Ceramic Capacitors

Ceramic capacitors have found extensive use in well logging tools and surface electronics.  The old familiar ceramic disc capacitors were ubiquitous historically, with the mil. spec CK (BX) style molded ceramic capacitors becoming common in tools built in the 1970's and 1980s during that boom cycle in the oil patch.  Surface mount ceramic chip capacitors are becoming more common in the present generation of logging equipment, but leaded ceramic capacitors are still seen.  The Electronic Industries Association (EIA) classifies ceramic capacitors into four groups (Classes 1-4), and into types within the classes.  In general, the lower the class number, the better the overall characteristics, but the larger the size for a given capacitance value.

Class 1 Capacitors

Class 1 ceramic capacitors include the C0G types (also called NP0 from the old military "negative positive zero" standard, but said obsolete term is disdained by EIA purists), and various temperature compensating types.  For lower values (.01 uF and below) the use of C0G capacitors in critical downhole applications is a no-brainer as long as other considerations like voltage rating does not select against them.  Size becomes an issue in higher value C0G ceramic capacitors, and in fact it is difficult to find C0G ceramics above about .047 uF.  As can be seen from the table below, the tolerance is usually specified for only the 25-85C range, but the 30 ppm C0G tolerance is sometimes specified to 125C, and this would be a preferred part for downhole applications (200C parts are available from a few manufacturers and rarely 10 ppm tolerance parts are seen, but these extremes are not usually necessary and are of course expensive).

The "EIA Class 1 Capacitor Codes" table below shows the Class 1 EIA naming convention.  Not all types are commercially available (there would be a very limited market for a C0N device for instance).  EIA lists about 200 preferred devices, but only a couple of dozen are readily available.  The "Common Class 1 Caps" table below shows some of the commonly available versions along with the obsolete industry terminology which continues to be popular, probably because it is more intuitive with its use of "N" and "P" to denote negative and positive temperature coefficients.

Class 2 and Class 3 Capacitors

Class 2 and higher ceramic capacitors have a host of problems not seen in the Class 1 ceramics, including much worse temperature drift.  The higher K material like X7R was once considered too unreliable to use in critical applications, but quality has improved considerably.  Aging is a problem with these capacitors since the crystal structure slowly changes to a lower K form, but changes back after several hours at 125-150C, a characteristic with interesting implications for deep hot logging applications.  Class 2 and higher ceramic capacitors do have a size advantage, and the higher K versions can even compete with electrolytic capacitors in applications where low ESR is more important than bulk capacitance.

The "EIA Class 2 and Higher Capacitor Codes" table below shows the Class 2 and higher EIA naming convention.  X7R, Z5U, and Y5V are the most common versions available in the marketplace, but others are manufactured.  The X7R is a good candidate for signal coupling (DC blocking), power supply bypassing and other non-critical applications in downhole tools.  The mil spec molded CK (BX) ceramic capacitors, long a favorite in downhole tools, are equivalent to X7R.  A good rule of thumb is to use the lowest K material available and to use Class 2 or Class 3 ceramic capacitors at a low percentage of rated voltage.  The "EIA Tolerance Codes" table below shows the code and corresponding accuracy used with the EIA specification scheme for all classes.

Class 4 Capacitors and Closing Thoughts

Class 4 consists of the barrier layer reduced titanate ceramics.  These capacitors have fairly lousy electrical characteristics, but high Ks made them useful at one time.  The modern multilayer ceramics have pretty much made Class 4 ceramics obsolete.

Construction techniques for ceramic capacitors have changed dramatically over the last few decades.  The old single layer ceramic disc capacitor has given way to the multilayer ceramic capacitor (MLCC) with its alternating layers of ceramic and electrode material, each only a few microns thick.  This technology is the secret to increased capacitance in less and less space.  Surface mount (SMD) ceramic chip capacitors are made by the billions every year, and even us recalcitrant types have begun to use them in downhole designs, cost being the principle motivator in applications like high voltage power supply filtering and line driver coupling.

Ceramic capacitors are a good choice for a variety of logging electronics applications.  However, test oven trials may be necessary to qualify a particular part for a particular application since each manufacturer has their own ideas about ceramic formulations, and the host of other variables that determine how a capacitor will behave in the harsh conditions encountered in deep well logging.  Variability may even be seen in a single line of parts from the same manufacturer; for example, the molded mil spec BX parts from some manufacturers will actually exhibit NP0-like characteristics for the lower capacitance value parts.

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Last 10-20-10