow I worked with called them pythons because of the lump in the middle of a snake after a large meal. That is the name I always use. It seems to have caught on with my friends. :D
     
  14. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    I did find one, it's a 9v, 850 mA and it does work (that solved the power problem).
    However, the machine is still not working.

    So I opened it up, and one of the boards has what appears to be 2 bulging caps.

    I will check that out.

    Thanks to Crutschow for the info on using a higher rated mA "wall wart".

    Gary
     
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Agree. The term I've seen most for something like a laptop battery charger is "desk-top supply". A wall wart needs a wall.

    To other responses, while most ww's today are switching, anyone with "a bunch" of ww's probably has mostly linear ones from the past. Most of those were not regulated, so a 9V output would be 9V *only* at the 800 mA load, and could be 12V or more at lower loads. This matters if you use a larger one, such as 9V at 2A. At 800 mA it will be more than 9V, and that might be a problem.

    ak
     
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  16. BobTPH

    Active Member

    Jun 5, 2013
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    I've always called the laptop supplies "bricks", because the early ones were about that size and weight. On the other hand, I bricked my brick could be confusing...

    Bob
     
  17. wayneh

    Expert

    Sep 9, 2010
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    Yup, they're bricks to me. I guess we're in the age when we should say "old style" wall wart when referring to the traditional transformer in the wall socket. Today's default wall wart is a SMPS.
     
  18. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Didn't think of this earlier, but I just right now used the term "brick" to refer to a laptop power supply.
     
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