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12 volt DC to 9 Volt AC

Discussion in 'Electronic Design' started by Eric, Apr 15, 2004.

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  1. Eric

    Eric Guest

    OK, did a search, didnt come up with anything. I'm wondering how
    hard/easy it will be to make something that can convert 12 volt DC to
    9 Volt AC. It needs to be fairly regulated and clean, as its for a
    piece of audio equipment, but not super sensitive like a computer.

    What im doing is trying to use an Alesis Nanoverb in my car. It has a
    'wall wart' power supply of 120 volt in and 9 volt AC out. Not sure
    what frequency AC out. Im assuming 60 hz. I dont really think using
    a 12 volt to 120 volt setup up inverter will work that well. Maybe
    thres another component that has 12 volt DC input... hmm. Im pretty
    good at basic electronic projects. Not sure about this one, so thats
    why I'm asking here. Any thoughts?
  2. Ian Stirling

    Ian Stirling Guest

    It will almost certainly do no damage to connect it directly to the
    car battery.
    9V AC at peak is some 13V or so.
    On a +10% AC line, 14.5V.

    It will almost certainly work just fine.
    What current is it supposed to draw, and what is it?
    Does it have any sort of motor?
  3. You probably don't need to put in 9 V AC. The unit obviously has a
    rectifier on board, and you can feed that with DC of either polarity.
    The only way it won't work is if the unit has two rectifiers, one
    producing positive DC and the other producing negative DC.

    Since the peak voltage of 9 V r.m.s. AC is 12.7 V, you appear to be able
    to just feed your 12 V vehicle supply to the unit, even though that is
    about 13.8 V when the battery is being charged. BUT that 12 V supply has
    lots of nasty transients on it and your unit won't be protected against
    those. I suggest you put about 10 ohms in series with the feed from the
    vehicle and a 12 V zener diode across the input to your unit.
  4. j.b. miller

    j.b. miller Guest

    Ian makes the ASSumption that the device takes the AC and converts it to
    ONLY one value of positive DC.
    WRONG and very,very dangerous ASSumption.
    Without schematics, no one KNOWS what's going on. Early modems converted the
    AC into 2 or 3 DC levels(+12,+5,-12) for true RS-232 use.
    I have a piece of proprietary equipment that does the same(+60,+5,-60).
    So either open up the device or get schematics, just plugging in and turning
    it on could be a very costly mistake.

  5. The wall wart also provides galvanic isolation. Really, just go buy a
    cheap inverter and be done with it.

    Best regards,
    Spehro Pefhany
  6. Also, even if the system is wired as Ian assumes, if it is using 'full
    wave', rather than half wave rectification, the average current through the
    pair of diodes 'in use' when operating off DC, is higher than when running
    on AC, which can again cause problems. Basic rule of thumb, is if you don't
    know what is going on inside a unit, 'assume the worst'...

    Best Wishes
  7. I read in that j.b. miller

    This is a piece of audio gear. The most complication is that it has +
    and - rails.
  8. I read in that Spehro Pefhany <[email protected]
    Why would you need galvanic isolation for a bit of audio kit in a car?
  9. I read in that Roger Hamlett <[email protected]
    This is a piece of audio processing gear. If it takes 100 mA that will
    be tops. Risk to diodes - zilch.

  10. Signal ground is connected to chassis ground at the audio input and
    output jacks, so there's the possibility of ground loops with the rest
    of the system.

    I also think the likelyhood is high that they are generating +/-
    rails, otherwise they would probably have designed in a DC-output

    Best regards,
    Spehro Pefhany
  11. Ian Stirling

    Ian Stirling Guest

    Plus, the RMS current through the diodes will probably be smaller.
    Instead of conducting 1A for 10% of the time, any bridge diodes will
    conduct 200ma continuously.
    It's probable that they'd run cooler.
  12. Eric

    Eric Guest

    To answer your questions- it is a signal processor for audio. It adds
    reverb, delay, chorus etc. It draws probably less then 1 amp. It is
    not a power amplifier and no it does not have any motor.

    So you're saying I can plug 12 volt DC directly into the terminal that
    calls for 9 volts AC?
  13. Tim Mitchell

    Tim Mitchell Guest

    No, as others have said, it uses the AC to generate + and - rails. DC
    won't work. I used to own one and occasionally mixed up its AC power
    supply unit with DC ones.
  14. Ian Stirling

    Ian Stirling Guest

    I was saying it probably won't damage it, which is not quite the same thing.
    However, as others have raised, you can't safely plug it into other car
    audio stuff, as the grounding would be wrong.

    However, I see that someone actually has owned one, and tried this without
    it working, so no.
    An inverter looks like your easiest option.
  15. Tam/WB2TT

    Tam/WB2TT Guest

    Go open up your Nanoverb, and see what it does with the 9VAC. If the 9V goes
    into a diode rectifier, and nowhere else, you can power it from 9 - 10 VDC
    as well, provided the case is negative (Like from a 7810).

  16. I read in that Spehro Pefhany <[email protected]
    You may well be right. But using an AC wall-wart makes some safety
    compliance issues simpler.
  17. It's possible. It MIGHT not work (if the unit generates + and - rails
    internally), in which case disconnect at once. But you are *unlikely* to
    cause any damage. The best thing to do would be to try to get
    information form the manufacturer on the subject.
  18. Interesting. Could you expand on that a bit?

    Best regards,
    Spehro Pefhany
  19. Tam/WB2TT

    Tam/WB2TT Guest

    As others have mentioned, make sure no (-) voltage is generated. They may
    have one side of the AC grounded, and +/- half wave rectifiers, or some
    other arrangement. They may even use a voltage doubler. You would have to
    run the thing off the AC and see if there are any negative voltages with
    respect to ground, or any voltages around 20 or so.

  20. I read in that Spehro Pefhany <[email protected]
    I'm thinking back to a project several years ago, involving CSA
    certification. AC-output wall-warts were treated much less stringently
    than DC output ones. I suppose the justification was that AC output
    devices were just a 'transformer in a box', whereas DC output devices
    would have additional electronics of unknown extent and raise unknown
    safety issues.

    I don't know whether the CSA requirements are still similar, nor whether
    current UL requirements are similar or not.
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