Please ID this compnent (cap?)

Discussion in 'General Electronics Chat' started by ehhsims, May 24, 2016.

  1. ehhsims

    Thread Starter New Member

    May 24, 2016
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    [​IMG]

    This is an amplifier for measuring very low currents (~pA) by measuring the voltage across a big high-precision feedback resistor (RH2HVS, 1e+11 ohms). The resistor and this other silver component (KP F) are both inserted into sockets (not soldered) so they can be easily swapped out.

    What is the silver thing, I am guessing a capacitor (making for a sensitive RC circuit) but the "KP F" lettering is not getting me anywhere. It looks like foil encased in glass. Any ideas what it is (and where I might be able to get more)?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
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    Hello,

    It looks like a styroflex capacitor.
    Those are often used in circuits that need high temperature stability.
    There might be more info on the other side of the component.

    Bertus
     
  3. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    Another word for that is, "polystyrene". Very low leakage, rather stable amount of capacitance across time and temperature.
    The main problem is that the label is on the other side.:(
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    Could it be a compensation cap in a current to voltage opamp circuit?
     
  5. ehhsims

    Thread Starter New Member

    May 24, 2016
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    Thanks!

    Yes the board is normally located inside a housing that is evacuated to 1e-3 bars, and heated a few degrees above room temperature for stability.

    The resistor Johnson noise would presumably be reduced by cooling, but then condensation happens...
     
  6. ehhsims

    Thread Starter New Member

    May 24, 2016
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    Ah - on Vishay.com I found that "KP" = polypropylene.

    An this:
    "Polypropylene (KP) films are used in high frequency or high voltage applications due to their very low dissipation factor and high dielectric strength. These films are used in AC and pulse capacitors and interference suppression capacitors for mains applications."

    Now to get the capacitance value....I may have to pull it off the board.
     
  7. ehhsims

    Thread Starter New Member

    May 24, 2016
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    OK - get this - the cap is a 2 pF Styroflex.

    I have googled and cannot find a supplier that carries these in 2 pF - anyone have any leads?
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    I seriously doubt that the capacitor is a 2 pF capacitor. There is 2 pf of stray capacitance in those leads. Get a grip on reality. If that value was measured then I would question the measurement method and or the instrument.
     
  9. ehhsims

    Thread Starter New Member

    May 24, 2016
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    2pF cap coupled with 1e+12 ohm feedback resistor - RC time constant is 2 seconds. It is an amplifier for measuring small currents (on order of pA) by passing the current through the resistor and measuring the voltage across.
     
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