dc dc converters - isolating voltages and currents

Discussion in 'The Projects Forum' started by BurningSmell, Feb 8, 2015.

  1. BurningSmell

    Thread Starter New Member

    Feb 7, 2015
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    Hi

    I've just embarked on a project where I need to broadcast an mp3 or wav file at intervals at loud volume in a remote setting. My attempt at a solution has been to hack an mp3 player, take the signal to a 12v car amp IC and output to a horn speaker, all powered by a 12v deep cycle leisure battery.


    I opened a cheap mp3 player and found a Li battery at around 4.5v. So I removed the Li battery and wired an adjustable dc dc converter to step down from 12v and ran the mp3 from the resulting output. I wired the 12v amp IC on a parallel circuit from the same leisure battery.


    I still managed to fry the mp3 player with this set up. Can anyone shed light on why this occured? Presumably the frying is a result of current overload. Is this because of the draw on the battery from the 12v part of the circuit? I noticed that the dc dc converter is described as non isolating. Is thee such a thing as an isolating converter, and would this allow tje mp3 to funtion in its own little world protected from large voltages and currents on the other side of the converter?


    I did have a few other problems with the circuit before the mp3 blew- like getting the correct wiring for a sinlge audio input and speaker output on the tda7370 amp but I'll take this one step at a time! My rudimentary electronics seemed fine in theory but converting it to a practical circiut seems to be getting more complicated- I would be very grateful of any help or ideas - maybe theres a completely different way to approach this!
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,061
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    It is usually best to prove out one step at a time. First step would be to power the MP3 player and make sure it is stable.

    Then add an amplifier.
     
  3. Denesius

    Member

    Feb 5, 2014
    89
    14
    It's hard to say why the mp3 player was fried without knowing what the setup looks like. It could be as simple as having the DC-DC converter output reversed, as there are such things as inverting converters. Since it's not isolated, ground is ground and the output is neg 4.5 volts, so instant fry. If the converter is isolated, then the output can be used, as long as you pay attention to polarity. To have the amp work properly, you usually need a common ground between the player and the amp, so a non-insulating converter becomes irrelevant.

    Before using multiple mp3 players to test & confirm your setup, may I suggest investing in a cheap DVM? You can even borrow one, since nothing you're doing is going to damage it. Once you know the output and polarity are correct, there's nothing wrong with your setup.
     
  4. BurningSmell

    Thread Starter New Member

    Feb 7, 2015
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    Thanks for taking the time to reply. I've attached a general layout of the circuit to give you some idea of what I'm trying to achieve. The DC DC converter is well marked as + in -in, +out -out, so i'm sure its connected well. I adjusted the built in pot until the multimeter read 4.5v between the output pins. I was confused as to how to ground everything, but you mentioned that the mp3 player and amp will share a common ground - does this mean the -in, - out of the converter are also common? And then theres the audio output. I've stripped back an audio jack lead connected to the mp3 player to reveal red, white and yellow wires. I've connected red as the signal to the amp in, white as a ground to -ve battery and left the yellow one hanging around looking a little lost for the time being.

    If anyone is able to confirm the basic layout is sound, advise on the grounding and confirm the audio out wiring is correct I would be very grateful!
     
  5. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,041
    1,669
    Did you use the correct capacitors on the amp IC inputs to isolate their IC side voltages from the MP3 devices Outputs?

    In order to cook your MP3 unit you either had to give it way too much supply voltage, voltage of the wrong polarity or have fed power backward into it through one or both of the audio outputs.
     
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  6. BurningSmell

    Thread Starter New Member

    Feb 7, 2015
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    Ah! Yes, never thought about current returning back down audio out. I suspect this is it. I'll recheck all the caps and connections and the ic wiring. Once the mp3 is working and producing a signal i should be laughing!
     
  7. Denesius

    Member

    Feb 5, 2014
    89
    14
    Come back here when you burn out the next one!!!
    I would not tie pins 8 & 9 together. Tie pin 8 (Pwr Gnd) to the common ground on battery and converter. Tie pin 9 (Sgn Gnd) to the shield or ground of the MP3 player audio output only
     
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  8. BurningSmell

    Thread Starter New Member

    Feb 7, 2015
    4
    0
    That sounds like good advice! I guess if these grounds were meant to be common, they'd be tied within the icrather than on separate pins. I'll report back after my next soldering session!
     
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