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Power adapter modifications. + & - ??

Discussion in 'General Electronics Discussion' started by unlimited, Nov 22, 2014.

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  1. unlimited

    unlimited

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    Nov 22, 2014
    I'm an amateur punter on here trying to find some assistance with a challenge I've come up against with little something I'm putting together.

    The idea is to by pass the need for 9V batteries and or inverters and 240 supply alternatives to this audio mixer.

    http://www.behringer.com/assets/1002B_P0A04_M_EN.pdf

    I have a 12V battery source and have been attempting to connect that to the 9 Volt battery terminals in the mixer via this power adapter.

    http://www.electusdistribution.com....ywords=mp3478&SPECIAL=&form=KEYWORD&SUBCATID=

    The mixer uses 18v from two separate 9v batteries with a third battery providing 9v power to a phantom powered microphone line.

    I've modified the output lugs on the power adapters to plug directly into the 9v battery terminals in the mixer switched them on and nothing happened. I have since diagnosed that the two batteries are in a 9v+ and 9v- arrangement that normal 9v batteries can handle however and the power adapters I have will only work in the + and hence the system is not working.

    So I need to asses if anyone knows of a relatively cheap 12v-9v power adapter that works in the + and the - or advice on how I might make a custom circuit of my own to address this challenge.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,482
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    Jan 21, 2010
    If I understand your question correctly, you want a "negative voltage regulator". Something like a 7909. However, if you are using those cheap switchmode DC-DC converters (which I WOULD NOT advise for the phantom power to the mic, or indeed for your mixer) then there are configurations which invert the output voltage. This would allow the one 12V battery to produce both +9V and -9V.

    Hmmm, that "regulated car battery adapter" is almost certainly a switchmode device. For audio applications -- which are also low current -- a 7809 +ve regulator and a 7909 -ve regulator would be the best (quietest) bets. This would require two batteries.
     
  3. unlimited

    unlimited

    2
    0
    Nov 22, 2014
    Thanks for your reply.

    They look to be suitable how would I install them? inline with the existing regulators? Or would I need to build my own regulators with these installed?

    I'm not uber worried about hum as the mixer itself is rather low end.

    I can get current from two batteries that could be isolate by a diode?
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,482
    2,830
    Jan 21, 2010
    Firstly, the 7809 and 7909 three terminal regulators are not complete regulators, but componets that go into a circuit to make a regulator. In this case you only require 2 capacitors for each circuit. It's probably worth googling the datasheets (LM7809 and LM7909 are their full names). These are really easy to build, but you need some matrix board or strip board and a very little bit of experience. negative regulators are not something you're likely to fin as a module of (say) eBay, so making one is probably the practical option.

    The batteries need to be completely isolated from each other because you'll connect the +ve of one to the negative of the other (connecting them in series) so that (from the common junction) you have +12V to the end of one battery and -12V to the other.

    Be aware that lead acid batteries can deliver *enormous* currents and I would advise placing an inline fuse in series with each battery -- yes, they can melt wires and cause serious burns.

    Actually, I slightly tell a lie. Here is a fully built up +/-9V regulator. The AC input could be connected to the +/-12V supply created by two 12V batteries (don't worry that it's not AC). This would not work at high currents, but should be OK for low currents.
     
    unlimited likes this.
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