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Multi-rail DC supply from a Wall-wart - (was How To: DC +/- Power Supply?)

Discussion in 'Electronic Basics' started by Gerhard v d Berg, Nov 9, 2004.

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  1. Multi-rail DC supply from a Wall-wart

    Warning - this circuit requires a little bit of electronic
    knowledge and some soldering skills. Please ask a knowledgeable
    person to help if you do not understand the circuit or risks as
    mains voltage circuitry is involved.

    The circuit provides a 'high' current output at a nominal voltage
    as well as 'low' current auxiliary supplies at minus and at double
    the nominal output voltage (if only one auxiliary supply is
    required, the components for the other can be left off).

    The 'low' current supplies have limited capability due to the
    half wave charge pump technique employed (50-100mA). The main supply
    should be limited to 60% of Wall-wart's rating as Wall-warts are
    normally heavily over rated.
    The power supply circuit normally forms part of the user's circuit.

    This supply was quite popular in the late 70's. From a single
    transformer winding it provided 5v DC for a microprocessor circuit
    and +- supplies for analogue interface circuits and/or an RS-232
    comms buffers (3-terminal regulators were used to regulate the outputs).

    Start by buying one of the 4.5v to 12v DC switchable Wall-warts (<$10)
    as it simplifies the selection of the correct output voltage(s).
    Also ensure that the Wall-wart's case can be opened as it requires
    a small internal rewire change (this might be illegal in some countries).

    Next open the Wall-wart's plastic casing.

    ( WARNING === WARNING === WARNING )
    ( DO NOT PLUG WALL-WART INTO MAINS SOCKET WHILE IT IS OPEN !!!! )
    ( MAINS VOLTAGE IS PRESENT INSIDE WALL-WART WHILE IT IS PLUGGED )
    ( INTO A MAINS SOCKET !! = MAINS EASILY BLOWS HUMAN FUSE !! = :)
    ( The human fuse has proved to be very difficult to repair - and )
    ( replacement through cloning, is still in an experimental phase )

    Unsolder DC output wires across Wall-wart's smoothing capacitor
    (normally 1000uF 25v or 16v) and wire it directly across suitable
    secondary transformer windings. Select windings to provide a
    suitable AC (Alternating Current) voltage on output leads to
    generate the most appropriate DC output levels for the circuit below.
    (It is an AC voltage, so the polarity of leads is not an issue).

    I prefer to wire the output permanently to the correct transformer
    windings and not to the output of the voltage selction switch.
    It averts the problem of people that adjusts the switch and select
    a too low or high an output voltage.

    Close the Wall-wart and as a pre-emptive measure
    LABEL THE WALL-WART AS:

    = NON STANDARD =
    = AC OUTPUT =

    Plugging a DC Wall-wart output into the circuit blow will do no
    harm (provided the voltages are not to high) but the charge-pump,
    auxiliary supplies will not function.

    First test the AC output voltage with a suitable AC voltage meter.
    Connect the output to the multi-rail DC supply circuit below
    and test the DC output voltages (if necessary select a more
    suitable transformer secondary winding tap).

    All diodes are 1N400X (nominally 1N4001)
    C1, C2, C3 and C5 should be 200uF or larger depending on the
    output current and minimum ripple requirements of the
    -V and +2V supplies.
    C4 should be 2000uF or larger depending on current and minimum
    ripple requirements.
    C1 and C3 should have a voltage rating of 2X V +50%
    All other capacitors should have a voltage rating of V +50%.

    Regulators with suitable heatsinks can be used if regulated output
    voltages are required. Low dropout regulators, large capacitors
    and suitable transformer output voltage selection can minimise
    heatsink requirements.


