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What's the most temperature stable capacitor?

Discussion in 'Electronic Components' started by Nemo, Sep 9, 2007.

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  1. Nemo

    Nemo Guest

    I've been asked to design a low pass filter, but, it's to work over the
    range -40C to +65C (maybe +75C). The frequencies of interest are around
    2Hz so it looks like I'll need capacitors in the range 1uF to 4u7. Oh,
    and it's meant to be small so surface mount components would be best
    (but I'd be happy to sacrifice that requirement if necessary).

    What's the best kind of capacitor to use here? I don't think the usual
    dielectrics - X7R, etc - are characterised to -40C and I'm sure they
    will be 20% or more out by that temperature. Are there more exotic
    capacitors available, perhaps from non mainstream suppliers, for this
    kind of application, that might reach 1uF?

    Thank you

    Nemo
     
  2. One of the stable ceramic dielectrics is COG which has a temp. coefficient
    <30 ppm/C but you will not get near 1uF.
    Maybe you can mix some some plastic dielectrics like polyester and
    polypropylene. This is not SMD!
    Polyester is +400(+-200) ppm/C at 1 kHz, polypropylene -200(+50,-100) ppm at
    1 kHz according to the Evox-Rifa site.

    Try 1/3 polyester and 2/3 polypropylene and you might get something you can
    use.
    The capacitance will change over time but if you temperature cycle the
    capacitors from lowest to highest a few times before you use them they will
    stay more stable.

    Best regards
    Stig Carlsson
     
  3. Nemo

    Nemo Guest

    Stig Carlsson suggests
    ....
    Comprehensive answer with nice plot twist at the end - thank you -
    sounds extremely useful!
     
  4. Use smaller components, and synthesise the required value. Possibly use a
    gyrator, and an inductor.
    Or go for a complete active filter design. The most _accurate_ solution,
    will be to do this using a DSP (at this frequency, you could even
    potentially use quite normal processors, rather than DSP designs). With
    crystal timing, it'll beat anything possible with pssive components.
    You are not going to get small accurate bipolar capacitors. Even if you
    sacrifice the 'size' requirement, the cost will be silly....

    Best Wishes
     
  5. Hal Murray

    Hal Murray Guest

    How sharp a cutoff do you want?

    Can you do it in the digital world? (Quartz crystals are pretty
    stable over temperature.)
     
  6. Tilmann Reh

    Tilmann Reh Guest

    Also have a look at the MKM capacitors by WIMA - they are made of such a
    mixed dielectric, especially for (relative) temperature independent
    capacity.

    Tilmann
     
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