Basic question on capacitors in parallel

Discussion in 'General Electronics Chat' started by J0ker, Apr 25, 2012.

  1. J0ker

    Thread Starter New Member

    Apr 18, 2012
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    Hi everyone!

    I'm learning the basics and I struggle a little bit on the "why" of some components. For example:

    [​IMG]


    Excuse me for the Paint Brush diagram :) . In this circuit, i can assume that when DC provides briefly less voltage, the capacitor will act as a "temporary supplier" to the lamp, so we won't even notice right?

    Now, in this diagram, is the capacitor useful for any other situation? Does it make any difference if, for example, there's a high voltage spike in the main supply?

    Thank you!

    PD: the circle represents a lamp or something like that
     
    Last edited: Apr 25, 2012
  2. #12

    Expert

    Nov 30, 2010
    16,261
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    It depends on the size of the capacitor compared to the current used by the lamp and the time called, "briefly".

    The basic equasion for changing voltage on a capacitor is Vo = dV x e^-t/RC
    Volts out equal change in voltage times e to the negative time over resistance capacitance.

    Plug in "briefly", apparent resistance of the bulb, capacitance, and "slightly less voltage" to get the droop of voltage feeding the lamp.

    If you start with 12 volts and the apparent resistance of the bulb is 12,000 ohms (for a .001 amp current draw), briefly is a millisecond, and the capacitor is a whole farad, you won't notice any change. If "briefly" is half a second at zero volts and the capacitor is 100 picofarads, you will see it go dark.

    A capacitor of the right size can absorb fast spikes on the power line by 99% or better. It all depends on how much, how fast, and how big the capacitor is.
     
  3. steveb

    Senior Member

    Jul 3, 2008
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    Another consideration is the series resistances in the capacitor and the battery. Neither device is perfect and there is an effective series resistor on both devices. Often these values play an important part in figuring out what will happen. Draw the resistors in to help visualize what effect they have.
     
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  4. #12

    Expert

    Nov 30, 2010
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    Good point. For "Basic Light Bulbs and Batteries 101" the resistances that are not shown in a common schematic don't matter much but, the time will come when they do matter. Get in the habit of drawing parasitic capacitances and equivalent series resistances. They sometimes start mattering at only a few kilohertz.
     
  5. J0ker

    Thread Starter New Member

    Apr 18, 2012
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    Thank you, now what about the second part: in this diagram, is the capacitor useful for any other situation different than covering up a "brief" lack of voltage?

    Would a capacitor make any effect on a circuit such as that besides what we just discussed?
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Did you notice the "sticky" on decoupling or bypass capacitors at the head of this forum?
     
  7. J0ker

    Thread Starter New Member

    Apr 18, 2012
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    I just did and also checked the website in it...but i just find myself in the same point :(

    I'm having a hard time understanding the role of capacitors in parallel. Maybe it's just the physics of it (i.e. why they act like that) what's driving me crazy.

    Any resources or answers to stuff my head into?
     
  8. steveb

    Senior Member

    Jul 3, 2008
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    I think the reason why you are having trouble is that you drew a very simple circuit that does not contain any realistic nonideal behavior. Consider that there will be a wire or printed circuit trance making connections from power to load. This wire will pick up noise. It will also have parasitic inductance and resistance. Both the capacitor and the battery have source resistances. Also, in general the load may have changing current demands over time.

    Try drawing a circuit that includes all of these effects. Imagine what happens when a load changes its required current demand. The line inductance will resist the current change and generate a voltage drop. Also, the line resistance implies a changing voltage drop with changes in current. This means that the load will see a changing voltage over time. Also, there is noise pickup on the line and this also presents voltage variations to the load. So, by placing additional capacitance on the line, both big ones farther from the load and small ones closer to the load, one is giving the load a better approximation for a DC power source.

    There are additional situations one can consider, but hopefully this shows why the parallel capacitors are important. To really appreciate this, try building real circuits and leaving the caps out. You will see that even simple circuits become unreliable, or completely stop working.
     
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I understand your frustration - particularly as you are apparently just starting out in learning electronics. Sometimes a lack of knowledge in the field makes it difficult to ask the 'right' questions.

    You originally asked :

    "Now, in this diagram, is the capacitor useful for any other situation? Does it make any difference if, for example, there's a high voltage spike in the main supply?"

    The other posters made some important points concerning the disparity between your rather basic circuit representation and the physical reality that exists in a circuit that one actually constructs from components.

    One would normally (& wisely) capacitively decouple real DC power supplies and key active components (such as digital logic IC's) to mitigate the problems that parasitic circuit elements (e.g. wiring or PC board track inductance & resistance etc.) can impose on circuit performance.

    The (undesirable) possibility of a high voltage spike appearing at the output of a real power supply could well be the reason for the inclusion of a parallel capacitance - but this would normally be a component included in the design of a typical DC power supply anyway. A good designer would hope to anticipate and effectively eliminate the likelihood of DC power supply output voltage spikes entering the circuit being powered.
     
  10. J0ker

    Thread Starter New Member

    Apr 18, 2012
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    Thank you for trying, electronics is turning out very difficult for me, and I really don't know why. I'm a former programmer and a math freak, I never used to need explanations to understand stuff, maybe I'm getting too old. I'm not going to give up anyways.

    I'll keep hitting my head against a couple books i'm following even though i didn't grasp part of the basics. I have a huge feeling that no book explains electronics as they should... but if all the lights are coming to me, maybe i'm the one going the wrong way.

    I'm finding kind of useful also this applet http://www.falstad.com/circuit_es/

    F$%&!
     
  11. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I imagine if you hopped in the time machine & went back to the day you wrote your first computer program you would be amazed at the difference in sophistication in your last program. Once the aha moment arrives you'll be cruising - especially with that strong math background.
     
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