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Capacitors

Discussion in 'Electronic Basics' started by Jeffrey Turner, Mar 5, 2004.

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  1. First, those caps aren't in parallel there's the 5V regulator between
    them. Second, paralleling caps is done either to allow more charge
    storage (the capacitances add) or because of the frequency
    characteristics of the capacitors (a large value electrolytic for
    smoothing out the voltage and a small value cap for shorting high
    frequency spikes). Putting capacitors in series is rarely done as it
    gives you an overall smaller capacitor value, but it may be used when
    the full voltage is above the rating of either capacitor or if a
    specific value of capacitance must be achieved. Putting capacitors in
    series gives an overall capacitance of C1*C2/(C1 + C2) [similar to
    resistors in parallel].

    --Jeff

    --
    A man, a plan, a cat, a canal - Panama!

    Ho, ho, ho, hee, hee, hee
    and a couple of ha, ha, has;
    That's how we pass the day away,
    in the merry old land of Oz.
     
  2. electricked

    electricked Guest

    Hi all,

    I'm looking at the following website:
    http://electronics.howstuffworks.com/digital-electronics4.htm

    The two caps are connected in parallel. I'd like to know what's the
    difference in terms of practical applications of connecting the caps in
    parallel and series and how the circuit works.

    TIA!

    --Viktor
     
  3. The first cap is across (in parallel with) the input of the 7805
    and the second is across (in parallel with) the output. The caps
    would only be in parallel if there was only wire connecting both of
    their legs.
    They cannot mean a simple transformer. The input to the 7805 has to
    be a DC voltage. The capacitor is just to smooth the input.
    The capacitor on the output helps smooth out noise from the 7805. If
    your load might draw more current than the 7805 can supply you should
    use a different regulator or build your own. This is an easy to build
    switching regulator:

    http://www.romanblack.com/smps.htm
    At high frequencies, an ideal capacitor behaves like a short circuit.
    Electrolytics don't have very good response to spikes but ceramic caps
    work well in those applications. If there are really severe spikes
    then there are products like transorbs (and other things) that are
    designed to short them effectively. I designed with such things when
    I was designing burglar alarm systems that had to survive nearby
    lightning strikes, but those are beyond what most electronics require
    in normal operation. But it is not uncommon to put a small capacitor
    across the power supply pin of a chip to eliminate problems with RF.

    --Jeff

    --
    A man, a plan, a cat, a canal - Panama!

    Ho, ho, ho, hee, hee, hee
    and a couple of ha, ha, has;
    That's how we pass the day away,
    in the merry old land of Oz.
     
  4. electricked

    electricked Guest

    First, those caps aren't in parallel there's the 5V regulator between
    Thanks for the fast reply, Jeff!

    Hmm. They are not in parallel? I'm seeing it like this. The first cap is the
    main circuit coming from transformer. Then we have the regulator in parallel
    with the first cap and the second cap in parallel with the regulator. Am I
    reading this right? So since the first cap is in parallel with the regulator
    then it would smoothen out the voltage before it reaches the regulator. If
    the transformer (let's say 12V AC transformer) connection doesn't have a
    bridge rectifier, wouldn't the voltage be close to +12V in half cycle
    and -12V in the other half of the cycle with the smoothening cap in place?

    As far as the second cap which is in parallel with the regulator, I'm
    thinking if the voltage goes below 5 volts for some reason and the second
    cap has charged up, it would drive the voltage output back to 5V. Is this
    correct?

    One more question having to do with one of your questions. If there's a
    voltage spike and I use a small cap value, how would the cap short the
    spike? I'm interested in what the cap would do to get rid of the spike.

    Thanks!

    --Viktor
     
  5. electricked

    electricked Guest

    I'll keep reading trying to understand the practical uses for a cap.

    Thanks Jeff!

    --Viktor
     
  6. The name 'capacitor' derives from the word capacity. It 'stores' energy,
    acting like a little, weak, quickly discharged, quickly recharged battery.

    For your little circuit, the capacitor on the left of the regulator is used
    to store energy, since the power line is AC, and so 'rectifying' it leaves
    gaps when there is no energy being fed into the regulator. The capacitor
    supplies energy during those gaps.

    There are several reasons for the capacitor on the right side of the
    regulator. The first reason is that, again, capacitors are charge storage
    devices, and can thus respond to requirements of the circuit for charge much
    more quickly than the regulator (although not for as long, obviously.)

    The second reason is that the 7805 is really an amplifier, and amplifiers
    will oscillate in particular situations where some of the energy they are
    directing gets back into their input. Capacitors will soak up some of the
    energy by storing it, thus decreasing the feedback and preventing
    oscillations. This is particularly true of energy at higher frequencies,
    which turns out to be more problematic.

    On the input of integrated circuits, capacitors are again used as storage
    devices; digital ICs can suddenly draw large amounts of current, and if
    there isn't a capacitor across their power supply, the voltage of the power
    supply would droop in the region near the IC because of this. A capacitor
    will supply this relatively shortlived requirement for extra current, and
    thus prevent the power supply from drooping. This is good, because a droop
    might cause effects in nearby circuit elements.

    Another reason capacitors are used is in timing circuits. If you have a
    capacitor that is charged up to a voltage V, and you discharge it through a
    resistor of value R, then you can predict how long it will take for the volt
    age to drop to a given value using a simple formula. Thus, capacitors, in
    conjunction with resistors, are useful for creating timers of various kinds.

    Yet another reason folks use capacitors has to do with a phenomena called
    resonance. It turns out that there are devices called 'inductors', which,
    when put in a circuit with a capacitor, will 'resonate' at a frequency that
    can be predicted based on the values of the capacitor and inductor. This
    means that oscillators (called LC oscillators) with an extremely stable
    frequency can be built using capacitors.

    Capacitors, in circuits with resistors and inductors, can also be used to
    'filter' signals, allowing certain frequencies through, and supressing
    others. There is a whole science devoted to designing these 'passive
    filters'. Radios (used to) consist of passive filters, in which the
    frequency you want to listen to is selected using one of these filters, then
    passed through other circuitry to pick out the information from this
    frequency.

    Anyway, these are some of the uses of capacitors. They also make nice
    jewelry, and my youngest daughter used to call ceramic caps 'lolly pops' due
    to their shape.

    Regards,
    Bob Monsen
     
  7. electricked

    electricked Guest

    The name 'capacitor' derives from the word capacity. It 'stores' energy,
    Thanks Bob! Your post was definitely informative! I wish more people would
    explain what you just did when they explain capacitors before they start any
    of the math. If I were to write a tutorial on caps (and I WILL once I know
    more about them) I would explain certain applications as you just did, I
    would then show an example how to make a circuit using caps and then explain
    their values and math behind it; i.e. In series you use C1*C2/C1+C2 and in
    parallel you simply find the capacitances to get the final capacitance, or
    energy to be stored.

    Once again, thanks Bob! great info...

    --Viktor
     
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