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Cheap 120VAC to 3.3VDC power supply

Discussion in 'Electronic Design' started by Nevo, Dec 1, 2006.

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  1. Nevo

    Nevo Guest

    I'm looking for the cheapest, smallest way to power a 50mA 3.3V circuit from
    mains power. I'd like the whole circuit, including the power supply, to fit
    in a matchbox.

    I've always used transformers in my designs but they're both too expensive
    and too big for what I want to do. I recently discovered a number of
    "off-line" switching regulators from various vendors. Are these the correct
    solution to the problem? Even these require a fair number of external parts.

    I'm sure I'm not the first person who's faced this problem or asked this
    question. I suspect there's a "standard" answer out there, but searching is
    difficult given how generic the search terms are.

    Thanks,

    -Nevo
     
  2. TuT

    TuT Guest

    If you don't need an isolated supply, you could use a capacitor as a
    lossless series element - by my reckoning you'd need about 1uF to deliver
    50mA. Problem is the cap will be very transparent to mains spikes, so you
    need to do something limit the current surges through the cap and resulting
    voltage surges at the 3.3V supply. You also need some form of rectification
    to ensure the cap conducts AC current, otherwise it will just charge up to
    the peak supply voltage. I have seen commercial circuits using this kind of
    approach, though, personally, I'd tread very carefully.

    Try Googling for "capacitive series elements" or something similar.
     
  3. Henry Kiefer

    Henry Kiefer Guest

  4. jasen

    jasen Guest

    if you don't need iosolation from the mains use a capacitor to limit the
    current to about 70ma, a rectifier, and a 3.3V zener.

    1.5u .------------+---[33R]-+--+
    || / \ | |
    250VAC --[22R]---||---<BR >-. 1000u === .->Z-' load
    || \ / | 10V | | 3V3
    c1 `---|--------+----+-------+
    ---------------------' c2

    use chunky diodes in the bridge, power-on inrush can exceed 15A for a few microseconds.
    - 1n4001 would probably do, 1n914 probably not

    a bleed resistor across c1 would probably be a good idea too.

    c1 should have 'X2' safety rating. and will probably take up most of that
    matchbox space


    here's an LTSPICE file for a 4.7V design (no 3.3V zener model) don't use
    1n914 diodes in the rectifier of a real one.

    --------8<----- cut here save as filename.asc
    Version 4
    SHEET 1 880 680
    WIRE 64 96 -16 96
    WIRE 176 96 144 96
    WIRE 368 96 240 96
    WIRE -16 144 -16 96
    WIRE 304 144 272 144
    WIRE 368 144 368 96
    WIRE 464 144 432 144
    WIRE 272 272 272 144
    WIRE 304 272 272 272
    WIRE 464 272 464 144
    WIRE 464 272 432 272
    WIRE 512 272 464 272
    WIRE 656 272 592 272
    WIRE 768 272 656 272
    WIRE 464 304 464 272
    WIRE 768 304 768 272
    WIRE 656 320 656 272
    WIRE -16 368 -16 224
    WIRE 368 368 368 272
    WIRE 368 368 -16 368
    WIRE 272 416 272 272
    WIRE 464 416 464 368
    WIRE 464 416 272 416
    WIRE 656 416 656 384
    WIRE 656 416 464 416
    WIRE 768 416 768 384
    WIRE 768 416 656 416
    WIRE 272 448 272 416
    FLAG 272 448 0
    SYMBOL voltage -16 128 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value SINE(0 315 50)
    SYMBOL res 160 80 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R1
    SYMATTR Value 22R
    SYMBOL cap 240 80 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C1
    SYMATTR Value 1.5µ
    SYMBOL diode 304 160 R270
    WINDOW 0 32 32 VTop 0
    WINDOW 3 0 32 VBottom 0
    SYMATTR InstName D1
    SYMATTR Value 1N914
    SYMBOL diode 368 160 R270
    WINDOW 0 32 32 VTop 0
    WINDOW 3 0 32 VBottom 0
    SYMATTR InstName D2
    SYMATTR Value 1N914
    SYMBOL diode 304 288 R270
    WINDOW 0 32 32 VTop 0
    WINDOW 3 0 32 VBottom 0
    SYMATTR InstName D3
    SYMATTR Value 1N914
    SYMBOL diode 368 288 R270
    WINDOW 0 32 32 VTop 0
    WINDOW 3 0 32 VBottom 0
    SYMATTR InstName D4
    SYMATTR Value 1N914
    SYMBOL cap 448 304 R0
    WINDOW 0 -43 36 Left 0
    WINDOW 3 -33 63 Left 0
    SYMATTR InstName C2
    SYMATTR Value 1000µ
    SYMBOL zener 672 384 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 -76 69 Left 0
    SYMATTR InstName D5
    SYMATTR Value 1N750
    SYMATTR Description Diode
    SYMATTR Type diode
    SYMBOL res 752 288 R0
    SYMATTR InstName R2
    SYMATTR Value 80R
    SYMBOL res 608 256 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R3
    SYMATTR Value 33r
    TEXT -50 506 Left 0 !.tran 0 2 0 .001 startup
    -------------------8<------ cut here

    Bye.
    Jasen
     
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