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The smallest 240vac to 12vdc power supply

Discussion in 'Power Electronics' started by BlinkingLeds, Apr 18, 2014.

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  1. BlinkingLeds

    BlinkingLeds

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    Feb 23, 2013
    Hi.
    I'm thinking of way of making a really tiny 240vac or 240vdc to 12vdc power supply. I need it to be less than 20x20x20mm (yeah i know :) ) if it can be done smaller it would be even better. i've searched for transformers but i can't seem to find a transformer that small on digikey.
    The power supply will supply a circuit of picaxe (08m2 smd) or pic (haven't figured that out yet) that will drive an irf740 with variable duty cycle (adjusted by a potentiometer) at 100-600hz. and a resistor/zener to give the picaxe the 5v. So maybe a switching voltage regulator circuit without the actual controller? Like an oscillator fet choke capacitor circuit that will work on a fixed freq?

    I have worked before with smd components like SOT-23 fets and the tiny resistors/capacitors that are less than 1mm long so you can design a circuit that uses the smallest components available.
    Thanks.
     
  2. duke37

    duke37

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    Jan 9, 2011
    I do not think this can be done. You will want an isolated supply and there will be minimum distances specified for insulation. That is probably the reason you cannot find a very small transformer.
     
  3. KMoffett

    KMoffett

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    Jan 21, 2009
    You didn't say what your current requirement is.

    Ken
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Yes, we really need to know how much current (or you could specify the power) you need at the output.

    Have a look at some teardowns of compact USB power units and chargers made by Apple and others. These have two PCBs in them, with SMT devices on both, and they supply 5V at around 1A maximum, i.e. 5W. They're pretty compact, but I think they're still bigger than you want. You might be able to make the PCBs smaller, but the transformer will be the limiting factor.
     
  5. BlinkingLeds

    BlinkingLeds

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    Feb 23, 2013
    Hi thanks for the reply. I know there is no such transformer that's why i thought that it could be done with a simple circuit like this : http://schmidt-walter.eit.h-da.de/smps_e/abw_hilfe_e.html but i have never done anything like this
    As for the power requirements i don't know but i think it's much less than 1w because it only needs to supply power to the 08m2 smd picaxe , (i can't find anywhere how much is it's current consumption) which will drive the gate of the irf740. so something like 50-100ma should be fine right?
     
    Last edited: Apr 18, 2014
  6. KMoffett

    KMoffett

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    Jan 21, 2009
    Not sure what your actual project is (It really helps to elaborate), but that is a dangerous design for a beginner. It can put the "hot" side of the line directly to your circuit common. You would also need to rectify and filter (large components) before that circuit comes into play.

    Unless the 20x20x20cm is an absolute, get a tiny, switch mode wall wart. I frequently gut them to get a small AC-DC power supply PCB that I can fit into projects.

    Ken
     
  7. BobK

    BobK

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    Jan 5, 2010
    If the only thing you are powering is a 5V PIC, why would you use a 12V supply? That is just wasteful with no advantage. Use a 5V adaptor. The one Amazon supplies with Kindle looks like a sightly large wall plug.

    Bob
     
  8. KMoffett

    KMoffett

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    Jan 21, 2009
    This is the typical SMPS removed from cellphone charger wall warts. I use them for PICAXE projects.
    100-240VAC >5VDC/750mA
    Ken

    SMPS5V750mA-1.jpg SMPS5V750mA-2.jpg
     
  9. BlinkingLeds

    BlinkingLeds

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    Feb 23, 2013
    i have a 250vdc led light panel and i want to make a dimmer for it but i want it to look like a normal dimmer so that's why i need it to be as small as possible (and for my future picaxe and other space-limited applications that i don't know yet but it's a challenge for me to make everything as small as possible i'm planning to build a cnc soon to make my smd boards better because i can't make routes smaller than 0.5mm with the old etching process without them being washed away.) . I don't worry about the isolation because i don't plan to put any hands in there after i build and test it :) . For the testing purposes i have a 500w isolation transformer.

    Bob you're right i don't need the 12v but i don't know if the irf740 will work with 5v at it's gate.

    KMoffett i'm not American but that looks a bit large like 40x20x20 but it's quite good for the other stuff . I have one like that too but i don't use it when testing because i don't know if it's safe enough.
     
  10. KMoffett

    KMoffett

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    Jan 21, 2009
    More like 30x55x20mm. Gate threshold for the irf740 looks like 2-4VDC, so 5V might not work.

    Ken
     
  11. BlinkingLeds

    BlinkingLeds

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    Feb 23, 2013
    yes i also thought so that's why i wanted a 12v ps and a resistor/zener regulator to down the voltage to 5v for the picaxe shame they don't make those things at 12v
     
  12. davenn

    davenn Moderator

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    you don't understand the full purpose of the isolation ... it also protects the users of the circuit by stopping mains voltage accidentally appearing on the low voltage DC output

    what you are suggesting doing with that linked circuit is extremely dangerous

    Dave
     
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    As others have said, you MUST have electrical isolation if you're powering a circuit that is not fully enclosed and insulated to relevant safety standards and/or has any external connections. No doubt it's a legal requirement; it's also a moral obligation. To my mind, there are very few rationalisations that could excuse you from this requirement, and you haven't given any of them yet.

    In any case, a suitable inductor is likely to be only slightly smaller than a suitable transformer. Also the input smoothing capacitor will be the next smaller item, and that's required in either case.

    Regarding the need for 12V. There are many low-Vgs MOSFETs available that will saturate fully or nearly fully with a gate voltage of 5V.

    You still haven't told us how much current or power you need.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Another answer is to run your pic from 5V then set one pin to do a constant frequency output (from several to a hundred kHz) and use a circuit like this one (as seen here, from an idea I found here):

    uC_Boost.png

    This circuit will generate a voltage equal to the lowest breakdown voltage of the mosfet, the diode, or the capacitor (mine generated well in excess of 90V when operated without feedback)

    You need to add other components to regulate it (there are many ways). None of the components are critical, and the inductance of L1 can certainly be much lower.

    However starting with 12V and using a simple 5V regulator would be simpler if power consumption was not an issue (yeah, and a logic level mosfet is an even better one!).
     
  15. BlinkingLeds

    BlinkingLeds

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    Feb 23, 2013
    Thanks.
    Kris that's the problem i don't know how much power will the smd picaxe (i can't find any specifications for it) and the gate of the fet require.
    Steve i did a quick search in digikey for a 1-2mH inductors and there are very small smd ones with only 7x7mm which is great
    Daven i know that working with uninsulated mains voltage is extremely dangerous but do i have a choice? and even if the mains voltage make it to the dc rail who will touch it?it's gonna be closed inside the electric box in the wall.
     
    Last edited: Apr 19, 2014
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The gate of the mosfet (if driven correctly, and not at high frequency) will require almost no power at all.

    A picaxe also draws bugger all. Allow 20mA. If you've got any LEDs, they'll use more power.

    You can use an inductor ranging from about 40uH to 2mH. Lower inductance will be smaller. Lower inductance will also require higher frequency and/or lower duty cycle
     
  17. BlinkingLeds

    BlinkingLeds

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    Feb 23, 2013
    I don't have any leds so that's the cirucit? but what will drive the q1?
     
  18. davenn

    davenn Moderator

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    Sep 5, 2009
    you have to be kidding !!

    you ALWAYS have a choice, you should NEVER be putting anyone at risk of electrocution
    you need to redesign the circuit to comply with safety criteria

    and with that I will close the thread, since you wont listen to the safety advice

    Dave
     
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