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12V adapter + resistor = 9V adapter?

Discussion in 'General Electronics Discussion' started by Pharaday, May 23, 2016.

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  1. Pharaday

    Pharaday

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    Jan 18, 2016
    This one seems kinda silly, but a buddy of mine suggested it and it seemed so crazy it might just blow my electronics... I mean work. I want to add some resistors to a 12V adapter to make a 9V adapter. I know that the amps is what I need to look at. I've seen one working, tested volts etc. but I figure he musta got lucky with the amps. What must I take into account to do this? I'm trying to power a flange pedal that wants 9V but it doesn't say how many amps it wants. It can also be powered by a 9V battery, if that halps. If it doesn't halp, well there it is anyways!
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    Although you can drop the voltage using a resistor, it also drops the available current considerably
    and the resistor will get really hot. There will be absolutely no voltage regulation and the output voltage
    will depend on the load resistance.
    a resistor and a 9V Zener diode would be a bit better but still no stable regulation
    a 7809 regulator is the best way if your expected current drain will be 1A or less


    Dave
     
    hevans1944 likes this.
  3. Alec_t

    Alec_t

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    I can't believe a flange pedal designed to run from a 9V battery (presumably a PP3 size) would draw more than a few mA. Therefore a zener-based regulator would probably suffice. A 7809 looks like overkill. Do you have a schematic for the pedal?
     
  4. BobK

    BobK

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    HellasTechn likes this.
  5. Alec_t

    Alec_t

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    Be aware that the LM7809 has a quiescent current as high as 8mA (the 78L09 is lightly better at 6mA), so your pedal would need an on/off switch to avoid running the battery flat quickly.
    It's possible that your pedal would work ok from 12V directly, but without a schematic we can't be sure.
     
    Last edited: May 25, 2016
  6. BobK

    BobK

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    Alec_t,

    He is running it off a 12V adapter, not a battery.

    Bob
     
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  7. Alec_t

    Alec_t

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    So he is. :oops:
     
  8. Pharaday

    Pharaday

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    Jan 18, 2016
    Actually, I have only ever run it off a 9V battery OR, recently, off a 5V adapter. It works on a 5V adapter, but not as loud and speed doesn't go as high. I agree that it would probably work off a 12V adapter, but I'm afraid to try. I've blown things just trying willy nilly like that. It's a FAB flange, S/N: 7010909522, if that helps. I can post a pic of the circuit boards, would that halp?
     
  9. Alec_t

    Alec_t

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    Jul 7, 2015
    Why not buy a 9V adapter? A couple of diodes could then be used to steer power from that or the 9V battery as required, or a switched jack would do the job.
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    I answered this in post #2 :rolleyes::rolleyes:

    thread closed
     
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