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Power supply question

Discussion in 'Electronic Basics' started by OverTork, Apr 12, 2004.

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  1. OverTork

    OverTork Guest

    Is it feasable to connect a 0-120 volt variac directly to a 120/12 volt
    transformer to get a 0-12 volt power supply. The variac puts out 3 amps and
    the transformer has a 25 amp output rating. The output will be rectified,
    filtered and fused for 15 amps as that's all I will be needing. The reason
    for this approach is I would like to use parts I have on available. This
    will be used for anodizing aluminum.

    Thank you in advance.
  2. John Larkin

    John Larkin Guest

    Sure, should work fine.

  3. John Fields

    John Fields Guest

    With 120VRMS out of the variac, you'll get about 15.5 Peak DC volts
    out of the bridge. I don't think you'll need the filter caps, since
    you'll be anodizing with the DC out of the rectifier and what'll be
    doing the work is the _average_ current going into the workpiece. In
    addition, not having to filter will save you some money and will
    reduce the load on the transformer and the variac. As far as fusing
    goes, you should fuse on the mains side of the variac so that if
    anything _does_ go wrong nothing downstream from the mains will be
  4. CFoley1064

    CFoley1064 Guest

    Subject: Power supply question
    Variacs have always been used for plating power supplies -- you'll be fine.
    Just remember you're limited to the output current rating of the variac on the
    primary of the transformer, and the current rating of the secondary on the
    output side of the transformer. That means the variac is good for 3A, whether
    it's set for 120VAC out or 20VAC out. I would put a 3A fuse in front of the
    variac, just to be safe.

    By the way, be careful about fumes -- they'll corrode the surface of the
    variac, causing discontinuities which lead to burning of the contact points.
    Wipe the contacts occasionally with a soft, clean cloth, and keep the plating
    fumes away.

    Good luck
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