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100K capacitor = 100pf?

Discussion in 'General Electronics Discussion' started by TallPaul, Apr 16, 2018.

  1. TallPaul

    TallPaul

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    Oct 5, 2014
    I'm replacing ceramic disc caps in an 80's amplifier and I pulled 5 that had CGE 100K markings. They all measured 100pF. According to the Internet 100K is suppose to be 10pF. They are 10mm wide and thats kind of big for a 10pF. It only has to deal with 15v from the regulator. All the other ceramics of different values measured correctly according to the markings. Could these capacitors drifted that far in 30 years? The capacitors are CGE ceramic discs with epoxy coating.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Agreed.
    No way.

    Shouldn't be necessary unless these are clearly defect.
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009

    It's a trap that young players can fall into
    it depends on if the 3 digit code is being used eg
    100 = 10pF
    101 = 100pF
    102 = 1000pF etc

    or if the cap is being marked in pF which is also very common, in which case 100 = 100pF
     
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  4. Harald Kapp

    Harald Kapp Moderator Moderator

    8,797
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    Nov 17, 2011
    ... which is the likely explanation here.
     
  5. kellys_eye

    kellys_eye

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    Jun 25, 2010
    I'd like to see a picture of the capacitors in question.

    My own opinion is that they should be 0.1μF

    15V?? Regulator??? We're talking something like a pre-amp supply in a solid state circuit so 100pF?????????

    Could be a problem with the way they're being measured????
     
    hevans1944 likes this.
  6. Audioguru

    Audioguru

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    100k picofarads is 0.1uF. The datasheet for a Texas Instruments uA7815 shows a 0.1uF capacitor at the output.
    100pF will not work. 10pF is stray wiring capacitance.
     
    hevans1944 likes this.
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    8,797
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    Nov 17, 2011
    Assuming the measurement is correct, then the letter K is for +- 10 % tolerance.
     
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  8. Robert_fay

    Robert_fay

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    Jun 15, 2017
    100k could be correct for 100pF. Some caps only change to the 3 digit when they go over the 100pF rating. So a 10pF would be marked as only 10 and the K would be tolerance 10%.
     

    Attached Files:

  9. kellys_eye

    kellys_eye

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    ....a 10mm wide capacitor on a 15V regulated supply line and 100pF??? Something is wrong with this... all the 100pF capacitors I have (rated at 50 to 100V) are 5mm in diameter and around 1mm thick.
     
  10. Robert_fay

    Robert_fay

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    Jun 15, 2017
    Any chance for a picture of the markings and cap. May clear up some options!
     
  11. Cannonball

    Cannonball

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    May 6, 2017
    100k, the 100 is the value of the capacitor in pfs.
     
  12. BobK

    BobK

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    All my 10pF caps are marked 10 not 100. All under 100 are marked with pF in 2 digits. All my 100pF ones are marked 101. So 100 and 101 might very well both mean 100pF.

    bob
     
    davenn likes this.
  13. TallPaul

    TallPaul

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    Oct 5, 2014
    And the winner is 100pF. I'm using a Fluke multimeter. They don't look like cheap junk ceramics. Back in the 80's this amp was $600. I had to replace the twin power caps and I don't do anything half a--. I replaced all the electrolytics with Nichicon Muse and all the ceramics with silver micas
     
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  14. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Which Fluke? Does it read accurately with other known-capacitances?

    I'd like to see the schematic for the circuit in question to see how they can justify using 100pF on a 15V regulated line.....
     
  15. TallPaul

    TallPaul

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    4
    Oct 5, 2014
    Finally found the schematic and the Fluke was correct. 100K for CGE brand is 100p. I doubt if this cap company is in business anymore. Since it was early 80's it was pre-China junk era. More than likely they were made in the US
    100p.JPG
     
    Harald Kapp likes this.
  16. kellys_eye

    kellys_eye

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    Jun 25, 2010
    I call 'misdirection'...!!!

    your schematic shows the capacitor to be RF suppression/decoupling across the inputs of the op-amp - perfectly suited to 100pF.

    But, back to the original 'fault' - these capacitors rarely, if ever, go faulty. Even if they changed value by 500% it would likely make no appreciable difference to operation.
     
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