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Need AC power supply help

Discussion in 'Power Electronics' started by Truckdrivingfool, Sep 30, 2012.

  1. Truckdrivingfool

    Truckdrivingfool

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    Sep 30, 2012
    I've gotten into making guitar amps and have few successful 9vdc LM386 amps under my belt and decided to move on to a kit I found on Ebay. I have a great understanding of DC but my AC knowledge is limited to replacing light fixtures/switches and other things around the house.

    My problem is understanding what I need for a power supply for the above linked kit. I'm trying to figure out if a simple supply like this will work? I've also been reading about power loss and wonder if I should go with something like this. Or do I need to be looking for an actual transfomer.
     
  2. davenn

    davenn Moderator

    13,349
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    Sep 5, 2009
    hi there
    welcoem to the Electronics point forum :)

    neither of those, you need a transformer centre tapped on the output for 12V-0V-12V
    you are not going to find one like that in a ready made plugpack You will have to buy the bare transformer, get a power cord fuse holder and fuse etc and build it up inside you amplifier housing. Select a transformer with 120VAC or 240VAC depending on the country you live in
    like this circuit I have drawn for you :)

    [​IMG]

    cheers
    Dave
     

    Attached Files:

  3. davenn

    davenn Moderator

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    Sep 5, 2009
  4. Truckdrivingfool

    Truckdrivingfool

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    Sep 30, 2012
    Thanks Dave for the welcome and reply,

    I'm trying to understand to avoid having to ask the same questions in the future so bear with me.

    On the transformer, I now know I obviously need some clarification on the way the ratings are listed. The ones I had been looking at were showing 12v not the 24 like the one you linked (Thanks for that too) would those 12v ones have come out to actually being 6v once they were split? Perhaps they were not a center tapped style?

    On the actual hook up of the transformer looking at the PDF and your diagram I know the hot and neutral will go to pins 2&4
    1. Does it matter which goes to which pin?
    2. Where does the ground from the wall go to? To the ground connection on the PCB connector in the amp circuit? Or are those connections the 6 8 10 pins out of the transformer?
    3. The main power switch I have some DPST micro switches rated 3A @ 250VAC/ 6A @ 125VAC these should be fine there correct?
    4. What is the smallest safe wire size I can use to make the fuse/switch connections between the wall cord and the transformer?

    I've got a million more questions but those should cover me for now. I would take any recommendations on reading that would explain circuit design (I guess) that would explain the way AC circuits work. The most I really understand is to think of the hot and neutral in AC like the hot and ground in a DC circuit. However I'm not even sure of that. :eek:
     
  5. Truckdrivingfool

    Truckdrivingfool

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    Sep 30, 2012
    Just found the tutorials section and will look in there but still open to other sources of reading.
     
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    Yes you need a centre tapped secondary with 12V for each half so as the linked PDF and my circuit shows there will be 3 terminals on the secondary 12V-0V-12V

    no

    it goes to the transformer casing using a eye terminal under one of the nut/bolts holding the transformer in the case, it also goes to the 0V terminal of the transformer

    DPDT switch toggle or rocker type ... rocker is the most common for power switches
    those ratings are ok that you showed :)

    the fuse should be an either a PCB mounted fuse holder or a chassis mounted holder
    the wiring between the input power cable, switch, fuse and transformer should/can be the same thickness and insulation type as the mains power cable

    no thats not correct :) tho we refer to mains power wires as Hot ( Active or Live) and Neutral, this is very different to a Dc supply where you have + and - wires

    in an AC supply AC = Alternating Current the voltage polarity and current direction is changing (alternating) at 60 times a second (60 Hz)
    The Hot/Active/Live wire is the one that is above earth potential. That is at any home power meter/fuse panel, the neutral and earth are connected to a common buss bar, so the neutral is effectively at earth/ground potential

    in a DC = Direct Current supply, there is no changing of polarity of the supply so the + Volts is always just that and the negative is the 0V line

    Dave
     
  7. Truckdrivingfool

    Truckdrivingfool

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    Sep 30, 2012
    I get that there is always potential on both in AC but have always pictured the mechanical connection/flow to be like DC. I know its a very basic grasp and probably not correct but in a layout way of thinking it's how I've always thought of it.

    I'll read through the stuff in the tutorials and see if can get my view corrected and come back to ask if I'm still confused.:)

    Thanks again.
     
  8. davenn

    davenn Moderator

    13,349
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    Sep 5, 2009
    OK here's some pics of some mains wiring in a commercial unit
    Note that the transformer is prewired not solder lugged as in that PDF
    also the secondary is just one winding ... not centretapped

    you can see that the cabling is a reasonable gauge ... I will often take a bit of 3 core appliance cable and
    strip off the outer sheath to expose the 3 inner insulated wires ... it makes idea hookup wire
    I know in the USA they commonly use Black and White for hot and Neutral) dunno what colour your earth wire usually is ? green ? in Australia we use the European standard Brown = Hot, Blue = Neutral and Green with yellow stripe = Earth


    [​IMG]

    [​IMG]

    [​IMG]

    The last pic to show you what a rocker switch looks like, they come in various sizes and ratings


    Dave
     

    Attached Files:

    Last edited: Oct 1, 2012
  9. Truckdrivingfool

    Truckdrivingfool

    31
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    Sep 30, 2012
    Thanks, doesn't get any clearer than that.

    Good to note the European color coding as I didn't know that. Yes, here in the states black - hot, white - neutral, and ground is most commonly just bare copper at least for in the wall house wiring. Then typically inside appliances/fixtures ground is green.

    On to read more. :)
     
  10. Truckdrivingfool

    Truckdrivingfool

    31
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    Sep 30, 2012
    Well the kit showed up today so it's time to get off the pot and order a transformer.

    I'm still trying to understand the ratings, I'm trying to keep this as cheap as possible so I poked around Mouser. Just so I know I understand before I order one the 24v in the rating of the one you listed is the total output of the secondary coil and that will yield 12v on each pin once it splits. Or should I be looking at one that lists the secondary as 12v?

    Also just as an example to make sure I'm reading things right, looked at this one and deduced it would be too small since it's output is only .12A and I need a full 1A.
     
  11. Truckdrivingfool

    Truckdrivingfool

    31
    0
    Sep 30, 2012
    Never mind I finally found a good explanation of center tap workings though google and get it now.

    $4 amp gonna cost me 25 to make it work oh well still cheap in the overall scheme of things.

    I also see now that I need 2.5 to 3 VA for this.
     
    Last edited: Oct 2, 2012
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,214
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    Jan 21, 2010
    If you have a 12V transformer already, why not try something like this.

    The link shows 15 volts rather than 12, but the concept is the same. Depending on the actual requirements, the 12 volt regulators may not be required.
     
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