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Capacitors in series instead of single capacitor, why?

Discussion in 'Electronic Basics' started by drgonfly, Nov 7, 2003.

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  1. drgonfly

    drgonfly Guest

    I'm a complete newbie to electronics, so sorry is this is a stupid question but
    in looking at schematics in an attempt to see how a power supply works, I
    notice in some setups, right after the bridge rectifier, people use a single
    electrolytic capacitor (3300uf or so), while others use 2 capacitors in series.
    Can anyone explain why they don't just use a single capacitor? They are usually
    something like 1000uF each, so why not use a single 2000uF capacitor? When is
    it better to use 2 capacitors instead of a single cap? Does using 2 capacitors
    help smooth out the voltage better then a single capacitor?

    Again, sorry if this sounds stupid to you. I'm just trying to learn a little
    something about this. Thank you for your time.
  2. Capacitors are put in series for three main reasons. One to get a
    no-preferred value, two, to get double the voltage rating, three, to
    make a large bipolar cap from unipolar caps.

    Kevin Aylward
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  3. John G

    John G Guest

    2 1000mfd capacitors in series gets you only 500 mfd not 2000 mfd.
    The maths would take too long for me to explain.
    Resistors in series ADD.
    Capacitors parallel ADD.
  4. In electronics, 'series' has a very specific meaning. Series means
    connected start of one to end of other, forming a single path. This
    reduces the effective capacitance, but increases the voltage rating.

    I am guessing that you are asking about two capacitors connected in
    parallel (side by side across the rectifier output rails). Sometimes
    capacitors are connected in parallel (which adds their capacity) to
    approximate a larger capacitor but one that has a higher ripple
    current capability than a single larger unit might have, or to fit the
    total capacitance into a space restriction (like keeping the maximum
    component height below some limit).

  5. Study the rectifier configuration. Quite often two capacitors are put in
    series to perform a voltage doubling function (that is, provide a DC voltage
    of magnitude equivalent to the peak to peak voltage of the AC waveform minus
    two diode drops). Usually the arrangment is shown where the two capacitors
    are stacked vertically on top of each other and the connection between the
    two of them goes to one of the transformer's secondary pins. The other
    transformer pin of the secondary is then fed to two diodes, one going to the
    + side of the top capacitor (diode cathode to + of capacitor), and the other
    going to the - side of the bottom capacitor (diode anode to + of capacitor).
    The DC output voltage is then taken from the - side of the bottom capacitor
    and the + side of the top capacitor. This can be a handy arrangment.
  6. capacitor).

    Whoops. In the last line above replace the "diode anode to + of capacitor"
    to "diode anode to - of capacitor".
  7. Fred Abse

    Fred Abse Guest

    "Dual voltage" arrangements often use a voltage doubler arrangement for
    115 volts and a bridge rectifier for 230 volts, connecting the reservoir
    capacitors accordingly.
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