    . Multi-rail DC supply
    . C1 || +
    . ,----||--------o------------o---------- +2V
    Wall-wart . | || | |
    . | D1 | |
    . | +---|>|---' | +
    . | | =====
    . | | | C3
    . | | D3 |
    ------- ---->>--o----|------o---|>|----o----|----o----- +V
    )( . | | | D4 | | |
    AC )( . | | |--|---|>|----' | | +
    )( . | | | | D5 | =====
    Mains )( . | | | '---|<|----| | | C4
    )( . | | | D6 | | |
    ------- ---->>--|----o---o------|<|----o----o----o--o-- 0v
    . | | | |
    . | | | + GND
    . | | D2 =====
    . '----|---|<|---| | C5
    . | | |
    . | + || | |
    . +----||---o------------o---------- -V
    . C2 ||
     
  2. Snip ...

    Take 2
    I left out two diodes in the charge-pump circuits.
    Nobody queried or corrected me - the hawks in the group is slipping up !
    Maybe there is no interest :-/

    I reposted the whole text and circuit with the corrections and add a
    reference.
    - - -

    Multi-rail DC supply from a Wall-wart.

    This supply was quite popular in the late 70's. With regulators
    it provided 5v DC for microprocessor circuit and a +- supply for
    analogue interface circuits and/or an RS-232 comms buffers
    from a single transformer winding.

    The circuit provides a 'high' current output at a nominal voltage
    as well as 'low' current auxiliary supplies at minus the nominal
    supply as well as a 'low' current supply at double the nominal voltage.

    Buy one of these 4.5v to 12v DC switchable Wall-warts
    as it simplifies the selection of the correct output voltage(s).
    Also ensure that the Wall-wart's case can be opened as a small
    internal change is required.
    Open the case (do not plug it into socket while it is open !!!!

    ( MAINS VOLTAGE PRESENT INSIDE WALL-WART WHILE )
    ( PLUGGED INTO MAINS SOCKET !!!!!!
    ( MAINS EASILY BLOWS HUMAN FUSE !!!!!! :)
    ( Human fuse has prove to be very difficult to repair -
    ( replacement through cloning, is still in experimental stage)

    Unsolder DC output wires across Wall-wart's smoothing capacitor
    (normally 1000uF 25v) and wire it directly across the transformer
    windings. Select windings to provide a suitable AC (Alternating
    current) output to generate the most suitable DC outputs for the
    circuit below.

    I prefer to wire the output permanently to the correct transformer
    windings and not to the switch output. It avert problems when
    people adjust the switch and thus select a too low or high voltage.
    (View circuit using notepad or a fixed pitch font such as courier)

    . Multi-rail DC supply
    . C1 || +
    . ,----||--------o----|>|-----o---------- +2V
    Wall-wart . | || | D2 |
    . | D1 | |
    . | +---|>|---' | +
    . | | =====
    . | | | C3
    . | | D3 |
    ------- ---->>--o----|------o---|>|----o----|----o----- +V
    )( . | | | D4 | | |
    AC )( . | | |--|---|>|----' | | +
    )( . | | | | D5 | =====
    Mains )( . | | | '---|<|----| | | C4
    )( . | | | D6 | | |
    ------- ---->>--|----o---o------|<|----o----o----o--o-- 0v
    . | | | |
    . | | | + GND
    . | | D7 =====
    . '----|---|<|---| | C5
    . | | |
    . | || + | D8 |
    . +---||----o----|<|-----o---------- -V
    . C2 ||
     
  3. Lord Garth

    Lord Garth Guest

    <snip>

    The plural police have flagged your sentence above as incorrect. The
    correction is as follows, "Nobody queried or corrected me - the hawks
    in the group are slipping up !"

    :)
     
  4. john jardine

    john jardine Guest

    [clip]

    There's always interest else there'd be no group!.
    I saw it but didn't notice any diodes missing. If a newcomer to electronics,
    then those diodes are very important but like many people here, over the
    years I've looked at literally 1000's of PSU circuits, hence will only
    casually 'scan' a circuit to see if any new idea has turned up. With
    experience it takes about 2 seconds to do this.

    But ... From long and close familiarity with these things, it's just like
    reading a line of text. Words can be misspelt, whole words, characters, even
    sentences can be missing, yet the -meaning- still shines through. Simply a
    case of pattern recognition.

    Occasionally, some weird novelty circuit comes along, at which point
    everything stops while pleasure is taken sucking the goodness out of it
    (may take an hour).
    The circuit is then no longer oddball and it and its variants (with and
    without crucial diodes etc) can henceforth be spotted and scanned quickly.
     
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