DATA SHEET: CAPACITORS #1

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From: Paul Harden (pharden@aoc.nrao.edu)
Date: Fri Mar 10 1995 - 11:21:27 EST


DATA SHEET de NA5N: CAPACITORS (part 1 of 2)
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CONTENTS: Brief description of major capacitor types (non-electrolytic)
           Capacitor identification and coding schemes
           Temperature Characteristics

    ************ CAPACITOR TYPES: (non-electrolytic) ************

DISK CERAMICS consist of two metalic plates whose area and spacing deter-
mines the capacitance, separated with a ceramic film dielectric and housed
in an epoxy molding.
>Advantages: inexpensive, small, high C per unit size
>Disadvantages: high capacity changes over the temperature range
>Uses: blocking, coupling, bypassing and energy storage applications
>Use only NPO (see below) for critical timing or oscillator circuits

MONOLYTHIC/MULTILAYER CERAMICS are very similar to disk ceramics in
characteristics and usage. The major difference is monolythics are made
from MULTIPLE layers of electrodes and dielectric films to yield high C in
small packages. For this reason, they are also called MULTILAYER ceramics.
The multilayered element is usually thermally fused to yield a
chip ... which by itself is packaged as a surface mounted chip capacitor,
or surrounded by an epoxy moulding to form a monolythic capacitor.
>Advantages: inexpensive, small, high C per unit size
>Disadvantages: high capacity changes over the temperature range
>Uses: blocking, coupling, bypassing and energy storage applications
>Use only NPO (see below) for critical timing or oscillator circuits

POLYESTER FILM capacitors use layers of metal and polyester (mylar)
dielectric to make a wide range of capacitances in a small package and
have become the standard for DC applications.
>Advantages: inexpensive, small, high C per unit size
>Disadvantages: high capacity changes over the temperature range and
    high loss factors, limiting their use at high frequencies
>Uses: blocking, coupling, bypassing and energy storage applications

POLYCARBONATE FILM capacitors are layers of metalized film and poly-
carbonate dielectric for an almost ideal capacitor. These caps have become
the standard for Mil-spec film dielectrics.
>Advantages: high insulation resistance, minimal capacitance change with
   temperature and low loss (dissipation factor).
>Disadvantages: costly (3-5 times a disk ceramic)
>Uses: blocking, coupling, bypassing, frequency discrimination (like RC
   filters), critical timing circuits, precision oscillator circuits, and
   operation in high temperature environments.

POLYPROPYLENE FILM capacitors use layers of metal and polypropylene
dielectric for low moisture absorption and high breakdown voltages.
>Advantages: Low loss factors and good capacitance stability over the
   temperature range for high frequency applications, H.V. circuits.
>Disadvantages: Polypropylene has a low dielectric, resulting in larger
   physical sizes for comparable capacitances and working voltages than
   most other capacitor types. About twice the cost of disk ceramics.

METAL vs METALIZED FILMS. Capacitor electrodes have traditionally been
made from metal alloys. Metalized films use vacuum sputtering techniques
to coat a dielectric film with a metal composition to form high C on very
thin pieces of film, making for very small sizes. They also tend to be
"self healing" in that a high voltage arc will vaporize the metal deposit
but will be contained by the dielectric, such that the damage remains
localized. An arc will thus not short the capacitor, a common problem
with older metal alloy types.

    **************** CAPACITOR IDENTIFICATION ******************
Capacitors are identified by:
1. Direct value marking (i.e. "27pF 35V")
2. EIA Identification Coding (i.e., "104M"= .1uF, 20%)
3. Color coding schemes (pretend this is red-yellow-orange!)

Since color coding schemes are disappearing except for some foreign made
tantalums, and direct marking is fairly obvious, only the EIA coding
scheme is described.
                               ********
Typical EIA coded capacitor: * Z5U * ---> Temperature characteristics
                               * 224J * ---> Value and tolerance
                               ********
                                | |
TEMPERATURE CHARACTERISTICS:
  Min. Temp. Max. Temp. Cap. change over temp. range
 (degrees C) (degrees C) (+/- percentage)
  X=-55C 2=+45C 4=65C A=1% B=1.5% C=2.2%
  Y=-30C 5=+85C 6=+105C D=3.3% E=4.7% F=7.5%
  Z=+10C 7=+125C P=10% R=15% S=20%
                                    T=-33%,+22% U=-56%,+22%
                                    V=-82%,+22%

THEREFORE, a Z5U has an operating temperature range of +10C to +85C
and its capacitance changes from -56%C (at +10C) to +22%C (at 85C).

VALUE AND TOLERANCE:
  Example: "2 2 4 M"
            --- - --------> Tolerance Codes (+/- %)
             | | F=1pF or 1% G=2pF or 2% (if C>10pf, then %)
             | | J=5% K=10% L=15% M=20% N=30%
   Capacitance Number P=0%, +100% W=-20%, +40%
     in pF of zeros Y=-20%, +50% Z=-20%, +80%

THEREFORE, a 224M = 22 0000pF = .22uF, 20% tolerance

QUICK REFERENCE CHART (Decade values)
  1R0 = 1pF 103 = .01uF
  100 = 10pF 104 = .1uF
  101 = 100pF 105 = 1uF
  102 =.001uF 106 = 10uF
                     1010= Some numbers used on 10 meters!

AFTER ALL OF THAT ... when ordering capacitors, the THREE most common sold
by most mail order vendors are
        Z5U (Class 1) ........ +10C to +85C, -56%,+22%
        X7R (Class 2) ........ -55C to +125C, 15% (fairly flat <100C)
    and NP0 (Class 3 or C0G) ... -55C to +150C, <1.5% (ideally flat)

The "N" codes indicate the amount of NEGATIVE capacitance change per
degree C.
Thus, NP0 = 0 ppm/deg C
      N150 = -150 ppm/deg C
      N750 = -750 ppm/deg C ... ETC.

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WARNING: The Surgeon General has determined that chewing on orange
tantalum "gum drop" capacitors may be hazardous to your health.

GL and have fun, Paul NA5N (pharden@nrao.edu)


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