All politicians promise but some deliver & a few never. Ceramic capacitors resemble the later in the world of electronics.
Capacitor, also known as condenser, is a device with two electrodes (two conductors) and a dielectric in between. The use & application of this simple device is as same as the use of salt in cookery.
The classroom theory on this component is quite simple and straightforward. However, recently it turned out to be that this simple device seriously defies its straightforwardness and demands much more respect and attention.
The premise for this post is ceramic capacitors which are very prevalent.
Capacitors come with their ratings imprinted on them, same as candidates before an election. For example, 2.2uF/1206/250V/… The value 2.2uF is not gospel truth. In fact it is the first lie that the device (or the manufacturer) professes.
The capacitance of a capacitor is true only when it was just manufactured, at room temperature, at the atmospheric pressure, at no physical deformation & more interestingly at NO DC VOLTAGE across the capacitor. If I may rephrase, the rated capacitance is valid only when all these conditions are met at the same time. Under these circumstance, can a capacitor be ever trusted?
Following is a short explanation of degradation of capacitance in a capacitor.
From the hour it was born, capacitance of a capacitor decays. This decay is significant in the first 100 hours while the decay slows down as it usually follows a logarithmic curve. For example, a 2.2 uF capacitor can be 2 uF after 4 days of its birth and 1.8 uF after 1 year. Remember that this is still without any voltage applied to it !
However, thanks to the dielectric properties, the aging can be reset to zero by heating the capacitor upto 150 deg C for several minutes. This information should be obtained by the manufacturer.
2. Size and form
2.2 uF ceramic capacitor with different sizes can perform differently and give different tolerances. This should be kept in mind in your designs.
3. DC bias
This is the funniest part of all of a capacitor. Although, a capacitor may come as 2.2uF/250V as soon as it is applied 250V dc across the device, capacitance can drop up to 70%. This means that 2.2uF might be actually 0.6uF at its rated 250V dc. This is a serious point to note in the critical engineering designs.
Temperature is another factor that affects the capacitance. However, on this, the manufacturers are open and the coefficient is usually noted.
There is an intriguing phenomena that when the DC bias already degrades the capacitance, the effect of temperature rise is next to nothing. I presume that the property of dielectric that both DC bias & temperature rise affect is the same.
Pressure is another factor that affects the capacitance of a capacitor. But usual engineering work are not intended for high pressure applications. A lot of R&D is involved in the pressure related capacitors.
There we come to the end of a practical preview of the crooked capacitors. Also note that a capacitor can accumulate charge even if it was completely discharged due to a phenomena called dielectric absorption. Therefore discharging capacitive components before handle is & always will be a good engineering practice.