The ceramic capacitors that you work with in the lab have two or more alternating layers of a metal acting as the electrodes and a ceramic acting as the dielectric. The capacitance measured in farad represents the charge stored in a capacitor at a particular applied voltage. The quantity should be a constant for a particular capacitor at all values of applied voltages and temperatures.
While working with Class I capacitors, you may find that their capacitances do not deviate from the expected values. However, Class II and Class III capacitors do show a marked deviation from the rated values. These capacitors have greater volumetric efficiencies, however. This means that they offer higher capacitances compared to the volume occupied by the capacitors.
An alphanumeric code of three characters designates the type of a class II capacitor. The first and second characters of the code indicate the lower and upper limits of temperature and the third character specifies the change of capacitance within the range.
Take the case of X7R, which is a popular Class II capacitor. The letter X indicates a lower limit of temperature of -55°C and the number 7 indicates an upper limit of +125°C. The third character R points to a change in the actual capacitance by +/- 15% from the rated value while the device is working within the temperature range defined above.
Deviation of Capacitances
In other words, you can expect that an X7R capacitor of a rated value of 4.7 microfarad might show a capacitance of 3.9 microfarad, while working under these temperature limits. However, it is a common occurrence to find that capacitors in certain circuits show a much more remarkable drop from their rated values. An X7R capacitor can exhibit a drop of 20%. Certain other Class II capacitors may show a drop as significant as 80% of their rated values of capacitances.
The real fact is that the rated capacitance value of a capacitor holds for a particular value of the applied voltage, also called the DC bias voltage. If the bias voltage is different from the specified value, the capacitor will offer a capacitance that is different from the rated value.
For instance, if you choose a capacitor of 4.7 microfarad designed to operate at 16 V, it may offer a capacitance as low as 1.5 microfarad while working at 12 V.
The code used to identify the capacitors does not indicate the exact variation of capacitance with the applied DC bias voltage. However, it is a known fact that Class II capacitors designated by the letter X are the most stable. The capacitors designated by the letter Y are less stable under adverse environmental conditions while the Z capacitors are the least stable.
To understand the problem, you need to study the data sheet for capacitors, which indicates the variation of capacitance with the applied bias voltage. The data sheet illustrates another interesting fact regarding capacitor sizes. A larger capacitor offers a greater capacitance at a particular DC bias voltage than a smaller one identified by the same alphanumeric code. Hence, you can expect a better performance with a larger capacitor than with a smaller capacitor of the same code. A possible reason for the fact could be that manufacturers have to compromise on the material while making smaller capacitors of the same code.