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Trying to mod plug pack to output DC from AC...

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  • Trying to mod plug pack to output DC from AC...

    I have a plug pack that puts out 15V AC, 1100mA. I wish to use a simple full-wave rectifier circuit with a smoothing capacitor to produce about 12V DC, nothing less than 600mA. (It's to power a cheap chip amp(s) which take ~7W all up)

    If I was to use the circuit in the picture, roughly what sizes of components would I need?

    Also if the amp only draws (eg.) 50mA, will the 1100mA burn it out? Or will it just "take what it needs"?

  • #2
    Re: Trying to mod plug pack to output DC from AC...

    No image attached.

    Without a basic understanding of electronics...at least ohms law, I'd stay away from the electronics. At the very least you'll want to have a multimeter on hand for building any circuit.

    15V(RMS) AC supply will give you a whole lot more than 12VDC with a full-wave rectifier.
    I'm not deaf, I'm just not listening!

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    • #3
      Re: Trying to mod plug pack to output DC from AC...

      it's probably not worth the cost of parts for only 7W. I'd recommend either a 12VDC 1A

      http://www.parts-express.com/pe/show...number=120-050
      for $10

      or a 12VDC 1A regulated (=better) adapter

      http://www.parts-express.com/pe/show...umber=120-1085

      for only $14.

      regulated input power is going to result in cleaner overall operation in most cases, and because chipamps generally take in excess of 12V, it's unlikely to have a 12V regulator built in already. and at 7W (12V * 600mA =~ 7 volt-amps DC, so i'd aim for 800mA current capacity or above. running it on the 1A supply should be fine. i've also used the 24V unregulated ones for a small project, and it's done pretty well).
      TM (RS125/TB25-302), TM RS150S / Vifa BC25SC06, RS150-4 / Vifa ML-TL, 3CR-AL Ultra Budget: Electric Blue TM, TMM, MTM, Dragonflies , Mounties: Mini HT Satellite,sub for minions

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      • #4
        Re: Trying to mod plug pack to output DC from AC...

        That is good advice. I'd recommend the regulated supply, I've measured enough wallwarts to know that 12V is generally 12V at full load. With only a small load, the poor little transformer makes a considerably higher voltage, and can harm some devices that can't work with such a high voltage.

        On the other hand, the linked regulated supply gives this warning as well...odd...
        I'm not deaf, I'm just not listening!

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        • #5
          Re: Trying to mod plug pack to output DC from AC...

          Does the chip amp use a virtual ground? If so, I think the wall wart needs to have an isolated ground.

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          • #6
            Re: Trying to mod plug pack to output DC from AC...

            Originally posted by patrickm View Post
            it's probably not worth the cost of parts for only 7W. I'd recommend either a 12VDC 1A

            http://www.parts-express.com/pe/show...number=120-050
            for $10

            or a 12VDC 1A regulated (=better) adapter

            http://www.parts-express.com/pe/show...umber=120-1085

            for only $14.

            regulated input power is going to result in cleaner overall operation in most cases, and because chipamps generally take in excess of 12V, it's unlikely to have a 12V regulator built in already. and at 7W (12V * 600mA =~ 7 volt-amps DC, so i'd aim for 800mA current capacity or above. running it on the 1A supply should be fine. i've also used the 24V unregulated ones for a small project, and it's done pretty well).
            He is in Australia, he'll probably want to buy from his continent, rather than pay shipping all the way from Parts Express in Ohio.

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            • #7
              Re: Trying to mod plug pack to output DC from AC...

              Ok, so it looks like I may have to buy a $20 plug pack (regulated) from Jaycar - they should be good enough value with all the plugs (7) you get with it; I was just trying to get around the problem the "cheap" way.

              As for the (crap?) chip amps, they are the TDA2822m and TDA1905, both with only +ve and GND for power, both DC. Power output is (1W + 1W) and 5W respectively.

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              • #8
                Re: Trying to mod plug pack to output DC from AC...

                Originally posted by Dirk View Post
                Does the chip amp use a virtual ground? If so, I think the wall wart needs to have an isolated ground.
                That doesn't matter. ;)

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                • #9
                  Re: Trying to mod plug pack to output DC from AC...

                  I think I've found a way around it - the plug pack was originally from some computer speakers, and I'd blown the IC amp due to "hot-wiring", plus I'd done stuff to the drivers during my "intro to speakers", so no point really fixing...

                  Instead I've just reconnected the power into the PCB, and used the holes from where the IC got its power from. It means I'll have a 11 x 9cm PCB to deal with, but it should work out OK.

                  Also, the 25V 3300uF capacitor was heating up with a 1k resistor as load (testing for when power is "disconnected" and plug pack still on). So much for thinking it would handle 21V...

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