[Beginner's Question] Capacitors

Discussion in 'General Electronics Chat' started by mrobd, Jun 15, 2011.

  1. mrobd

    Thread Starter New Member

    Jun 15, 2011
    1
    0
    Hello.
    I'm want to make a DCDC converter circuit and I have a question about it. Here is the design:
    [​IMG]
    I guess the capacitors are here used to smooth the current? Why are the arranged this way (always connected to GND)? Why on both sides (on the input side and on the output side)? Could the capacitors be only on the +12V side without being connected to the GND? If not, why not?

    Thanks for any answers!
    Rob
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    A 7805 is a linear voltage regulator - not a DC - DC converter. The data sheet will explain the placement and sizing of the capacitors (use a search engine and "7805" as the search term).
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hi Rob,
    Welcome to the Forums.
    The circuit you have posted is simply a linear 5v regulator, and not a DC-DC converter. Linear regulators are cheap and easy to use, but quite inefficient, particularly if the input to output voltage differential is high. You have 12v in, and 5v out, so for every Watt of power dissipation in the load, you will have 12/5 of that power, or 2.4 Watts, expended in the regulator itself as heat for an efficiency of ~29.4% - not very good.

    DC-DC converters are switching supplies; they use an inductor and a switch (transistor or MOSFET with control circuitry) to efficiently convert one voltage to another; efficiency can be above 94% in many cases.

    But to answer your capacitor question....
    Wires have inductance. If the wires are more than a few inches long, the inductance of the wire will cause poor transient response at the other end of the wire; it takes time for current to start flowing through an inductor, and the same amount of time to decrease that same amount of current. The capacitor acts sort of like a bucket that you'd use for washing a car; it's a ready supply of water and you don't have to concern yourself with how much water you can get out of the hose after the bucket is filled; you have a ready supply of water.

    The 78xx series of regulators require at a minimum an 0.33uF capacitor from input to GND, and a 0.1uF capacitor from out to GND, otherwise the regulator may oscillate unpredictably at high frequencies (in the MHZ range; I have seen this happen). Adding a 10uF capacitor from OUT to GND will help considerably with transient response (removing momentary power "sags" or "peaks") when the circuit using the power has a change in requirements.

    Larger capacitors are good at storing large charges, but aluminum electrolytic capacitors (most frequently used for values >=1uF) don't have very good high frequency characteristics due to the parasitics involved (resistance, inductance of longer leads and large plates). So, a pair of caps are used, the small one usually being ceramic or metal poly film, and a larger aluminum electrolytic. And no, you couldn't replace a 0.1uF and a 10uF cap with a 10.1uF cap; they serve the same basic function but in different frequency ranges.
     
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