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Newbie power questions

Discussion in 'General Electronics Discussion' started by enigma65, Aug 22, 2013.

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  1. enigma65

    enigma65

    3
    0
    Aug 22, 2013
    Hi all, hope you can help me on some basic questions...

    I have AC to DC power supply as follows:

    Input from wallwart: AC to 18v DC @ 1000ma

    Multiple Outputs:
    9v DC @ 100ma
    9v DC @ 500ma
    12v DC @ 100ma
    18v DC @ 100ma

    I need to take a feed from one of those outputs and get 5.1V DC @ 2A. Is this even possible?

    If not what about tapping directly into the 18V @ 1000ma input instead of one of the outputs.

    Any info would be appreciated but keep in mind my knowledge of electronics is minimal.
     
  2. Kam

    Kam

    19
    0
    Aug 21, 2013
    if the maximum current the transformer can supply is 1000mA
    then you cant draw 2000mA from it.

    Is it unregulated ? if it is then when you hook it up you will get a
    voltage drop anyway so 9v might drop down to around 5v
     
  3. enigma65

    enigma65

    3
    0
    Aug 22, 2013
    As far as I can tell from the specs the voltage at the outputs is regulated. I just thought that since I need only 5v of the 18v, somehow those 13 extra volts could be harnessed into more amperage.
     
  4. Kam

    Kam

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    Aug 21, 2013
    It doesnt work that way unfortunately

    you could easily drop the voltage to 5v using something like
    an LM7805 which would give you 5v out from 9-18v input but that again is rated
    at 1A

    maybe make something up using a battery/s ?
     
  5. enigma65

    enigma65

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    Aug 22, 2013
    Thanks Kam... I guess that's why nobody else invented this "great idea" I had...

    You know those boards that guitar players use that are loaded with different effects pedals? Well a lot of us are now using a tablet (iPad or similar) mounted on our mic stand to display set lists, song lyrics, etc... The pedals on the board are generally powered by a DC power supply brick that typically takes a 18V 1000ma input and feeds it to a bank of outputs as mentioned above to power different pedals with pigtails (short power cords). Plugging in an iPad or other tablet requires access to an additional AC outlet. My bright idea was a box (just like an effects pedal) with USB ports on top that could feed off the power supply "brick" and somehow transform the low amperage of those outputs to 2 amps due to only needing 5V instead of say 12 or 18.

    As they say... Oh well!
     
  6. Kam

    Kam

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    Aug 21, 2013
    nothing stopping you making one from scratch,
    sounds like a great idea though.

    hope you get something sorted :)
     
  7. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    You can get buck convertors which can take a high voltage and convert it to a lower voltage with higher current. The efficiency is about 80%. I have never used or made one.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    11,413
    2,619
    Nov 17, 2011
    You're not going to be happy with this adapter and your output requirements.

    9v DC @ 500ma makes for 4.5W. This is the maximum power you can draw from a single output. Even if you connect both 9V outputs in parallel (I do not recommend that because it can give you trouble due to load sharing problems) there would be 9V*600mA=5.4W.

    You require 5.1V*2A=10.2W. No regulator on the world can give you this power from a 4.5W source (not even at 100% efficiency). You will have to get a more powerful adapter.

    You could use the 18V/1000mA input source (18W) and a buck-converter (step-down converter) to get 5.1V/2A. A commercial DC/DC converter is 30€ and up. You may try to build one yourself.
    However, buying a suitable wall wart is probably the least expensive thing to do.
     
  9. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    The problem with a buck conveter is that it is supplying the full current during the ON period. The average current will be less than the output current, but the peak current will not. If it switches fast and has a large enough capacitor on the input, it could work though.

    Bob
     
    Last edited: Aug 23, 2013
  10. Yaser43082

    Yaser43082

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    Jul 20, 2013
    So from what I understand, you have 18 volts DC rated @ 1A and you want to go to 5.1 Volts rated at @ 2A. This is really a two part question.

    1) To go from 18 volts DC to 5.1 DC, you will need to design a Buck Convertor
    At this stage of the game, you circuit sees 5.1 Volts @ 1A

    2) Next you will need a Current Amplification system. You can do this with very basic transistors. Pay attention to the Beta value in the data sheet. Beta is dependent both on Temperature and Collector Current. This will get you to the 2A required
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

    11,413
    2,619
    Nov 17, 2011
    Excuse me, but that is nonsense.
    While a transistor can amplify current (i.e. collector current vs. base current), no transistor circuit can deliver more current than the source itself.

    If you have [email protected], then this is equal to 18W. You can use a step-down converter to create [email protected] from that. The original post is ambiguous on this point: the table lists [email protected], the text says [email protected] What is correct? With [email protected] it is impossible to drive this scheme.
     
    Last edited: Aug 27, 2013
  12. Yaser43082

    Yaser43082

    11
    0
    Jul 20, 2013
    Harald, thanks for the correction. Perhaps I did not express the solution well enough
     
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