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Choosing ceramic capacitors

Discussion in 'Electronic Design' started by Andrew Holme, May 17, 2009.

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  1. Andrew Holme

    Andrew Holme Guest

    If I search Farnell (Newark) for 1uF 0603 ceramic chip capacitors, I get a
    choice of 8 suitable parts. Rated voltages vary from 6.3V to 25V. Prices
    vary from 0.01 GBP to 0.26 GBP. Makes are Kemet, Murata, AVX and Yageo.
    How do I choose? Which is the best make? In general, is it best to buy the
    lowest rated voltage? Are more expensive capacitors more reliable? Should
    I just roll a dice?

    TIA
    Andrew.
     
  2. Nico Coesel

    Nico Coesel Guest

    You should compare the specifications of the dielectric. As a rule of
    thumb the working voltage should ba half the rated voltage. When the
    applied voltage comes close to the rated voltage, the capacitance
    decreases dramatically.
     
  3. Andrew Holme

    Andrew Holme Guest

    All but one are X5R and 6.3V is already twice my Vdd!

    The one is X7R at 0.099 GBP.
     
  4. Ecnerwal

    Ecnerwal Guest

    Depends a lot on what you are doing. IMHO:

    Stocking hobby bench parts - aim for a tradeoff of high voltage and low
    price (assuming you care about low price - if you care to believe that
    the most expensive part is best, then buy that) - pick your point on the
    curve, given that higher voltage parts will work at lower voltages, and
    the physical size is the same. Buy at least 100 for the price break -
    they don't take up much space.

    If building for a particular project where cost is more the object and
    you won't be looking at different voltage applications, the math works
    differently, though if reliability is key then using 10V parts on 5V is
    generally better than using 6.3V parts on 5V.

    If you can actually find country of origin information on the parts and
    you can avoid Chinese ones, that is worth doing.

    Personal preference is to stop downsizing at 0805, but I'm doing hand
    assembly and rarely need the smallest possible board. Ease of hand
    assembly wins, and 0805 is enough easier than 0603 that I generally
    avoid smaller packages.
     
  5. Hello Andrew,

    The X7R material is less voltage and temperature dependent if both materials
    are rated for the same voltage.

    Best regards,
    Helmut
     
  6. Guest

    Ok, opposite question, whats the WORST dielectric with respect to
    applied voltage, A idea for a VCXO comes to mind.

    Steve Roberts
     
  7. Jim Thompson

    Jim Thompson Guest

    I doubt that you want the worst. You only want a small tuning range.
    right? Dielectric voltage sensitivity can be a double edged sword...
    distortion products at large signal swing.

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona 85048 Skype: Contacts Only | |
    | Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
    | E-mail Icon at http://www.analog-innovations.com | 1962 |

    Stormy on the East Coast today... due to Bush's failed policies.
     
  8. whit3rd

    whit3rd Guest

    There are ferroelectric ceramics that have very high capacitance,
    but take a persistent stored charge if taken to high voltage.

    There is hysteresis.

    The temperature effects are horrid, too (a buddy once got a stack
    and tried to match them with a bridge; his warm hands caused
    nonrepeatable capacitance measurements).

    The "C" in VCO stands for "controlled", you do NOT want to use
    those worst dielectrics. For power filters where hysteresis energy
    loss is
    a good thing, they're fine.
     
  9. Looking at Digikey in-stock parts, I'd pick the Taiyo Yuden 1.0uF/10V
    X7R at $40 US for a reel of 4,000 (0.0066 GBP pp).

    Good dielectric, widely useful voltage rating, reliable brand, good
    price, and lots in stock.


    Best regards,
    Spehro Pefhany
     
  10. James Arthur

    James Arthur Guest

    These are pretty bad:
    http://www.avx.com/docs/Catalogs/cy5v.pdf

    Z5U might be worse.
     
  11. Tim Williams

    Tim Williams Guest

    Wow, those are almost as good as hyperabrupt varactors, a decade's
    capacitance in just a few volts. At constant temperature. Or PTC/NTC
    thermistors, for measuring temperature (at constant voltage)...

    Tim
     
  12. Guest

    Perfect, Thanks, Steve.
     
  13. Jim Thompson

    Jim Thompson Guest

    Wheeeeee ;-)

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona 85048 Skype: Contacts Only | |
    | Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
    | E-mail Icon at http://www.analog-innovations.com | 1962 |

    Stormy on the East Coast today... due to Bush's failed policies.
     
  14. qrk

    qrk Guest

    Beware of Murata's X7R material, it has a higher voltage dependence
    than most other manufacturers.
     
  15. Hello Mark,

    Thanks for the tip.
    I will have a closer look next time when I choose a ceramic capacitor.

    Best regards,
    Helmut
     
  16. James Arthur

    James Arthur Guest

    I've got a reel or two rated 6VDC. If you use the 10% to 20% of WVDC
    region you get ~50% change per volt!

    I've been meaning to use them as varactors some day...

    Cheers,
    James Arthur
     
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