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Thread: MIDI Schematic replace polarized cap

  1. #1
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    MIDI Schematic replace polarized cap

    Hi,

    I'm building a MIDI-controlled LED project and I would like to know if it's all right to replace the polarized 0.1uF capacitor going from +5v to ground (in the PJRC-recommended MIDI circuit) with a 0.1uF non-polarized ceramic capacitor. The smallest polarized cap I have is 1uF and I'd like to not have to buy more capacitors if possible.

    Thanks,
    Alex

  2. #2
    Senior Member PaulS's Avatar
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    I'm pretty sure that the 100nF cap mounted on the board, shown on this page, is not a polarized version. A 100nF ceramic cap will suffice here.

    Paul

  3. #3
    Senior Member PaulStoffregen's Avatar
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    It's not a polarized capacitor. There is no "+" or "-" shown on the schematic.


  4. #4
    Senior Member PaulS's Avatar
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    I think I know where the confusion comes from: long time ago I was tought at school this:
    Click image for larger version. 

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    The curved line on the polarized cap indicated the negative pin.

    Agree, an explicit '+' sign is more clear...
    Paul

  5. #5
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    ... and more to the point, itís a decoupling capacitor, so either your unpolarised 0.1uF one or your polarised 1uF one will be fine

  6. #6
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    Quote Originally Posted by PaulS View Post
    I think I know where the confusion comes from: long time ago I was tought at school this:
    Click image for larger version. 

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ID:	24594

    The curved line on the polarized cap indicated the negative pin.

    Agree, an explicit '+' sign is more clear...
    Paul
    This is why I was confused.

    Thanks everyone, it's working now.

  7. #7
    Senior Member PaulStoffregen's Avatar
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    While polarized vs nonpolar isn't important, this capacitor should be a ceramic or plastic film type, because those have the lowest impedance at higher frequencies. Aluminum electrolytic and tantalum capacitors are not as good for this usage.

    It should be connected as reasonably close as practical to pins 5 & 8 on the optocoupler.

  8. #8
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    I used a ceramic one. It's a pretty small dev board so none of it is very far from the optocoupler.

    Here's a picture of the result!

    Click image for larger version. 

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    Thanks again for the help!

  9. #9
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    Some plastic film are stacked, some are wound, and some wound types can be much more inductive and
    not likely to be much use for high speed decoupling.

    Standard practice is MLCC for high speed decoupling, and usually its the cheapest/smallest option too.

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