capacitor farad rating

Discussion in 'General Electronics Chat' started by s214519, Dec 13, 2014.

  1. s214519

    Thread Starter New Member

    Dec 10, 2014
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    If a circuit diagram calls for a certain capacitor you could use a capacitor with a slightly higher farad rating and it would work the same right?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Depends entirely on the application, some are very value specific.
    Max.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If the capacitor sets the frequency of oscillation or a time delay, then the capacitor directly influences that parameter.

    If the capacitor is used as filter, bypass, or coupling cap, then it is not so critical.
     
  4. s214519

    Thread Starter New Member

    Dec 10, 2014
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    A circuit I'm using calls for a couple capacitors that I can't seem to find. Since most capacitors have a tolerance of +-20%, as long as the capacitor is within a couple pF it should be fine right?
     
  5. MrChips

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    Oct 2, 2009
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    A 10pF capacitor with 20% tolerance maybe.

    Otherwise your assumption is incorrect.

    Show us the circuit with the capacitor in question and then we can discuss the tolerance required.
     
  6. s214519

    Thread Starter New Member

    Dec 10, 2014
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    The ones I can't find are the 203pF, 151pF, 103pF, 102pF, 222pF, 104pF, and 501pF
     
  7. MrChips

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    Whoever chose those component values must be nuts. Just round the last digit to zero.

    Let's do one at a time. First the transmitter:

    [​IMG]

    Item Value Substitute Comment
    C1 203pF 200-100,000pF non-critical filter cap
    C2 47μF 10μF-100μF non-critical supply filter cap
    C3 47 47 non-critical
    C4 68 68 non-critical
    C5 203 200-100,000 non-critical filter
    C6 68 68 non-critical
    C7 203 200-1000 non-critical HF filter
    C8 151 150 non-critical
    C9 151 150 tuned tank circuit will depend on L3 in any case
    C10 103 100 tuned tank circuit will depend on L3 in any case
     
    Last edited: Dec 14, 2014
  8. s214519

    Thread Starter New Member

    Dec 10, 2014
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    Thanks Mr chips! That's pretty much what I thought. The only ones that are important to have really close capacitance to what's written are the ones from L3 to ground.
     
  9. MrChips

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    Oct 2, 2009
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    There is a strong possibility that someone transcribed the values incorrectly from another schematic:

    Item Marking Value Comment
    C1 203 20nF non-critical filter cap
    C2 47μF 10μF-100μF non-critical supply filter cap
    C3 47 47pF non-critical
    C4 68 68pF non-critical
    C5 203 20nF non-critical filter
    C6 68 68pF non-critical
    C7 203 20nF non-critical HF filter
    C8 151 150pF non-critical
    C9 151 150pF tuned tank circuit will depend on L3 in any case
    C10 103 10nF this one does not seem right
     
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  10. MrChips

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    BTW, for a 27MHz crystal you want to choose one of the following frequencies used for 27MHz RC band:
    • 26.995 MHz -- Channel 1 (Brown)
    • 27.045 MHz -- Channel 2 (Red)
    • 27.095 MHz -- Channel 3 (Orange)
    • 27.145 MHz -- Channel 4 (Yellow)
    • 27.195 MHz -- Channel 5 (Green)
    • 27.255 MHz -- Channel 6 (Blue)
     
  11. s214519

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    Dec 10, 2014
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  12. MrChips

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    As far as I know 27.000MHz is not a legal frequency.
    Operating at this frequency will interfere with CB Channel 4 operating at 27.005MHz.
    There is a 10kHz separation between legal alloted channels in the 27MHz band which has been allocated for CB and RC.
     
  13. s214519

    Thread Starter New Member

    Dec 10, 2014
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    Well it's +-20ppm.
     
  14. MrChips

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    That doesn't matter. You need to understand frequency spread caused by modulation.
     
  15. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Read the bill of materials again. I do believe the third numeral is a power of 10.
     
  16. s214519

    Thread Starter New Member

    Dec 10, 2014
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    Ok MrChips, sorry. I understand what you're saying now.
     
  17. s214519

    Thread Starter New Member

    Dec 10, 2014
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    Shouldn't there be a receiver crystal in the receiver circuit? Or does this circuit not need one?
     
  18. s214519

    Thread Starter New Member

    Dec 10, 2014
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    If anyone has any suggestions on a completely different transmitter/receiver feel free to tell me. I just need at least 4 channels.
     
  19. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Get yourself some of these. Do not get the ones without the encoder/decoder chips.

    I am using 4 of the 315MHz transmitters and one of the receivers. I mounted the receiver inside a weatherproof fullsize 1/2 wavelength vertically-polarized Dipole antenna which is mounted on the outside front of my metal hangar building. I am able to open/close a swing gate, two garage doors, and the aircraft hangar door from a distance of about 100m from the inside of of a car.

    Gate015a.jpg

    It is all about how good the receive antenna is, and where it is mounted....
     
    Last edited: Dec 15, 2014
  20. s214519

    Thread Starter New Member

    Dec 10, 2014
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    Both the transmitter and receiver should have a crystal to set them at the same frequency right?
    And is this a better circuit for what I'm trying to do?
    Screenshot_2014-12-15-02-26-18.png
     
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