Basic cap question

Discussion in 'General Electronics Chat' started by Mossen, Jul 30, 2011.

  1. Mossen

    Thread Starter New Member

    Dec 21, 2010
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    I'm thinking about building this circuit and I'm just looking through my parts to see which pieces I already have. Anyway on the schematic it notes that some of the caps are NP. Are NP caps generally interchangeable with regular caps, or do they serve a completely different purpose altogether? For instance, how are those 100nF caps functioning in the circuit? Why can't they be combined with the 200uF cap right next door?

    [​IMG]
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    NP equates to Non-polarized. For the best performance you will need to use genuine non-polarized capacitors of the specified value.

    hgmjr
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    This is a basic question about power supply filtering. Capacitors have different characteristics besides capacitance values. The large polarized electrolytic capacitors, e.g. 220uF/35V are good at smoothing low frequency fluctuations but are lousy at high frequencies. The 100nF capacitors help to suppress high frequency spikes. You need both. You cannot combine the two as if they were capacitors in parallel.

    In this audio power amp application, you could eliminate the 100nF caps and you wouldn't notice the difference.
     
  4. Mossen

    Thread Starter New Member

    Dec 21, 2010
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    OK thanks for that explanation. But if I wouldn't notice the difference with or without those 100nF caps, why are they there?
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    There are almost unlimited ways to design circuits. You will find in every more complex schematic components you could take out or modify their values. The 100nF are recommended in the datasheet and the designer usually refers to the datasheet. The circuit may work without them or may not in worst case conditions.

    From the datasheet:
    "Proper layout of the printed circuit board is very important.While the LM1875 will be stable when installed in a board similar to the ones shown in this data sheet, it is sometimes necessary to modify the layout somewhat to suit the physical requirements of a particular application. When designing different layout, it is important to return the load ground, the
    output compensation ground, and the low level (feedback and input) grounds to the circuit board ground point through separate paths. Otherwise, large currents flowing along ground conductor will generate voltages on the conductor which can effectively act as signals at the input, resulting in high frequency oscillation or excessive distortion. It is advisable to keep the output compensation components and the
    0.1 µF supply decoupling capacitors as close as possible to the LM1875 to reduce the effects of PCB trace resistance and inductance. For the same reason, the ground return paths for these components should be as short as possible"
     
  6. Mossen

    Thread Starter New Member

    Dec 21, 2010
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    Wow, thanks! All of this is so complex...I have a technical background but wrapping my mind around electrical components has been really difficult for me. So many things to consider....:confused:
     
  7. gootee

    Senior Member

    Apr 24, 2007
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    Note that in the case of those 100 nF (0.1 uF) and 220 uF decoupling caps, they should be connected as close as possible to the chipamp's power pins.

    Decoupling caps can be thought of as small point-of-load power supplies. When the chip's power pin needs a sudden rise in current, the inductance of the power supply conductors could prevent the current coming through them from changing fast-enough, and would also cause a voltage spike to be induced on the power rail, according to V = L di/dt. But the decoupling capacitors can supply the current almost instantly, since there is no long conductor's inductance in the way. If high frquencies were involved, you would have to pay a lot more attention to the inductance and placement of the decoupling caps.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Why are you talking about the power supply capacitors when the only capacitor marked NP is the 2.2uF/16V NP input coupling capacitor?

    This capacitor has AC across it so it must be NP.

    Many products have NP electrolytic capacitors and Digikey sells many.
     
  9. Mossen

    Thread Starter New Member

    Dec 21, 2010
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    Audioguru, your response brought up a question for me...why are some of the caps marked with their polarity, some are marked NP, and some aren't marked at all...? Is this error/carelessness or...? I downloaded this schematic from the NatSemi website so I doubt it but, just wondering....
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Electrolytic capacitors are used for power supoply filtering of low frequency hum. They have high values, small size and are inexpensive. Their values have a wide tolerance. Most are polarized but NP ones are available.
    A schematic will always show the polarity if it is polarized and will always show NP if they must be NP.

    A ceramic capacitor has low values and is non-polar. It is good at very high frequencies so it can be used in radio circuits and as a power supply high frequency filter. It can prevent logic and opamp ICs from oscillating.

    A metalized plastic film capacitor has values up to about 1uf (huge and expensive ones up to 22uF are used in speaker crossovers) and is used for coupling in audio circuits. It is non-polar.
     
  11. Mossen

    Thread Starter New Member

    Dec 21, 2010
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    OK, cool, so for the caps that aren't marked at all in the schematic (except for their value) ... what does that mean? What type of capacitor do I use? Is it an oversight by the author?
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    You need to learn about which type of capacitors are used where in circuits.
     
  13. Mossen

    Thread Starter New Member

    Dec 21, 2010
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    That's one of the reasons why I'm asking this question, and one of the reasons why I joined this forum.
     
    Last edited: Aug 3, 2011
  14. bertus

    Administrator

    Apr 5, 2008
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