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120 VDC

Discussion in 'Electronic Basics' started by Chris W, May 8, 2005.

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  1. Chris W

    Chris W Guest

    I was recently told that if rectify the 120VAC mains voltage and then
    add a capacitor to smooth out the wave, I will then have something
    around 167 VDC. Is this true? If it is true, what is the easiest way
    to get a smooth 120V DC voltage?

    --
    Chris W

    Gift Giving Made Easy
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  2. If you can find an 85 volt transformer, it would produce approximately
    120 volts DC with a capacitor filter. It would also provide isolation
    from the mains, so that a connection to ground would not be hit with
    the full current the line could deliver. The 120 volts will not be
    very smooth, however. It will have ripple at twice the line frequency
    when loaded and will sag as the load current increases.

    If you want a clean, accurate and stable 120 volts DC you will have to
    build or buy a regulated supply. For instance, here is one that is
    adjustable from zero to 120 volts DC, capable of supplying up to 1 ampere.
    http://shop.store.yahoo.com/webtronics/lowcosioubep.html
     
  3. Yes. The 120 VAC mains voltage is 120 VAC, RMS (root-mean-square). This
    means that if you connect it to a resistor then the resistor produces as
    much heat as if you connected it to 120 VDC.

    The peak voltage of a 120 V (RMS) sine wave is 120*sqrt(2) = 170 VAC, minus
    a couple of diode drops I suppose to get your 167. The rectifier detects the
    peak value of the sine wave.
    Use a transformer to turn your 120 VAC into ~85 VAC. An autotransformer
    (Variac) might be easiest. What are you trying to do?

    Jonathan
    http://cq.cx/
     
  4. John Fields

    John Fields Guest

    ---
    FWB
    +-----+
    120VAC----|~ +|----+---+----C E-------+---->120DC
    | | | | B |
    | | | [R] | [R]
    | | | | | |
    | | |+ +------+----C |
    | | [C] B--+
    | | | E |
    | | | | [R]
    | | | | |
    120VAC>---|~ -|----+---------------+----+----->GND
    +-----+
     
  5. John Fields

    John Fields Guest

    ---
    Or, perhaps,:


    FWB
    +-----+
    120VAC----|~ +|----+---+----C E----------+---->120DC
    | | | | B |
    | | | [R] | |
    | | | | | |
    | | |+ +------+----C |
    | | [C] B-->[POT]
    | | | E |
    | | | | |
    | | | | |
    120VAC>---|~ -|----+---------------+-------+----->GND
    +-----+
     
  6. Jamie

    Jamie Guest

    yes.
    120 * 1.414 = 169.68

    to get smooth 120, use a regulator after the
    fact.

    oh well.
     
  7. John Fields

    John Fields Guest

     
  8. Don Kelly

    Don Kelly Guest

    Possibly he was seeing your ascii diagram in a non-fixed font where it
    becomes quite garbled..
     
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