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Beginner transformer question

Discussion in 'Electronic Basics' started by Ant_Magma, Feb 26, 2006.

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  1. Ant_Magma

    Ant_Magma Guest

    I'm looking for a transformer that is capable of stepping down 240V
    from the wall outlet to maybe less than 20V depending on the type of
    voltage regulator. My objective is to produce 3.3V from the wall outlet
    to power my ICs.

    1) Most of the times i see 2 same values for the secondary voltage, eg:
    0-9,0-9 (9V). What does it mean? Does it mean i still get 9V output?

    2) If i want to calculate the output current, do i take the power
    rating lets say 3VA and divide with the output voltage, for example 3VA
    / 9V=0.33A?
  2. It means there are two secondaries of 9 volts, each. You can connect
    them in parallel to act as a single secondary with the full VA rating
    of the transformer and rectify them with a 4 diode bridge, or you can
    connect them is series to produce 18 volts with the full VA rating,
    and also rectify with a 4 diode bridge. Or you can connect them in
    series and half wave rectify the each half of the total winding with a
    single diode, and parallel the two diode outputs for a full wave 9
    volt output. This uses each secondary only half of the time, so the
    VA rating has to be derated a bit. But you get only one diode drop in
    the rectification process, instead of two. Or you can use a 4 diode
    bridge across the seriesed secondaries and have positive and negative
    outputs with respect to the center tap node between the two
    secondaries. The flexibility gained by having two equal secondaries,
    instead of one is the reason this construction is so common.
    That works if the transformer will be loaded with a resistor, but you
    have to derate this if you are going to be charging a capacitor input
    filter at the voltage peaks, only. You may get as little as half of
    that (as filtered DC) for the same transformer temperature rise.
  3. Ant_Magma

    Ant_Magma Guest

    I thought the rectifiers and diodes comes together with the

    So with the 2 secondary voltages it's up to me to 'mix n match' to get
    the desired output correct?
  4. If so, it is a power supply, not a transformer.
    Yes, you have choices.
  5. Ant_Magma

    Ant_Magma Guest

    Is there reference circuit i can refer to in building a 3.3V power
    supply from a 240V AC line voltage?
  6. How much DC current must this supply deliver?
  7. Pooh Bear

    Pooh Bear Guest
  8. ehsjr

    ehsjr Guest

    Here you go:

    240AC to 12VDC
    "Wall Wart"
    --------- -------
    | + |------+---Vin| LM317 |Vout---+-----+---> +3.3Volts
    | | | ------- | |
    240| |12 [C1].1uF Adj [220R] |
    | | | | | |+
    | - |------+ +-----------+ [C2] 1uF
    -------- | | |
    | [360R] |
    | | |
    +----------+-----------------+--- Gnd
  9. Ant_Magma

    Ant_Magma Guest

    Well, i'm guessing around 500mA?

    Coz i have 2 main components, 1 is the Ethernet transceiver and d other
    is the powerline module. The module states that it requires a current
    of 370mA or a max of 530mA to operate.

    I was wondering if this transformer will work?
    The code is 3473518. It has a power rating of 3.2VA and a 6V output.
    So 3.2/6= 0.533A

    Is it correct?
  10. Ant_Magma

    Ant_Magma Guest

    In my case i should just replace the wall wart with a 240AC to 12V
    transformer right?
  11. Sounds a bit light, to me. When you rectify the AC to DC and filter
    that with a big capacitor, the current all bunches up around the
    voltage peaks, so to get an average of 500mA, the pulses get up around
    a couple amps. It doesn't hurt to have a transformer with extra
    capability, and a larger one doesn't cost much more. I would go with
    one rated between 1 and 2 amps, so you don't have to worry about it
    getting hot. You'll have enough of that at the regulator, that wastes
    all the extra voltage.
  12. Chris

    Chris Guest

    Hi, Ant. Just out of curiosity, could you answer a few questions?

    * What's your major?

    * Frosh, Soph, Jr., Sr?

    * Is this the senior project?

    * When's it due?

  13. Rich Grise

    Rich Grise Guest


    ehsjr's suggested circuit [WILL YOU GOOGLEGROUPIES ***PLEASE*** LEARN
    uses a DC wall wart. It contains a transformer, rectifier, and filter
    capacitor. To use just a transformer in its place, you would have to
    provide these components yourself, as John Popelish has described.

    And I'm way too lazy to draw that for you, but try googling for
    "linear power supply basics" or "ac/dc tutorial" or any phrase with
    "basics" or "tutorial" in it.

    Good Luck!
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