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Filter capacitor in power supply is increasing the voltage

Discussion in 'Electronic Basics' started by Jon Danniken, Oct 24, 2004.

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  1. Jon Danniken

    Jon Danniken Guest

    I'm building an unregulated 12V power supply, but I've run into an issue I
    can't seem to resolve. The supply consists of a mains transformer that
    converts 120VAC into 12VAC. The resulting 12VAC is fed into a full-wave
    bridge rectifier, with the resulting output of ~12VDC.

    Now obviously I need some filtering on that 12VDC, so I added a capacitor
    between the (+) and the (-) output of the bridge rectifier. The resulting
    output is filtered, but as a result the voltage at the capacitor is now
    ~17VDC.

    I'm somewhat familiar with why this occuring, having built a voltage
    multiplier in the past, but is there any "trick" to prevent it from
    occuring, or possibly a way (other than a resistor network or a voltage
    regulator) to reduce it afterwards?

    Thanks for any suggestions,

    Jon
     
  2. Jag Man

    Jag Man Guest

    Why not use something like a 7812 regulator IC?

    Ed
     
  3. john jardine

    john jardine Guest

    Unloaded, the cap will always charge up to near the peak voltage of the
    incoming 12Vac. (1.4 X 12Vac is 17Vdc).
    Other than inductors or regulators then the best option is maybe just use a
    9Vac transformer.
    Bear in mind, the DC voltage can drop a few volts when you put load on the
    power supply.
    regards
    john
     
  4. Jon Danniken

    Jon Danniken Guest

    Thanks, John, the voltage did indeed drop down to ~12.5VDC when I applied a
    5A load. I'm curious about your statement regarding the inclusion of an
    inductor though. In what way would an inductor be applied to reduce the
    voltage?

    Thanks,

    Jon
     
  5. john jardine

    john jardine Guest

    Used in this situation the inductors are usually called "chokes". The idea
    is to connect a choke to the output of the main capacitor, then feed into
    yet another capacitor that supplies the load. It doesn't 'drop' the voltage
    like a switcher or regulator or resistor would. It simply acts with the
    extra capacitor to 'reshape' or 'mould' the nasty incoming waveform to
    something flatter and smoother. As ...

    ___ o/p V
    +-+--+--+------UUU-----+------o to load
    o-. ,--, A A | choke |
    AC in )|( |-+ | --- ---
    )|( -(-+ --- ---
    o-' '--' A A | cap1 cap2 |
    +-+-----+--------------+------o

    The choke and Cap2 form a very low frequency low pass filter acting as a
    'flywheel' energy store that can smooth out the effects of the peaky
    voltages that are charging up Cap1. Nett result is that the load voltage is
    a nice smooth voltage hovering somewhere near the average (not peak) of the
    incoming voltage. I.e near 12V.
    Problem is that to be effective (at mains frequency), the choke inductor
    needs lots and lots of Henries and ends up as some humungous, expensive lump
    of steel and copper that is similar in size and cost, to the power
    transformer. Hence at the high currents and low voltages of modern
    equipment, is a technique little used nowadays.
    regards
    john
     
  6. Jon Danniken

    Jon Danniken Guest

    Thanks again for your reply, John. I've got enough now to play around with
    componenets and look for results.

    Jon
     
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